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2nd EAGE International Conference on Fault and Top Seals - From Pore to Basin Scale 2009
- Conference date: 21 Sep 2009 - 24 Sep 2009
- Location: Montpellier, France
- ISBN: 978-90-73781-69-6
- Published: 21 September 2009
78 results
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Integrated Trap Analysis – Best Practices and Workflows for Evaluating Potential Fault Traps and Closures
Authors F.V. Corona, J.S. Davis and S.J. HipplerExxonMobil's Integrated Trap Analysis (ITA) is a set of best practices and technical workflows for evaluating trap and seal in specific business environments. The ITA workflows contain recommendations for the application of appropriate tools and technologies in fault interpretation, bed and fault seal evaluation, and volumetric assessment of potential traps and closures for all upstream business stages. ITA best practice workflows differentiate between time scales (exploration vs. production) and the technical workflow is adjusted to reflect the time scale being analyzed. As well, rather than forcing an improbable solution using a single technology (e.g., capillary seal analysis), ITA workflows require the analyst to integrate multiple approaches to seal evaluation, which allows solutions that incorporate multiple seal mechanisms. Application of ITA workflows and associated technologies has impacted business decisions ranging from frontier and mature basin exploration, to development plans in new fields, to infill and near-field wildcat drilling in and around mature fields. Implementation of the ITA best practices has improved efficiency and quality in fault interpretation and structural mapping, promoted consistent application of appropriate technologies for trap and seal definition, and improved linkage of trap and seal analysis with volumetric assessment.
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Seal Effectiveness Prediction Using a BP Proprietary Toolkit
Authors M.J. Osborne, T. Barwise and J.S. HallOutline of talk - Business importance of seal effectiveness - Leakage processes and evidence for them - Characterising mudrock seals - Data uncertainties and limitations - Introduction to BP proprietary toolkit- BP Petroleum Prediction Toolkit o Seal Risk basic o Seal Risk Advanced - Example of usage - Conclusions
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Stochastic Trap Analysis and Risking – A Multi-seed Stochastic Approach to Fully 3D (Geocellular-based) Trap Analysis
Authors J.C. Pickens, N. Smith, H. de Vries, H. Mehmet, J. Nieuwkamp, R. Bennett and O. HoutzagerTrap analysis is inherently a three-dimensional problem involving fault geometry, stratigraphy, and seal integrity. The complex interplay of these elements can make it very difficult to understand and visualize the controls on column height – even for a trap analysis expert. Compounding the issue is the uncertainty associated with reservoir distribution, fault offset and seal thickness. Despite the often equivocal nature of the input, fault seal analysis in the industry has historically been done in a deterministic manner with some attempt at varying the stratigraphic model to give low, medium and high cases. Further, the calculation of fault sealing potential has commonly been done on a per fault basis rather than within the greater context of a trap framework. There are, of course, vendor products and practitioners that address column height prediction in an integrated and perhaps stochastic fashion. However, it could be argued that these practices have created a perception that fault seal analysis is a black box affair, which is often perceived as an ‘art’ rather than a rigorous investigation.
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From Static to Dynamic Fault Retention Potential Evaluation – A New Approach
Authors J-M. Janiseck, T. Cornu and C. WibberleyIn exploration the classical analysis of the retention potential of a fault zone in hydrostatic domain integrates a geometrical analysis (relay zone/segmentation), a reservoir juxtaposition study, a fault rock properties description and a re-activation tendency. The evaluation of the maximum hydrocarbon column height retention is performed at the present time using the fault entry pressure. In overpressure domain the fault entry pressure can not be used. There is a necessity to use a more dynamic approach: the fault zone permeability. Furthermore a fault zone evolves through time: fault initiation, fault sliding, fault burial, fault reactivation. And during this "life" its impact on fluid flow varies. The characterization of the fault zone permeability through time is so necessary. As several parameters can modify the fault zone permeability (temperature, pressure, stress, diagenesis) a complete understanding of the fault zone permeability through time is needed. To evaluate these parameters evolution through time our current work integrates a combination of outcrops description, geomechanical experimentation and basin modelling.
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Integrated Top Seal Evaluation for Robust Solutions
Authors J.S. Davis, S.A. Barboza and X. LiuPrediction of hydrocarbon column heights and contacts requires simultaneous evaluation of capillary and mechanical bed seal properties together with trap geometry (crest, spill), estimation and quantification of uncertainties, and propagation of these uncertainties through the analysis. We developed a method for making a probabilistic determination of total seal capacity for a hydrocarbon trap, simultaneously considering both bed seal leak mechanisms (capillary entry pressure and hydraulic fracture pressure), and accounting for trap geometry. Assuming unlimited gas and oil charge and uniform seal properties, simultaneous evaluation of top seal capacity and geometric controls on hydrocarbon fill, leads to recognition of only six possible combinations of hydrocarbon fill control in two-phase (gas and oil) systems. Single phase systems are controlled by only one of the three leak mechanisms. Different combinations of top seal and geometric spill controls can achieve very similar final hydrocarbon distributions in hydrocarbon traps. Without investigating all possible controls it is easy to draw incorrect conclusions about controls on hydrocarbon fill in any single accumulation, promoting incorrect prognostications about undrilled traps. When considering field development plans that include pressure support, misunderstanding actual controls on fluid distributions could lead to unanticipated outcomes, such as leakage through the top seal.
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Integrated Quantitative Seal Assessment for Exploration Projects
Authors M. Brundiers and J. KonstantyToday seal evaluations are commonly applied within the industry: during the prospect evaluation process of an exploration project it is essential to predict the probability of the hydrocarbon column height based on seal predictions. The introduction of various quantitative prediction algorithms and related software has opened the way for industry application; however the quality of seal evaluations in exploration projects is still varying from qualitative estimates over inconsistent applications to deterministic capacity estimates. Oversimplification and biased contact scenarios can result in misleading volume predictions and it may have additional negative impact on the prospect chance estimates. To enable consistent prospect evaluation concerning column heights, WINTERSHALL has developed a corporate standard for seal evaluations which is systematically applied to all exploration projects. The methodology integrates top and fault seal workflows resulting in structural related hydrocarbon column heights. Examples from various basins will be used to demonstrate the great potential of integrated quantitative seal assessment (QSA), its limitations in carbonates and the application and integration of QSA in play evaluations. The influence of comparative probabilistic assessment of top and fault seal capacities and proper statistical treatment of seal capacity results play a pivotal role in corporate exploration decision making.
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Empirical Correlations to Shale Strength
Authors D.N Dewhurst, C. Delle Piane, M.B. Clennell, M. Josh, M.D. Raven and A.F. SigginsA suite of preserved shales from basins widely separated in space and time underwent multi-stage triaxial tests in the laboratory to evaluate static and dynamic mechanical properties. Coupled with these tests was a workflow for extensive characterisation of physical and petrophysical properties, including porosity, bulk density, specific surface area, electrical properties and microstructural evaluation. Good correlations have been found between shale strength (cohesion and unconfined compressive strength) and some physical properties and there are encouraging relationships between dielectric properties and both static and dynamic mechanical properties. Poor relationships were found to friction coefficient and also between properties previously thought good for predicting shale strength, such as P-wave velocity.
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Faults Enhancement Based on 3D Geological Model Analysis
More LessA methodology for faults enhancement from seismic volume is presented. This method proposes to detect the faults distribution on the basis on the 3D analysis of a continuous geological model. This geological model is obtained by using an innovative technique based on global optimization algorithms in the PaleoScan software. The analysis of the geo-model allows to detect the faults network with a high level of accuracy and to estimate the throw along each fault plane. This approach has been successfully tested on various data sets where the throw analysis was required to identify the sealing faults for geo-steering and well placement applications. A case study in Nigeria is presented.
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Multi-phase Flow Properties of Fault Rocks – Implications Prediction of Across-fault Flow During Production
Authors Q.J. Fisher, S. Al-Hinai, C. Gratonni and P. GuiseHere we present new data on the relative permeability and capillary pressure of fault rocks as well as the stress dependency of relative permeability. The high capillary pressures and low relative permeability of some well lithified cataclastic faults helps explains why some Rotliegend reservoirs in the Southern North Sea are so severely compartmentalised by faults. The low capillary pressure and high relative permeabilities of poorly lithified phyllosilicate-framework fault rocks may explain why it is often possible to obtain good history matches of production data from faulted Brent-type reservoirs without incorporating the multiphase flow properties of fault rocks into simulation models. The multi-phase flow properties of some samples are highly stress dependent even during elastic deformation. These new data suggest that it may be necessary to take into account the stress-dependence of fault rock flow properties when modelling across-fault fluid flow in compartmentalised petroleum reservoirs.
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Subsurface Observations of Deformation Bands and Their Impact on Hydrocarbon Production within the Holstein Field, Gulf of Mexico, USA
Authors S.J. Naruk, S. Wilkins, R. Davies, J. Bikun, O. Uzoh and L. JensenThe Holstein Field in the Gulf of Mexico consists of Pliocene, poorly lithified turbidite sands deposited in a ponded basin above an allochtonous salt tongue. Dense arrays of cataclastic deformation bands have been observed in core from wells that penetrate the K2 reservoir sand. The highest density of bands is located near the hinge of a monoclinal fold that divides the field into an up-dip terrace and a down-dip, steeply-dipping ramp. The predominant set of deformation bands strike parallel to the fold axis, and dip at both high and low angles to bedding. Their orientation, and offset of marker beds where present, suggest reverse shear. Restorations indicate that the deformation bands formed early during the burial process, and an inferred stress path suggests that high fluid pressures during the initial phase of burial was an important component. Reservoir permeability estimates from PTA well tests indicate a bulk permeability approximately one third of the reservoir core permeability. In comparison, the reservoir bulk permeability calculated on the basis of the deformation bands' actual permeabilities, thicknesses and densities, exceeds the well-test permeability by a factor of two. Additional factors are required to account for the well test results.
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Evidence for the Long-term Barrier Integrity of Ordovician Rocks – The Deep Geologic Repository Project, Ontario, Canada
Authors M.R. Jensen, J.F. Sykes, K. Raven, I.D. Clark and T. AlThe Nuclear Waste Management Organization in late 2006 initiated geoscientific investigations at the Bruce Nuclear site near Tiverton, Ontario, Canada, to verify suitability for development of a Deep Geologic Repository for long-term management of Low & Intermediate Level Radioactive Waste generated at Ontario Power Generation owned facilities. The Bruce Nuclear site, located on the eastern flank of the Michigan Basin, is underlain by ca. 840 m of Cambrian to Devonian age sediments that lie unconformably on the crystalline Shield. The DGR, as envisioned, would be excavated at a depth of 680 m within the massive Cobourg Formation, an Ordovician argillaceous limestone that is overlain by ca. 200 m of Ordovician shale. Field and laboratory studies conducted as part of a deep borehole drilling, coring, testing and instrumentation program are yielding evidence of an ancient, diffusion dominant groundwater regime that is resilient to repeated glacial perturbations. This evidence includes measurements of extraordinarily low rock mass hydraulic conductivities(≤10-12 m sec-1), matrix porosities (0.01-0.08) and diffusivities (Dp≤10-11 m2 sec-1), and observation of anomalously depressed environmental heads (ca. 250 m) and vertical environmental tracer distributions within the Ordovician sediments. This presentation describes program results relevant to understanding groundwater regime stability and DGR safety.
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Gas Migration into Low Permeability Callovo-Oxfordian Argillite
Authors J. Talandier, F. Skoczylas, B.M. Krooss and M. PratGas migration into argillaceous formations is an important issue to evaluate the perturbation induced by gas in an underground radioactive waste repository. The mechanisms controlling gas entry and flow into clay media are not fully understood yet and have to be investigated. In that context, Andra and its partners developed an experimental program to characterize the gas behavior of the Callovo-Oxfordian argillite which is the host rock for the French radioactive waste repository. Gas entry pressure and some key parameters such as gas permeability have been measured. We present first the main experimental results obtained on argillite. Then, the analysis of the results leads to discussions about the two-phase flow model used to predict gas migration at the repository scale.
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Laboratory and Field Experiments on Gas Transport in the Opalinus Clay of Northern Switzerland
Authors P. Marschall and L. HolzerComprehensive laboratory and field investigations on the gas transport capacity of Opalinus Clay were conducted in the Mont Terri Underground Rock Laboratory (URL) in the Swiss Jura mountains and in a deep investigation borehole in the Molasse Basin of Northern Switzerland. Field investigations included hydraulic packer tests and gas injection tests in boreholes. The laboratory investigations on Opalinus Clay drillcores from Mont Terri comprised microstructural analyses, determination of the capillary pressure relationship and gas permeability measurements in isostatic cells. In this context advanced techniques were utilised, allowing for 3-D visualization of the microstructure of solid matter down to the nanometer scale. Fluid filled rock samples were investigated with cryo-techniques (shock freezing), which reduced substantially the appearance of artifacts associated with the sample preparation. Thus the focused ion beam-nanotomography (FIB-nt) with a resolution in the order of 10 nm and Transmission Electron Microscopy (TEM) with an expected resolution < 5nm have been applied for 3-D characterization of the pore structure of Opalinus Clay and in particular for the determination of quantitative measures of the pore connectivity.
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Fault Reactivation and Fault Properties – 3D Geomechanical Modeling Approach and Application to Nuclear Waste Disposal
Authors R. Soliva, L. Maerten, F. Maerten, I. Aaltonen, L. Wilkström and J. MattilaWe present the preliminary results of a methodology that consists on modeling 3D fault reactivation and to relate both the computed fault slip and residual stresses to fault seal and leakage capacity. A series of geomechanical simulations is run on a 3D fault model, which integrates friction, cohesion, far field stress and fluid pressure as variables. The hundreds of simulations are analyzed as a whole in order to determine the likelihood of fault reactivation with respect to the variable parameters. Each reactivated fault is then analyzed independently. Fault sealing and/or leakage capacity can be estimated when the relationship between computed fault slip and/or residual stresses and fault zone deformation mechanisms is known. The methodology has been applied and tested on the Olkiluoto nuclear waste repository site, Finland. The method can be applied to petroleum reservoirs, where depletion can affects fault reactivation, hence fault sealing and leakage reliability.
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Dating and Constraining Leakage Rates from a Natural Analogue for CO2 Storage – The Little Grand Wash and Salt Wash Fault
Authors N.M. Burnside, B. Dockrill, Z.K. Shipton and R.M. EllamCapture and Storage of CO2 could decrease global carbon emissions on short time scales. Leakage rates from geological formations are poorly constrained, and are therefore an unknown factor in CCS feasibility studies. Natural CO2 reservoirs in the Colorado Plateau, USA, are analogues for geological storage. In places CO2 has migrated to the surface along fault zones, forming CO2-charged springs and geysers. Recent drilling for oil and water has also provided rapid pathways for CO2 migration. Uranium-series dating of travertine mounds along two fault zones in Utah has provided insight to the timing and rates of leakage along these faults. A continuous record of leakage has been preserved over the last 315ka, with the oldest dated mound at 413ka. The position of leakage has switched through time, and individual pathways have been utilised more than once for flow. Combining dates with volume measurements from these mounds it is possible to calculate the rates and volumes of flow in individual pathways and the long-term time-averaged CO2 leakage rate for the entire system. The observation that leakage of CO2-rich groundwater from a fault can occur for hundreds of thousands of years has implications for geological storage of CO2.
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Analysis of the Swelling Pressure Development in Opalinus Clay – Experimental and Modelling Aspects
Authors H. Péron, S. Salager, M. Nuth, P. Marschall and L. LalouiIn the context of nuclear waste geological storage, deep argillaceous formations are likely to be subjected to complex mechanical, hydraulic, and thermal loads. In particular, the argillaceous material can be firstly dried, and then re-wetted. During the latter process, the material experiences swelling and can develop swelling pressure if swelling deformations are constrained. In this contribution, the results of swelling pressure tests on shale performed in totally constrained conditions (isochoric tests) are presented. A constitutive model (ACMEG-S) is used to predict the value of the swelling pressure in such conditions. The model is made of two parts. The mechanical part addresses the stress-strain behaviour of the material, as a result of effective stress variation. An elasto-plastic approach is employed, and Bishop's unsaturated effective stress, which is a function of the degree of saturation, the suction and the externally applied stress, is used as the mechanical stress. The water retention part of the model defines the relation between the degree of saturation and the suction within the material. The results put into light some factors that control the swelling pressure value, in particular the degree of saturation and the plastic behaviour of the material.
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Simulation of Gas Migration in a Waste Disposal in Deep Clay Formation – What Pathways? What Time and Space Scales?
Authors J. Wendling, F. Plas and E. TreilleA radioactive waste disposal in a deep clay formation is mainly based on the low, water and gas, permeability of the host rock. This property returns to a low capacity of gases evacuation and hydrogen produced by anoxic corrosion and radiolysis of organic waste, will affect the repository on time. At first raises the phenomenological question at the different space scales: what processes? , what pathways? , what gas pressure? , what couplings? Secondly raises the question of the safety functions: opening of preferential pathways due to gas fracturation?, transfer induced by water displacement due to gas or transfer in gas form? A first answer was brought by Andra in the Dossier 2005-Argile which concluded on the feasibility of the disposal. Since then, Andra has continued (i) researches on characterization of gas migration processes, and (ii) numerical simulation of the production and the migration of gas. The numerical work is covering scales from the disposal cell up to the whole disposal and from operating period to long term post closure. These new results will in term contribute to update a pluriannual program of data acquisitions on fundamental mechanisms of gas production and migration.
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Structural Controls on Leakage from a Natural CO2 Geologic Storage Site – Central Utah, USA
Authors B. Dockrill and Z.K. ShiptonWe present outcrop studies of a leaky natural CO2 accumulation in central Utah, U.S.A. The fluid migration history through the faulted stratigraphy is established from field relationships. Fluids charged and diffused through a carrier bed, migrating up-dip to pool in the structural high created by the faulted, Green River Anticline. The fault forms a lateral seal, while overlying clay-rich cap rocks provide a transient top seal. Fractures in the damage zone to the fault compromise the sealing integrity of this top seal and enable vertical migration of fluids, preferentially around structurally complexities where the fracture network is most intense. The fluids subsequently charge a shallower sand-rich carrier bed and the process continues through the faulted sequence. The dependence of folding and fault-associated fracture permeability is paramount in controlling fluid flow in the study area and emphasizes the influence structure plays on the migration of fluids in sedimentary basins. These results emphasise the need for detailed fault analysis of structures within future engineered CO2 storage sites, and consideration of the burial history and timing of fault activity to assess the risk of CO2 leakage through cap rocks, fault cores and fault damage zones.
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Petroleum Leakage in the Snorre Oilfield, North Sea – A Case for Change in Cap-rock Wettability and Dynamic Seal Risking
Authors O.A. Jokanola, A.C. Aplin, S.R. Larter and K.K. KurtevSnorre is a leaky commercial oilfield that supports ~300 m oil column in the Tampen Spur area of the North Sea. The field contains undersaturated oil with no gas cap. Pervasive evidence of oil and gas leakage into the cap-rock is well documented (Leith and Fallick 1997, Bond 2000). Oil migration may alter the wetting state of the cap-rock. Today, the reservoir sands are ~12 MPa overpressure. Log derived pore pressure for the Shetland Group mudstone cap-rock indicates a normal compaction state. This pressure disequilibrium between the reservoir and the cap-rock suggest that reservoir overpressure may have post-dated compaction and geologically recent. Phase behaviour models of the petroleum fluids indicate that a gas cap should exist in the trap if the reservoir was normally pressured. The observed oil column is in equilibrium with the average column height potential of the cap-rock estimated from pore size distribution data. This may imply that the cap-rock is at maximum supportable column height today. In addition, subsequent petroleum charge has leaked into the cap-rock possibly at a rate similar to the reservoir charge rate. These data suggest that the Snorre field capped by oil-wet cap-rock is a case example of a dynamic seal.
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Design of a Complete Characterisation of Bentonite Behaviour under High Pressure and High Temperature
Authors M. Rizzi, S. Salager, P. Marschall and L. LalouiRepositories in deep clay geological formations are considered one of the most promising solutions for a sustainable management of High Level radioactive Waste (HLW). The Swiss HLW disposal concept consists in horizontal tunnels excavated at high depth in strongly over consolidated clay (Opalinus Clay) where Granular Bentonite is chosen for enclosing steel canisters containing the waste. This paper presents the theoretical approach and the research activities aiming at investigating the behaviour of this material. An elasto-plastic constitutive model taking into account coupled processes of stress, capillary pressure, and temperature is used. In this framework, from an experimental point of view, an exhaustive characterization is necessary in order to calibrate required parameters and to validate the model. Laboratory tests designed for this purpose are described. Particular attention is paid in investigating the swelling behaviour, which is among the features that distinguish materials potentially usable in nuclear waste disposals. First results confirm indeed that the chosen bentonite shows a natural and remarkable swelling attitude.
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Leakage Through Top Seals During Reservoir Production
Authors L. Grande and F. CuisiatSome drainage of pore fluids in the top seals might occur at the time scale of production due to changes of pore pressure in the reservoir. The resulting compaction may have consequences for the sealing capacities of the cap rock, as it could induce leakage along the well annulus. In order to investigate capillary effects and drainage of intra-reservoir and/or overburden seals, special laboratory tests have been performed in NGI's rock mechanics laboratory. Examples of the depth of influence of the reservoir pressure diffusion (depletion) inside a top seal are given on a field scale. Also the additional compaction due to drainage from the cap rock, under burden and mudstone intervals within the reservoir is investigated on a field scale with a 1D coupled consolidation program. The additional compaction and settlement are found to be significant and this may affect the wells or installations in an unfavorable way (well failure or leakage along wells etc). It is therefore important to characterize not only the hydro-mechanical behavior of the reservoir sediments during production, but also that of the near non-pay sediments.
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Hydraulic Top Seal Failure – Hydrocarbon Preservation in HP/HT Central North Sea, Viking Graben and Mid-Norway Regions
Authors S.A. O'Connor, R.E. Swarbrick, R.W. Lahann, J.P. Clegg and D.T. ScottPart of the risking strategy for prospects is an assessment of seal breach risk at top reservoir, i.e. when the seal may be breached by high pore fluid pressures causing hydraulic fractures in the top-seal. Prediction of seal breach through hydraulic failure involves pore fluid pressures reaching or exceeding the minimum stress plus the tensile strength of the seal rock. Analysis of the relationship between pore pressures and least stress has been undertaken in the HP/HT areas of the Central North Sea, Mid-Norway and Viking Graben to establish the key controls on hydrocarbon preservation. A strong relationship using aquifer pressures has been established in the Central North Sea, particularly when analysis is repeated at BCU and Base Chalk levels i.e. at shallower levels in the overlying seal than top reservoir. Geospatially, these relationships do not appear to hold true in the other areas. The result is a risking tool for exploration in certain basins, although the mechanisms involved remain unclear.
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Top Seal Assessment in High Pressure – High Temperature Plays – Evidence from the UK Central Graben
Authors D. Casabianca and J.W. CosgroveThe current methodology of predicting top seal integrity in high-pressure/high-temperature (HPHT) plays (Gaarenstoom, et al., 1993) is to assume that the caprock, defined as the low matrix permeability formation immediately overlying the reservoir, is the seal (e.g. the Kimmeridge Clay Formation above older sandstone of the UK Central Graben Mesozoic play fairways). This study challenges this assumption and proposes the existence within the caprock of a fluid waste zone consisting of a system of fractures cutting from the reservoir up into or even through the caprock and therefore charged with reservoir fluids. Because of the waste zone the caprock cannot seal the underlying reservoir fluids. Seismic and elastic logs data help identify any fracture waste zone and also any stratigraphic boundary capable of containing it (or arresting its upward propagation).
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Predicting Fault Zone Architecture in the Subsurface from Outcrop Analogues and the Expected Impact on Flow
Authors R.K. Davies, R.J. Knipe, C. Souque, M. Welch, H. Lickorish and C. TueckmantelThe importance of defining the petrophysical properties of fault rocks within fault zones for reservoir flow modeling is well understood. A common observation, however, from well-exposed outcrop examples of faults is a zone of imbricated lenses and splays along the fault, which show a more complex architecture than can be captured with existing models. Modifications of the algorithms and models for the fault zone development are shown based on the outcrop examples that incorporate these fault zone complexities.
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Fault Facies and Analysis of Fault Heterogeneity Through Rock Lenses
Authors A.B. Braathen, R.H. Gabrielsen, E. Bastesen, M. Lindanger, J. Clausen and J. TverangerWe analyze fault envelope heterogeneity with implications for fault seal predictions, emphasizing fault core lenses as conduits for flow. The approach goes through 3D modeling of faults, applying the fault facies concept as a tool in statistical pattern recognition and database compilation. Various facies, such as fault lens, can be examined through shape, position, intrinsic deformation, and formation mechanisms. The outcome is statistical validated generic models for fault architecture that can be utilized in fault envelope design and subsequent flow considerations.
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3D Discrete Element Method Modelling of Fault Zone Internal Structure
Authors M.P.J. Schöpfer, C. Childs and J.J. WalshFaults are often simplified as planar structures but are, in reality, complex zones comprised of multiple slip surfaces that contain variably deformed rock volumes, ranging from intact fault bound lenses to fault rock (breccia, gouge). This sub-resolution structure has a direct impact on the juxtaposition geometries across faults and ultimately their impact on fluid flow. We use a commercially available implementation of the Discrete Element Method (DEM), which represents rock as an assembly of cemented spheres, to model the propagation of normal faults through mechanically layered sequences. The fault zone evolution observed in the models demonstrates the main processes thought to be the cause of internal complexity in fault zone structure and the model faults replicate a range of features observed in normal faults at outcrop; these include multi-stranded fault zones, relay zones, normal drag, asperities and corrugated fault surfaces. Systematic variation in the internal structure of model faults with both changes in the lithological sequence and confining pressure suggest that this type of modelling can provide a basis for evaluating the likely complexity of fault zone structure and associated sequence juxtapositions.
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Emergent Distributions of Stress and Strain in Fault Damage Zones
Authors H. Lewis, A. Paez, J. Ma and G.D. CouplesSimulation models of faulting based on realistic poro-plastic geomaterials develop complex but well-ordered strain patterns that are very similar to those observed in natural fault damage zones. The stress states in the simulations are complex, and exhibit substantial changes during the faulting process. The material properties of the materials in the FDZs are significantly altered as the deformation progresses. None of the current approaches to predicting fault stability or fault sealing is compatible with the conditions revealed by these simulations. There is a need to re-consider the basis for making such predictions to determine where the current methods might be approximately right, and where new methods are required.
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Using Probabilistic Shale Smear Factor to Relate SGR Predictions of Column Height to Fault-zone Heterogeneity
By G. YieldingThe Shale Gouge Ratio (SGR) algorithm uses the clay distribution through the wall-rocks, together with the fault displacement, to estimate an 'average' clay content at each part of the fault zone, completely ignoring the detailed fault-rock distribution. This average value is often correlated with particular fault-rocks observed in small-scale samples. However, probabilistic modelling of shale smear distributions shows that these can produce a variable sealing capacity, dependent on the arrangement of disrupted smears in the fault zone. The resulting SGR vs column height relationship is analogous to the conventional empirical calibration of SGR vs buoyancy pressure. However, it arises with only two components in the model fault zone: infinitely sealing clay smears and non-sealing remainder. Variable capillary threshold pressure of fault-rock is not required to explain the trend of trapped column height with SGR.
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Probabilistic Shale Smear Modelling – Scale Effects on Sealing
Authors T. Manzocchi, C. Childs and J. WalshSealing effects of multiple shale smears are poorly described by traditional algorithms (e.g. SGR, SSF etc). The newer PSSF algorithm contains a more representative fault rock model, and is directly applicable to some types of fault seal problem at some scales. However, it is an averaging algorithm and explicit object-based models of shale smears are sometimes required. Both the applicability of the different approaches, and the overall effects of shale smears on across-fault connectivity, are strongly scale-dependent.
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Fault-controlled Carbonate Cementation along Leaking Faults – Implications for Diagenetic Fault Sealing
By P. EichhublCarbonate fault cement can lead to effective sealing of faults that act as conduits for upward fluid flow. In conductive faults, fluid flow, and thus cementation, is typically focused along fault intersections, extensional steps, and fault terminations. Based on the analysis of four carbonate-cemented faults, I propose that carbonate fault cements form by 1. the microbial oxidation of hydrocarbons and 2. exsolution and degassing of CO2 during rapid upward fluid flow. Depending on the quantity and distribution of oil migration, microbial oxidation of oil can lead to discontinuous cements and thus ineffective seals. In contrast, oxidation of migrating methane is observed to lead to effective fault seal. Degassing of CO2 during upward flow can also lead to effective fault seal provided pressure gradients are steep in the flow direction. Under subsurface conditions, steep pressure gradients are expected for coseismic seal failure and rapid fluid flow.
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Heterogeneous Fault Zone Structure and Related Flow Localization Within Limestones
Authors J.J. Walsh, C.G. Bonson, C. Childs and T. ManzocchiHigh quality outcrop and mine data for normal faults within limestone sequences are used to highlight the inherently heterogenous nature of fault rock and fault zone structure. This heterogeneity is attributed to fault-related segmentation and refraction compounded by progressive breaching of segment boundaries (i.e. relays) and the removal of fault surface asperities with increasing displacement. Relay breaching and fault linkage is responsible for rapid changes in fault zone structure, with the generation of high fracture densities and increased host rock brecciation. Recent work on classic Zn-Pb Irish mineral deposits, hosted within Carboniferous limestones, suggests that the accentuated deformation associated with relay breaching and related fault bends is responsible for the creation of sub-vertical zones of high permeability which act as conduits for upward flow of mineralizing fluids from underlying basement rocks. These zones of enhanced permeability occur on a range of scales, most often below the limit of hydrocarbon and mineral exploration datasets. This talk considers the potential implications of the strongly heterogeneous nature of fault zone structure and related flow, for a variety of application areas, including hydrocarbon exploration/production and CO2 storage.
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Inferring Sealing Properties of Faults in Carbonates by Comparing Field Examples with Stress Evolutionary Models
Authors I. Mannino, P. Cianfarra and F. SalviniPermeability in carbonates is strongly influenced by the presence of fracture patterns. Carbonate rocks achieve fracturing both during diagenesis and tectonic processes. Spatial distribution of brittle deformation rules the secondary permeability of carbonatic rocks and therefore the accumulation and the pathway of deep fluids (ground-water, hydrocarbon). This is particularly true in the development of faults where damage zone and fault core show different hydraulic properties. In this work we studied the brittle deformation in carbonates related to fault kinematics to better understand the hydraulic properties of fault rocks. Quantitative analyses of brittle deformation from field measurements were compared to numerical models performed by FRAPtre software. This numerical tool allows to study the 4D evolution of stress/brittle deformation in fault related rocks.
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Fault Zone Structure and Diagenetic Evolution in Porous Carbonate Rocks, Provence, France
Authors R. Soliva, P. Labaume, L. Ducarme, J. Lamarche and J.P. MasseWe provide an analysis of normal fault zones exceptionally well exposed in porous calcarenites from La Fare les Oliviers (Provence, France). The fault zones crop out along their entire length due to their higher resistance to erosion compared to the porous host rock, and provide excellent exposures for detailed study. This high resistance to erosion is due to significant diagenetical and lithological modifications of the fault damage zone (i.e. pore cementation and sediment infiltration from the surface). The fault core zone is characterized by cataclastic shear and constitutes a vertical path for late gravity-driven oxidant fluids assisting karstification. The fault zone composition and structure is therefore the result of the interplay between the fault zone mechanical behaviour and the the Earth surface environement (marine vs continental).
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Are Cataclastic Shear Bands Fluid Barriers or Capillarity Conduits? Insight from the Analysis of Redox Fronts in Porous
Authors T. Cavailhes, R. Soliva, A. Benedicto, D. Loggia, R.A. Schultz and C.A.J. WibberleyThe effect of cataclastic derformation bands on fluid flow in a non saturated zones is still unclear. To discuss interaction between fluid flow and cataclastic bands in the vadose zone, we examine the spatial distribution of oxidation fronts around bands in the Turonian porous sandstones of Urchaux (SE France). Our study shows new field evidences that cataclastic bands behave as permeability barrier but also as capillarity conduits for wetweak fluid.
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Improved Implementation of Fault Seal Analysis in Reservoir Simulation
Authors S.R. Freeman, S.D. Harris, R.J. Knipe, K. Wood and V.S. O'Connorion of fault seal workflows in the reservoir simulation process is the inability to interactively update fault properties to fit to dynamic data. Often significant effort is expended in the generation of viable fault properties and geometries but this initial estimate will often be in error (due to the natural variability of the systems and the inaccuracy with which we can define the various parameters). When the reservoir engineers fail to generate a history match from this initial estimate the lack of geologically driven editing tools mean that often the geologically derived fault properties are replaced by uniform, fault wide single values which are editable. This major loss of data and understanding is due in part to a lack of editing tools available to the reservoir engineer that honors’ the geological form of the data. Providing tools that can allow interactive creation and editing of fault properties should allow for a far greater utilization and improved application of fault seal analysis in the simulation process. This enhanced integration of geological and reservoir engineering knowledge should ultimately lead to better management of the reservoir.
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Geomechanics Predicts the Characteristics of Overpressured Basins
By G.D. CouplesSharp pressure transition zones, and alterations of stresses within overpressure compartments, are features that are predictable and expected consequences arising from the geomechanical behaviours of typical mud-rich successions. A qualitative analysis based on poro-plastic material responses explains how seals form, how they fail, and how the stress state evolves within compartments. The analysis predicts that rocks within overpressure compartments may be very weak due to dilational deformations. These conditions pose major challenges to drilling and production operations.
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History Matching of Reservoirs by Modifying Fault Properties Using 4D Seismic Results – A Synthetic Study
Authors R. Villegas, C. MacBeth and A. BenguiguiThis work we proposes a new integrated methodology which includes geophysics, time lapse seismic data, and history matching to improve the spatial distribution of the fault properties. We also take into account during the history matching the prior probability distribution of the transmissibility obtained using 4D seismic and a prior reservoir simulation model. For updating the fault transmissibility values into the simulation model, we consider the prior probability distribution of the transmissibility values generated using the results of a previous methodology which uses time lapse seismic data and also involves the estimation of fault and matrix permeability to estimate the transmissibility multiplier (Manzocchi et al., 1999 and 2002). These data are then processed during the automatic history matching methodology proposed in this work to achieve fully structured 3D profiles of the underlying reservoirs which comply with the available prior information and which honour production data. Here we apply our methodology in a synthetic but realistic case.
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Fault Seal Analysis in the N-97 Oil Field in Wintershall Concession C97-I, Libya
Authors L.H. Bellmann, W. Kouwe and G. YieldingThe N-97 Oil Field is located in the western Sirte Basin, Libya. Differential fluid contact and higher aquifer pressure in the east of the field suggest sealing faults. However, the initial oil pressure is in equilibrium. There are two possible explanations: either the eastern part of the field could have a perched aquifer, or the trapping could be hydrodynamic, with east-to-west water flow and progressively lower pressures in each fault block from east to west, giving progressively deeper contacts. Physical properties of the fault rock were measured in the lab: pore size distribution, permeability and mercury injection capillary curves.These fault rock properties indicated that a single fault should not be able to support the difference in OWC. Fault seal properties have been modelled in conjunction with fault displacement indicating that a single fault hold an oil column height up to 350ft. The preferred explanation of the observed pressure differences in the aquifer is a hydrodynamic aquifer drive from East to West. Aquifer pressure drops occur at each fault resulting in a series of fault compartments with successively deeper contacts and lower aquifer pressure, but with a common oil gradient.
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Modelling of Pyrite Anomalies above Hydrocarbon Accumulations Due to Persistent Leakage Across a Non-perfect Top Seal
Authors E.O. Kudryavceva, P.Y. Legeydo, S.A. Ivanov, O.F. Putikov and P.C.H. VeekenThe geo-electric method detects Induced Polarization anomalies in the subsurface. In sedimentary rocks these anomalies are often situated in a halo above a deeper-seated hydrocarbon occurrence. An epigenetic alteration halo is stimulated by small but persistent leakage from the trap, due to diffusion and/or porefill perculation processes. Hence locally a reducing chemical condition is established below an effective regional seal, where in-situ pyrite crystals can grow. These crystals are easily polarized and detected by geo-electric investigation techniques. Mathematical modelling permits simulation of the micro-leakage and the geochemical processes triggered in the overburden. The influence of various rock physical parameters is examined. The consequences for the geo-electric evaluation method is demonstrated on the DNME dataset across the Severo-Gulyaevskaya oil-and-gas field, located on the Barents Sea shelf in NW Russia, as well as on a hydrocarbon occurrence in the Kaliningrad region.
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Assessing Fault Seal and Oil Preservation Risks Due to Fault Reactivation in the Timor Sea With an Automated Conceptual Model
More LessA conceptual model for risking trap integrity has been automated to derive a rapid, un-subjective, regional assessment of oil preservation. This methodology allows fast-tracking and high grading untested prospects for closer evaluation. Once validated such models can be easily applied to other suitable regions. A structural model has been proposed linking the magnitude of fault reactivation with the distributions of current and paleo-hydrocarbon accumulations in the Timor Sea, Australian North West Shelf. The model has been automated and applied to a regional 3250 km2 survey. The results find that the initial conceptual model is valid for all but one structure examined compared to the historical exploration success rate of 27%. The computational model was successful in identifying structures which have lost their entire hydrocarbon accumulation through fault seal breaching via reactivation, and in making reasonable approximations of the oil-water contacts in traps that have been protected from later tectonism and leakage. This model may be applied entirely or with modification in other regions adversely impacted by fault reactivation.
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Controls on Economical Hydrocarbon Accumulations in the Askeladd Field, Barents Sea – A Post-mortem Fault Seal Analysis
By A. BernalThe post-mortem study of a dry well is done by comparing the vertical and lateral sealing capacities of the faults bounding a dry structure with those of a proven gas filled structure. Comparison of buoyancy pressures measured in the discover well with threshold pressures calculated along the faults bounding the gas structure suggests that membrane seals estimated from shale gouge ratio are capable of holding the proven gas column. Lateral fault seal analysis in the faults bounding the dry well also suggests that they should be able of holding a gas column of similar dimension. Fault activity during Miocene time and the presence of seismic amplitude anomalies in sediments of Miocene age in both the dry an discover wells, the observation of a gas peak and oil inclusions in carbonate cement at the main fault and the presence of a paleo-hydrocarbon column in the reservoir in the dry well might suggest vertical leakage along the main faults bounding both the dry and gas filled structures. Considering the close geographical location and similarities in fault-zone processes and properties, fault seal analysis by itself does not explain why one well contains an economical hydrocarbon accumulation and the other not.
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Top Seal Property Evolution and Geohistories
Authors R.J. Knipe, G. Lewis, S. Muhuri, M. Johnson, M. Welch, G.M. Phillips, M. Hertle and D. CondliffeThe use of geohistory information to assess the best practice needed for the prediction of top seal properties in exploration is reviewed. An integrated approach combining petrophysical measurements, microstructural characterisation, down-hole tool data with basin modelling and tectonic setting is used to evaluate top seal property evolution. The results are used to highlight the importance of temperature – pressure histories for constraining top seal predictions. The study has incorporated top seal development from a range of basins. The identification and application of property-geohistory trends is used as a basis for a discussion on the prediction of sealing capacity and column heights.
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Fault Seal Risk Volumes – Assessing Uncertainty in Prospect and Field Development
Authors S.D. Harris, S.R. Freeman, V.S. O'Connor, P.W. Bradbury and R.J. KnipeFault seals can be both beneficial and/or detrimental to the generation of traps and the economic extraction of hydrocarbons from reservoirs. From exploration to development, the resolution at which fault behaviour needs to be understood varies, as does the data available to determine that behaviour. In this contribution we present new methods for mapping the potential seal risk of a spatially variable stratigraphic sequence. Both exploration and production scenarios are investigated and probabilities for different seal types from juxtaposition to membrane seals are calculated and mapped. Where more data is available (in production settings) permeability retardation or enhancement risk can be mapped over the reservoir area and through the stratigraphic volume. Risk mapping allows rapid assessment of fault-related reservoir compartmentalisation, which is often critical in more complex structural settings. Risk map interpretations can then be used to generate new faults in the grid that better honour likely layer connections while honouring the information gathered from the dynamic data. Overall the technique allows for a better understanding of the uncertainty due to faulting and hence a greater ability to focus potential new data acquisition, to direct further interpretation and to rapidly generate multiple realistic scenarios to mitigate these risks.
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Numerical Modelling of the Mechanical and Fluid Flow Properties of Fault Zones – Implications for Fault Seal Analysis
Authors J.H. ter Heege, B.B.T. Wassing, S.B. Giger and M.B. ClennellExisting fault seal algorithms are based on fault zone composition and fault slip (e.g., shale gouge ratio), or on fault orientations within the contemporary stress field (e.g., slip tendency). In this study, we aim to develop improved fault seal algorithms that account for differences in fault zone composition as well as deformation conditions under which the fault zone developed. The influence of composition and deformation conditions on the fluid flow properties of fault zones is investigated using discrete element simulations and laboratory experiments (cf. companioning paper by Giger et al.) of samples consisting of a low-permeability clay or shale layer, embedded in porous sandstone. A combination of discrete element and finite difference models is used to upscale the results and investigate the evolution of fault zone architecture and fluid flow properties of outcrop-scale faults. The fault seal algorithms are tested in a case study using finite element models of reservoir-scale faults.
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Laboratory Investigation of Shear and Compaction Bands – Compaction and Dilation Identification
Authors E. Charalampidou, S.A. Hall, G. Viggiani, H. Lewis, G.D. Couples and S. StanchisShear bands and compaction bands have been obtained in Vosges Sandstone cylinders tested in the laboratory under confining pressures of 30 to160MPa. The specimens have been heavily instrumented under load and subjected to pre- and post-deformation ultrasonic and Xray tomography. The primary purpose of this study is to add a detailed knowledge of the small-scale development of permanent dilational and compactional strains to our understanding of the mechanisms operative in shear/compaction band development and associated mechanical behaviours. This will inform predictions of changed fault zone permeability and lead to better prediction of fault seal or fault leakage under subsurface conditions. Samples were strain gauged and acoustic emissions were also recorded for some samples. Most were subjected to pre- and post-deformation ultrasonic and Xray tomography. Provisional assessment of the shear bands shows that material has densified but the velocities have also decreased. Open fractures are also seen on the processed Xray images. The samples show bulk compaction. The compaction band studies are less advanced, with acoustic emissions showing development of separated discrete bands that may have a shear component. This is an ongoing study and more robust conclusions, including thin section work, are expected by the Meeting.
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Anisotropy in the Petrophysical Properties of Deformation Bands
More LessMicroscopic examination of deformation bands reveals that the internal microstructure of different types of deformation bands varies along the bands at micro-scale. The variation can result in variations in petrophysical properties such as porosity and permeability. We have quantified this variation by utilizing our developed image-processing method and found out that porosity can vary substantialy and permeability by up to two orders of magnitude along a single deformation band.
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Characterization of Fault and Top Seals from Laboratory Experiments
Authors F.D.E. Cuisiat, E. Skurtveit and L. GrandeThe aim of the poster is to present recent laboratory experiments carried out at the Norwegian Geotechnical Institute to characterize the petrophysical, mechanical and hydraulic properties of faults and top seals. Three specific studies are detailed which address: 1) strength and deformation properties of argillaceous materials 2) formation of clay smear during faulting in sand - clay sequences 3) characterizing faulting during extension in unconsolidated sediments.
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Faults within Aeolian Deposits in the Upper Rotliegend, Poland
By A. PoszytekMicroimaging is a modern method of open hole logging. Imaging methods improve very fast and can be applied in all aspects of geology. Microresistivity measurements are oriented to the north, therefore the results can be presented as 3D images of rebored rocks. Microimaging can be used in the interpretation of sedimentological, structural geology and petroleum issues. It is helpful for the interpretation of detailed geology such as sedimentological features (bedding, current structures, size and shape of clasts, bioturbation, mass flow features, fluid escape structures, erosional surfaces) or structural data (natural fractures, microfaults, unconformities, induced fractures, breakouts). Faults within the aeolian succession of the Rotliegend in Poland have so far not been studied. This fact resulted from the low resolution of seismic data for the Rotliegend succession caused by the thick overburden of the Zechstein evaporitic series. Additionally, the faults are characterized by low amplitudes, therefore their interpretation on seismic logs is much more difficult. Microimage analysis allowed interpreting fractures and faults within the Rotliegend aeolian deposits in the Fore-Sudetic Moncline. Faults can influence on the decrease of reservoir properties within the aeolian succession.
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Evolution of Clay Smears and Associated Changes in Fault Transmissibility Using a New Direct Shear Fluid Cell
Authors S.G. Giger, J.H. Ter Heege, M.B. Clennell, B.B.T. Wassing, N.B. Ciftci, C. Delle Piane, C. Harbers and P. ClarkA new type of fluid cell has been developed to allow for direct shear deformation of very large and cohesive rock samples under sealed conditions. Rock samples consist of a low-permeability clay or shale layer, which is embedded in porous quartz sandstone to mimic a reservoir-seal pair. The cell is specifically designed to monitor changes of fault permeability, both across and along the evolving rupture surface, to displacements equivalent to several times the thickness of the argillaceous layer (i.e. SSF>6, c.f. Lindsay et al., 1993), and under stress conditions typical for burial depths of up to 2000 m. We present the general concept of the new testing equipment, and provide structural and flow data of evolving clay smear structures at controlled physical conditions. The results of the analogue experiments are integrated into a numerical modelling study (cf. companion paper by Ter Heege et al.) in an attempt to upscale our findings to reservoir conditions.
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Characterization of Fault Opening for Hydrocarbon Migration
Authors L. Zhang, X. Luo, G. Song, G. Qiu and G. VasseurIn order to characterize the sealing capacity of faults during hydrocarbon migration most studies use sealing indexes based on one or two parameters. However, these indexes, may successfully used in some areas but not in others, since fault sealing is the consequence of many geological processes that cannot be simply described by so few factors. We present an empirical method (termed fault-connectivity probability method) for assessing the long-term sealing capacity of a fault for hydrocarbon migration. This method is based on the observational evidence of the opening or closing behavior of the fault during the entire process of hydrocarbon migration. In practice, petroleum leakage through an element of fault is identified by the existence (or not) of hydrocarbon-bearing layers on both sides of this element. The data from the Wangjiagang Oil Field in the South Slope of the Dongying Depression in the Jiyang Sub-basin in Bohai Bay Basin, NE China, are used to develop this method. Fluid pressure in mud-rocks, normal stress perpendicular to fault plane, and clay smear are identified as the key factors representing fault seal capacity. They are combined to compose a non-dimensional Fault Opening Index, FOI. The values of FOI are calculated from the key factors measured on elements, and the relationship between FOI and fault-opening probability on any an element is established through a statistical analysis: when FOI is <1.0, the fault-opening probability is 0; when FOI is in the range (1-3.5), its relationship with fault-opening probability is quadratic polynomial; when FOI is > 3.5, the fault-opening probability is 1. Then, the values of fault-opening probability can be contoured on a fault plane, to characterize the variations of seal capacity on the fault plane.
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Differential Fracturing Pattern in Clay/limestone Alternations and Fluid Circulations in the Maltese Islands
Authors Y. Missenard, M. Rocher, P. Vergely, L. Casteleyn, P. Robion, M.E. Cushing, A. Bertrand and A. BenedictoDraining faults propagating from carbonates to clay layers could significantly impact the safety of nuclear waste storages, as investigated in the Bure Underground Research Laboratory (URL, France). In this study, we show that the draining properties of such faults seem to depend mainly of the fault throw magnitude. The results obtained on the Maltese Islands indicate that large faults (throw >50m) as well as small faults (throw <5m) does not lead to fluid circulation. On the opposite, intermediate faults propagates in the clay formation and lead to significant fracturation and fluid flows, including associated mineralisations.
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Predicting Spatial and Temporal Variations in Along-fault Fluid Flow
Authors A. McCay, R. Lunn and Z. ShiptonIn June 2008 the UK Government launched the White Paper ‘Managing Radioactive Waste Safely’ proposing construction of a deep Geological Disposal Facility. The GDF will be supported by an accompanying safety case that will include providing risk based estimates of the travel times for radionuclides leaving the GDF and travelling through the geosphere. This will require characterisation of the structural geological features surrounding the GDF in terms of location and associated permeability. Faults are heterogeneous in time and space. Few data are available on which to base quantitative predictions of long-term temporal variations in along-fault flow. No published research to-date enables quantitative predictions of temporal variations in along-fault flow. Episodic fault flow has been proposed by a number of authors to explain indirect observations of fault behaviour as diverse as hydrothermal springs, temperature anomalies, cementing phases, gas-related amplitude anomalies and earthquake triggering. We present here a summary of research from an innovative range of sources to support the hypothesis that along fault flow commonly varies over both space and time. We also present the results of field investigations of faults in mudstones to explore how field data can be used to characterise along fault flow.
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CO2 Plume Behaviour in a Cyclic, Fining Upward Clastic Sequence – Do the Claystone Intervals Seal?
Authors F. Schaefer and G.E. PickupWe numerically modeled CO2 plume behaviour in fining upward clastic sequences, using the Middle Buntsandstein of the North German Basin as an example. The aim was to examine how vertical and lateral reservoir heterogeneities influence CO2 plume development. CO2 injection was simulated through eight wells around a closed anticlinal structure, each injecting 500,000 m3 of CO2 per year over a period of 30 years. Sensitivity studies were carried out on several parameters. Lateral facies variations with high permeability contrast seem to inhibit CO2 flow. Where siltstones with a permeability of 1 mD hardly present an obstacle in graded sequences, it forces the CO2 plume sideways, effectively forming a barrier in rocks with a bimodal permeability distribution. In laterally homogeneous, but vertically fining upward sequences, the CO2 preferrably moves laterally and updip along the high permeability layers during the injection phase, forming a two-storey plume. However, after injection stops, most of the gas soon overcomes the finer grained, low permeability intervals and migrates vertically into the uppermost part of the reservoir and from there into the structural top. The structural top then fills from top to bottom, and not from bottom to top as one might expect.
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Is Gas a Problem for Long-term Safety of a Repository in Boom Clay?
Authors E. Weetjens and X. SillenPerformance assessment studies of the geological disposal of radioactive wastes in clay formations have repeatedly shown the paramount role of the clay barrier in providing long-term radiological safety. Therefore, due attention has to be given to the effects of disturbances of this barrier. Gas generation, predominantly caused by the anaerobic corrosion of metals from the waste forms and other repository components, is such a process that could perturb the clay host rock and the engineered barriers. The degree to which this may affect the long-term performance of a repository system will be explored in this paper, primarily on a methodological level. Assessing the importance of the gas problem for a repository in Boom Clay is a challenging task, because of its complexity and the significant uncertainties associated with both the source term and the gas transport processes. Assessments of the gas impact on long-term safety can be done based on a combination of mass-balance type calculations, the assessment of the water volume displaced by gas and complementary qualitative arguments. However, to underbuild statements on the safety of geological disposal, improvements are still desirable in understanding the phenomenology of the system and assessing the predictive capability of our models.
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Coupled Flow and Geomechanical Testing of Low Permeable Material – Application for CO2 Storage
Authors E. Skurtveit, E. Aker, M. Angeli and M. SoldalThe work presented is part of a national research project focusing on methodologies for testing cap-rock material and measure flow and mechanical properties that are important for the process of qualifying future storage locations. A flow through cell is used to flood a shale core where both axial and radial strains of the core are measured together with the acoustic velocities in the axial direction. The experimental results indicate that re-activation of micro cracks in shale plays an important role for the percolation and flow of CO2 in shale for the stress conditions given in the experiment. Current experimental experience raises several questions related to sample preparation, test conditions and flow mechanisms inside the sample during invasion and penetration of CO2.
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Assessing the Mechanical Impact of CO2 Injection on Faults and Seals
By B. OrlicTwo options for geological storage of CO2 are currently considered: storage in depleted hydrocarbon fields, which have a hydrocarbon proven seal, and storage in deep (>800m) saline aquifers, which lack such a seal. Pore pressure changes resulting from fluid extraction and subsequent CO2 injection into the reservoir induce stress changes that may mechanically damage seals, or trigger existing faults, creating the leakage pathways for CO2 escape from the containment. It is therefore required to predict the impact of CO2 injection and long-term storage on seals and faults. This is commonly done as a part of feasibility study carried out to assess the storage capacity and containment characteristics of the selected candidate site. In this paper we examine current practices for geomechanical evaluation of the mechanical impact resulting from pressure build-up on seals and faults. Discussion is supported by the results from recently accomplished studies of currently active and future potential storage sites, e.g. the Sleipner site located offshore in Norway (ongoing CO2 injection since 1996)and the De Lier depleted field located onshore in the Netherlands.
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Gas Injection Laboratory Experiments on Opalinus Clay – Experimental Set-up and Preliminary Results
Authors E. Romero, E.E. Alonso and P. MarschallPreliminary innovative tests are currently being performed on undisturbed Opalinus clay. These experiments intend to determine the conditions under which gas breakthrough processes occur, as well as to determine the relevant hydro-mechanical parameters that appear to affect these processes. An instrumented high-pressure triaxial cell is used, which was specifically designed to apply isotropic / anisotropic stress states while injecting gas at controlled volume rate. The study is mainly focused on maximum peak pressures, time for breakthrough, measured flows into and out of the specimen, preferential pathways created, induced desaturation of the sample, and relationship of initial peak pressure with total stress state.
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Gas Transfer in a Saturated Probe of Opalinus Clay
Authors A. Poller, G. Mayer, J. Croisé, B. Krooss, P. Marschall, P. Vogel and S. NiunoyaThe transfer of Argon and Helium gas through a sample of Opalinus Clay, a low permeability indurated formation, was investigated by means of injection tests in the laboratory. The experimental data were carefully analysed by means of numerical simulations, including two-phase viscous -capillary flow and diffusional transport.
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Wettability of Shales as Petroleum Reservoir Seals
Authors A. Borysenko, M.B. Clennell, M.I. Burgar, D.N. Dewhurst, R. Sedev, K. Liu, D. Mills, M. Raven and J. RalstonConventional surface methods, such as contact angle measurements, are combined with petrophysical probes, including nuclear magnetic resonance and broadband dielectric spectroscopy, to understand wetting mechanisms in clays and shales.
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Defining Critical Capillary Entry Pressures in Heterogeneous Mudstones – A Multi-scale Stochastic Approach
Authors K.D. Kurtev and A.C. AplinWe demonstrate a new methodology to define and upscale the Critical Capillary Entry Pressure (CEPc) of heterogeneous mudstones on a centimeter to meter scale. Two elements are involved: (a) quantification of CEPc of mudstones as a function of lithology and (b) definition of the texture or architecture of mudstone sequences, in particular the vertical connectivity of lithologies with the lowest and highest CEPc values. Since the CEPc values of coarse-grained muds can be two orders of magnitude greater than those of fine-grained mudstones, mudstone texture plays a key role in defining meter-scale CEPc. CEPc (and thus seal capacity) on a meter (and higher) scale can be very different from measurements obtained on a sample scale.
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Sub Log-scale Heterogeneity of Fine-grained Sediments – Implications for Effective Permeability and Top Seal Risking
Authors M. Drews, K.D. Kurtev, J. Ma and A.C. AplinEffective permeability plays an important role in prediction of leakage rates, seal capacity and quality. Current best practice uses conventional log data (15 cm as maximum resolution) to assess effective flow properties in seal sections and, with a maximum resolution of 15 cm, miss out the cm-scale sedimentological variability which is quite common in some mud-rich settings. The aim of this study was to identify and classify a reasonable number of different sedimentological heterogeneities and to investigate their impact on flow properties based on high-resolution data (microresistivity and core images). Four types of fine-grained sediments were defined on a basis of lithological variability and internal structure. Methods from geostatistics, such as the variogram as a descriptor for sedimentary structures derived from microresistivity and core images, were employed to quantify differences on a cm-scale between the four fine-grained sediment types. Furthermore, 2D flow models on a meter scale based on microresistivity and core images were used to determine ranges of effective permeabilities for the different types of fine-grained sediments. Finally, these effective permeabilities were upscaled in 1D and used to compare the differences due to the sampling rate of conventional and high resolution measurements.
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Risking Vertical Fault and Top Seal Using Gas Chimneys
More LessGas chimneys and related hydrocarbon migration pathways have been highlighted in seismic data using a supervised neural network, to visualize them in 3D and map their morphology. Using this information on over 125 fields and dry holes (containing effective reservoir)traps were divided into top seal and fault traps.Fault traps have been divided into three classes. Fault Flank Trap where the fault is involved in charging the trap from the flank are low risk high integrity traps. Fault Seal Traps show clear evidence of chimneys related to deep seated faults which terminate at the reservoir interval. They are high integrity traps. Fault Leak Traps have fault related chimneys observed at the crest of the structure. They are moderate to low integrity traps (LIT-MIT), and represent higher risk for vertical seal integrity. Top seal traps have been divided into four classes based on the nomenclature of Cartwright et. al. Gas Cloud Traps are low risk, but moderate integrity traps. Seepage Pipe Traps, Blowout Pipe Traps, and Mud Volcano Traps have increasing risk of seal failure, but can hold economic quantities of hydrocarbons, if buried or being recharged.
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Relationship Between Pore-scale Characteristics and Petrophysical Properties
Authors Z. Jiang, K. Wu, G.D. Couples, A. Ryazanov, M.I.J. van Dijke and K.S. SorbieFluid flow properties of geomaterials are fundamentally controlled by the characteristics of the pore system and the interaction of different fluid phases occupying complex pore shapes. Both the geometric and topological characteristics of the pore system are important, and each separately influences the flow of fluids. Here, we report a study in which we use network models to assess fluid flow through pore systems extracted from coarse-grained and fine-grained materials. Arbitrary changes to the pore systems, which mimic the effects of compaction, diagenesis, and possibly deformation, produce changes in the multi-phase flow properties, but the roles of pore size and connectivity vary in different materials. Continuing studies aim to identify predictive relationships between readily-obtainable descriptors and the hard-to-determine flow characteristics.
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Seals, Superseals and Faults
Authors J.W. Wheeler and K.E. WilliamsSuperseals are qualitatively different from conventional hydrocarbon seals because they confine water and abnormal pressure, not merely fluids that are immiscible with water. In the normal course of dewatering, common shales are largely invisible to expelled water. In some basins, sediment can accumulate so fast that thick common shales can interfere with dewatering sufficiently to cause transient overpressure. Superseals, such as the Woodford Shale, are impermeable to water and can retain overpressure indefinitely. The identification of superseals may have an impact on both the sequestration of carbon dioxide and the isolation of certain waste materials. Common shales are able to trap hydrocarbons because the pore entry pressures for fluids other than water are very high (a capillary seal), but the shale remains transmissive to water and to some solutes. A common shale seal is unsuitable as a barrier to carbon dioxide. A shale superseal will retain CO2. Horizontal pressure compartments are also very common and must also be accounted for in basin models. The sealing capacity of a fault is determined by the capillary entry pressure of the sealing rock unit (or fault gouge zone). The sealing capacity is increased where there is a pressure differential across the fault.
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Using 3D Geomechanical Modelling To Evaluate Stress-related Fault Seal Risk – A Timor Sea Case Study
Authors L. Langhi, Y. Zhang, A. Gartrell, J. Underschultz and D. DewhurstObservations from the Timor Sea suggest that the high incidence of breached Jurassic traps is a result of recent fault reactivation. Based on comparison between structural evolution and charge history, it has been proposed that the variation in strain accommodation between faults was critical in determining up-fault flow patterns. A geomechanical 3D model is used to simulate the response of trap-bounding faults to extensional reactivation. The model makes predictions on stresses/strains and fluid flow partitioning and is used to characterise the interaction between the deformation state and fault seal efficiency over an area including the Laminaria and Corallina oil fields. When integrated with well data and evidences of fluid remigration, the model results show their ability to constrain trap integrity predictions and isolate leaking/sealing fault segments. The model captures the inhomogeneous distribution of sealing fault segments driven by strain partitioning and structural architecture. It emphasises that fault tip interaction, fault growth process and fault plans arrangement all play a major role in controlling fault seal evolution.
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Fault Core Composition and Thickness-displacement Relationships of Extensional Faults in Fine Grained Carbonates
Authors E. Bastesen and A. BraathenConventional fault seal methods derive transmissibility properties of faults by calculating the shale gouge ratio along the fault. Fault thickness is a key parameter. However, the method does not consider intrinsic fault complexities, such as lenses, varying fault rocks and diagenetic elements. In this study we present a characterization of fault cores in carbonates, based on an analysis of fault composition with respect to the displacement/thickness relationship (D/T). Using a Fault Facies type description of rocks we have compiled and analysed the dimensions and composition of 100 faults cores from Sinai, Oman and Svalbard. These areas display thick succession of sedimentary carbonates truncated by extensional faults. Common volumetric elements of the cores (core facies) include fault rocks such as breccias, calcareous gouge and shale gouge (smear) and layers of calcite precipitated on the walls of slip-surfaces. The overall increase in thickness relative to displacement, showing a lower increase of fault thickness for large scale faults than for small scale faults, fits a power law distribution with an exponent lower than one. The individual fault core facies display different D/T than the overall D/T relationship, indicating that protolith, diagenetic effects and tectonic environment influence the observed thickness increase.
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Pedogenetic Effects in Fault Evolution
Authors I. Cantarero, A. Travé and G. AlíasDuring the late Oligocene – middle Miocene, an extensive deformation took place in the Western Mediterranean. In this process, main faults of NE-SW striking direction bordering horsts and grabens of the Catalan margin and minor faults perpendicular to the former were formed. We have studied some of these minor faults affecting Miocene conglomerates in the Barcelona Plain. The fault zone is formed by an absent damage zone and a poorly developed fault core. Their fault rocks are obliterated by later pedogenetic processes that have changed fault and fault rock properties and consequently fault rock classification. Nowadays fault rock is a cataclasite but because it is cemented by later pedogenetic processes, in fact, in origin was a gouge. When fault rock was a gouge (stage 1), and considering fault zone architecture, cross-fault flow could exist. During the second stage, after fault rock pedogenesi, new minerals reduced effective porosity and so permeability of the fault rock. The consequence was the seal of faults and compartmentalisation of flow. In conclusion, faults occurring in the upper meteoric environment can have other kind of processes responsible of their impermeabilization than deeper faults.
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Fault-zone Localisation in High-porosity Sandstone Reservoirs and Impact on Flow Efficiency
Authors C.A.J. Wibberley and E. SailletWe present results from a study of the distribution of deformation bands and larger ultracataclastic faults in a high-porosity sandstone reservoir outcrop analogue. We show that tectonic loading path and the nature of the stress changes causing deformation may strongly influence strain distribution. Localisation of deformation onto a smaller number of larger ultracataclastic fault zones is more likely in an extensional context than a compressional one because of the work-hardening nature of deformation band formation: In the extensional context, the differential stress required to continue deforming deformation bands is much less than in the compressional case, and may be achieved by subtle differences in stress variation around faults or small fluid pressure changes. Using new permeability measurements of host rock and deformation structures, we test models for the impact of these structures on flow rates in a producing field. Calculations of flow efficiency show that a small number of larger ultracataclastic faults may severely impede flow during production. Low-displacement deformation bands, with a much less reduced permeability however, will have a much lower impact on flow rates despite their higher densities. The impact of small deformation bands is likely to be greatest when produced by tectonic shortening.
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Stochastic Modeling of Permeability Reduction and Enhancement in Faults and Damage Zones
Authors T. Murray, G. Christie, J.P. Brown and S. TysonDuring appraisal, development and production of oil and gas fields a large amount of effort is applied to understanding the impact of changes in permeability around faults. In this paper we describe a methodology for calculating distributions of static permeability in and around faults incorporating, geomechanics.
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The Whitehouse Shore – A Natural Laboratory for Characterizing Fault Geometry, Heterogeneity and Permeability
Authors Y. Kremer, Z.K. Shipton, R.J. Lunn, C.A.J. Wibberley and J.K. InghamThe Whitehouse shore intertidal exposure (Scotland) provides an excellent field site to study small scale faults in sand-shale sequences. Especially interesting is the presence of paleo fluid flow indicators, which allow us to constrain the fluid flow behaviour of the faults and fractures at depth. Combination of field study, laboratory analysis and numerical modelling allows us to collect vast amounts of data on the geometry, heterogeneity and permeability structure of fault zones. These data will ultimately form the basis of a new workflow that allows the spatial variability of fault zones to be represented in reservoir models.
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Multi-episodic Fault Movements and Karstic Fills Along the Vallès-Penedès Fault, Coastal Catalan Ranges, NE Spain
Authors V. Baqués, A. Travé, A. Benedicto and P. LabaumeWe describe different fault rocks within a normal fault bordering the Neogene Vallès-Penedès basin in NE Spain. Through this fault, the Lower Cretaceous limestones of the footwall are in contact with the Miocene siliciclastic sediments of the hangingwall. In the Lower Cretaceous protolith, cataclasites and breccias are a mixture of tectonic and karstic products resulting from multi-stages movement and development of the fault. The localisation of frictional processes generated a cohesive fault breccia and the cataclasite during the progressive opening of the hydrological system. Later, when the fault reached the surface, meteoric diagenetic processes leaded to the formation of calcretes and karstic features and generating the breccia with the pisolithic matrix.
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Geomechanically-derived Fault Zone Petrophysical Properties – A Synthetic Fault Model Study
Authors A. Paez, H. Lewis, G.D. Couples and J. MaA 2D geomechanical model of a fault zone has been used to develop porosity and permeability distributions that reflect the geomechanical rock damage. A pseudo-3D geomodel, where fault displacment varies from zero to maximum has been created and a method developed to appropriately populate it with porosity and permeability values. Both dilational and compactional deformation occurs in this normal fault,as is expected in real subsurface faults, resulting in both increased and decreased (relative to the matrix) permeability values. Geometrically equivalent geomodels using a range of transmissibility multiplier to represent the fault have also been generated. These six geomodels (5 with transmissibity multipliers and 1 with geomechanically-derived poroperm) have then been converted into an industry-standard flow simulation that vary only in the treatment of petrophysical properties around the fault zone. The resulting flow simulations show that the geomechanically-controlled poroperm properties to a better job of representing the kind of complexity of e.g. across-fault pressure regimes and fluid saturations that are encountered in subsurface reservoirs.
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Fault Sealing Behaviour of Soft-sediment Extensional Faults in High-porosity Sediments from the Crotone Basin, Italy
Authors F. Balsamo and F. StortiThe hydraulic behaviour of deformation band-dominated fault zones is strongly controlled by the petrophysical properties of the protolith sediments, deformation intensity and environmental conditions of deformation. To provide further constrains on the relationships between fault structure, displacement and hydrology, we acquired structural, granulometric and permeability data from a total of 25 extensional fault zones developed in high-porosity sandy sediments of the Crotone basin, having mean permeability values in the 10E3 to 10E5 mD range. Fault core rocks developed by progressive grain size reduction and consist of foliated granular material and gouge lenses along striated slip surfaces. Mean fault core rock permeability ranges between 10E1 and 10E4 mD, although we recorded permeability values lower than 10 mD. Fault damage zones consist of closely spaced cataclastic deformation bands with different degree of complexity and mean permeability between 10E2 and 10E4 mD. By combining structural and hydraulic information, we obtained empirical quantitative relationships between bulk permeability variations, fault zone thickness variations, and fault displacement. Most of permeability reductions in both fault cores and damage zones occur at sub-seismic scale, whereas such a rapid permeability decay significantly decreases for displacement greater than 25-30 m.
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Quadshear – A New Kinematic Model for Clay Smear Development
Authors M. Welch, R.J. Knipe, C. Souque and R.K. DaviesA new kinematic model, based on the Trishear model, has been developed to simulate fault-propagation folding in ductile, clay-rich strata between two propagating faults, a process which often leads to the development of clay smears which can cause the fault to seal. The new model is used to determine the amount fault throw required for the continuous clay smears to break down, normalised to the bed thickness (this ratio is often known as the critical clay smear factor or CCSF). We find that the maximum length of continuous clay smear is primarily dependent on the position of the clay-rich bed in the stratigraphy, and the rate of propagation of the faults. The thickness of the clay-rich bed, the horizontal offset of the two propagating faults and the width of the fault-propagation folds have much less impact on the maximum length of continuous clay smear.
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The Development of Sub-seismic Fractures around a Fault – The Causes and the Implications for Fluid Flow
Authors D.C. Tanner and C.M. KrawczykThree parameters control the development of secondary sub-seismic fractures: Material properties, the fault displacement and its morphology. We demonstrate how restoring seismic faults with three dimensional geometrical kinematic modelling can generate information about the intensity and orientation of the fractures around the fault.
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Fault Compartmentalization of a HP/HT Field
Authors S. Hoth, S. Ottesen and T. OdinsenFault compartmentalisation maps have been produced for two reservoir horizons for a field located on one of the deepest fault blocks in the Viking Graben. Mapped fault rock properties in conjunction with Mercury injection data were used to calculate maximum Hydrocarbon columns and associated threshold pressures to obtain a static fault seal model. In general mapped fault rock properties indicate a high lateral and vertical variability. Key uncertainties comprise the structural interpretation and the relation between Mercury injection pressure and SGR. Given the presence of different Hydrocarbons, interfacial tension is expected to vary within the field significantly, thus giving rise to different column heights and threshold pressures for the same fault rock type. Thus, the structural compartmentalisation is superimposed by an HC property compartmentalisation and their spatial distribution is not expected to be the same. Given the HP/HT nature of the field, even low SGR fault rocks can sustain several 10's of meters of HC columns. The compartmentalisation maps in conjunction with the present Gas Water Contacts, the Gas Oil Ratio distribution, as well as the resulting communication paths, provide strong support for an upside potential for some compartments surrounding the discovery wells.
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Optimizing Fluid-rock Properties in Compartmentalized Reservoirs By Using Dynamic and 4D Seismic Data
Authors R. Villegas and Q. FisherIn this work we develop improved constraints on fault areas and define methods for history matching using both production and 4D seismic constraints, which will provide a more robust basis for production forecasting. We propose improved workflows to change the fault areas into the simulation model specifically by modifying the relative permeability near the faults and by considering two phase flow together with the 4D seismic data results.
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Integrated Hydrodynamic Analysis of Fluid Pressure and Leak-off Pressure Distributions to Evaluate Seal Effectiveness
By J.M. VerweijHydrodynamic methods are used to analyse the present-day fluid overpressure and leak-off pressure distributions in offshore Netherlands in combination with recently completed stratigraphic and structural maps, and knowledge on distribution and characteristics of oil and gas accumulations. This paper presents an overview of the results, including an improved characterisation of the permeability framework of offshore Netherlands with focus on the seal effectiveness of the low permeable units and identification of locations of seal breach risk.
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