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3rd EAGE International Conference on Fault and Top Seals
- Conference date: 01 Oct 2012 - 03 Oct 2012
- Location: Montpellier, France
- ISBN: 978-90-73834-35-4
- Published: 03 October 2012
68 results
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Column Height Controls in the Western Hammerfest Basin
Authors C. Hermanrud, K. Kristiansen, M. Halkjelsvik, A. Bernal and A. StrömbäckWe have compiled information on hydrocarbon column heights and structural setting for 16 drilled structures in the western Hammerfest Basin Barents Sea, most of which were dry or underfilled. The column height of the structures appear to be controlled by updip fault intersections, as the two structures without updip intersecting faults are the only structures that are filled to spillpoint. All of the dry structures that have leaked have intersecting faults updip of the well position. The nine underfilled structures are delineated by several faults. We have not yet been able to identify individual updip column-restricting fault intersections for these structures as several alternatives exist. Our work suggests that vertical fluid flow along intersecting faults is the main cause of leaked structures in the area, but that the road towards safer column height predictions is still elusive.
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A 3D Grid-based Workflow for Fault Seal Capacity Estimation and Risking in Exploration
By M. KentIn 2011, Maersk Oil contracted the reprocessing of seismic data for the entire Danish Central Graben. The Pre-stack Depth Migration (PSDM) data was reprocessed by Western Geco, to help mitigate any risk associated with opportunities within the Maersk Oil operated study area. An initial screening of the new seismic highlighted that the most interesting prospects would rely heavily on the ability of seismic scale faults to hold back a significant hydrocarbon column. Using Petrel 2011, in conjunction with the Structure and Fault Analysis (RDR) module, a complete structural analysis of the identified fault system was performed. The work required careful interpretation and analysis of the 3D fault system characteristics. This was accomplished by constructing a structural framework of the fault system and assessing the likelihood of fault seal using the Structure and Fault Analysis (RDR) module. This methodology allowed for the results to be integrated into a standard exploration risk and volume assessment workflow, enabling critical decision making on each prospect. This presentation will describe the workflow used; challenges faced and shed light on how the latest tools can be smoothly connected into the existing prospect risking practices.
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Fault Seal Analysis, UK Northern North Sea - The Alwyn North Statfjord Example
Authors O. Onyia, O. Bernet-Rollande and D. TaylorThe Alwyn North Field of the UK Northern North Sea, today considered a mature field, still possesses a number of as yet untapped prospects. This paper is an illustration of how, using fault seal analyses, the potential of these previously undrilled objects have been unlocked. The initial phase of exploration of the Alwyn North Field focused on “large” conventional plays, resulting in the discovery of the various Brent accumulations and the Statfjord Main accumulation: The northern and western panels of the Statfjord were concluded water bearing as a result of five unsuccessful penetrations of the Statfjord. Thus, the presence of smaller but nonetheless economic prospects was overlooked. Following the unexpected discovery of a 90m thick gas column in a western panel of the Statfjord by the N49, a series of studies on panel bounding faults of the western and northern Statfjord were undertaken. These studies focused on the sealing potential of these faults. They identified the strong probability that the previously overlooked panels in fact contained up-dip accumulations of hydrocarbons. This realisation inspired the start of a phase of “Near Field Exploration” that has led to five successful wells, which have been or will soon be put into production.
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The Capability of Faults and Top Seals in the Bunter Sandstone of the UK Southern North Sea to Seal Carbon Dioxide
Authors J.D.O. Williams and S. HollowayIn the UK sector of the Southern North Sea, the Triassic Bunter Sandstone Formation (BSF) is perceived as having a significant potential for the storage of anthropogenic CO2 within major structural closures. Although most of these structures are saline water-bearing, eight gas fields with BSF reservoirs have been discovered to date. The presence of natural gas fields demonstrates the capability of the various top seals to effectively seal significant gas accumulations over geological time-scales. However, during the formation of the domes, the BSF and its overlying top seals were subjected to extensional stresses and are therefore commonly transected by faults. The seismically resolvable structure of the BSF-reservoired gas fields are considered here, in order to assess the relationship between faulting and pre-production gas/water contacts. The assessment concludes that at least four of the fields have been affected by crestal faults which do not appear to have affected the sealing ability of the overlying top seals. Therefore, similar minor-offset faults occurring within the non-gas bearing structures may be capable of supporting CO2 columns that would exert pressures comparable to those exerted by the pre-production gas columns in the fields.
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Analysis of a Compartmentalized Bunter Structure in the Broad Fourteens Basin, Dutch North Sea
Authors P.E. Kraemer, A. Frischbutter, S. Doering, F. van den Bos and A.J. NieberdingUnderstanding the impact of faults on reservoir compartmentalization, hydrocarbon migration and fluid contacts is a key for the exploration and development in structurally complex areas of the North Sea and requires an integrated analytic approach. The structure discussed here is located in the Broad Fourteens Basin and underwent a complex multi-phase structural evolution of extensional episodes followed by contractional phases. Two different gas-water contacts were encountered in fault bounded compartments of Bunter reservoirs. Gas pressure points lie on the same gradient in both compartments whereas water pressure points indicate differently pressured aquifers. Another key observation is that the compartments are under-filled. The integrated trap analysis applied, allows proposing a two-phase scenario in order to explain the present day trap configuration and the observed gas-water contacts. Initially gas charging of the structure took place and resulted in a single gas-water contact. During the subsequent contractional event one compartment was uplifted along the main fault of the structure preserving the trap integrity. The gas pressure was able to equilibrate during geologic time across an upper low Vshale window but no pressure equilibration was possible within the water leg, leading to the water overpressures observed in the uplifted compartment.
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Cross-Fault Pressure Depletion, Zechstein Carbonate Reservoir, Weser-Ems Area, Northern German Gas Basin
Authors F.V. Corona, F. Brauckmann, H. Beckmann, A. Gobi, S. Grassmann, J. Neble and K. RoettgenA cross-fault pressure depletion study in Upper Permian Zechstein Ca2 carbonate reservoir was undertaken in the Weser-Ems area of the Northern German Gas Basin. The primary objectives are to develop a practical workflow to define cross-fault pressures scenarios for Zechstein Ca2 reservoir drillwells, to determine the key factors of cross-fault pressure behavior in this platform carbonate reservoir, and to translate the observed cross-fault pressure depletion to fault transmissibility for reservoir simulation models. Analysis of Zechstein Ca2 cross-fault pressures indicates that most Zechstein-cutting faults appear to act as fluid-flow baffles with some local occurrences of fault seal. Moreover, there appears to be distinct cross-fault baffling or pressure depletion trends that may be related to the extent of the separating fault or fault system, degree of reservoir flow-path tortuosity, and quality of reservoir juxtaposition. Based on the above observations, a three-part workflow was developed consisting of 1) careful interpretation and mapping of faults and fault networks, 2) analysis of reservoir juxtaposition and reservoir juxtaposition quality, and 3) application of the observed cross-fault pressure depletion trends. This approach is field-analog based, is practical, and is being used currently to provide reliable and supportable pressure prediction scenarios for subsequent Zechstein fault-bounded drill-well opportunities.
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Influence of Small-scale Fault Geometry on Across-fault Fluid Flow
Authors M. Giba and P. RuellandFew case studies have demonstrated the importance of small-scale fault zone complexities on across fault fluid flow and little progress has been made in modeling and implementing them in reservoir simulation models. In this study, we use a high quality 4D seismic dataset from offshore West Africa to investigate the relationship between small-scale fault zone complexities and fluid migration. A 4D seismic signal occurs across a relay zone of a normal fault where throws are the lowest (less than 10m) while reservoir thickness varies between 10 and 20m. The shape of the 4D seismic signal shows that aquifer water crosses low-displacement tips of overlapping fault segments. At several other faults (with higher displacements) at the southern rim of the field 4D seismic signals are bound by the faults and clear evidence for across fault water migration cannot be observed. Hence, the 4D seismic signal at fault F1 shows that northward water migration across the fault occurs only where throws are smaller than ca 10m.The investigated fault is then modeled using non-pillar based methods which permit geometrically accurate fault representation in a reservoir grid. Flow simulations are performed to reproduce fluid migration as observed by 4D seismic signals.
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Mechanical Modelling and Seismic Imaging of Fault Zones
Authors C. Botter, N. Cardozo, S. Hardy, I. Lecomte and A. EscalonaFaults are 3D zones of deformed rock that play a major role in controlling fluid flow in reservoirs. Fault zones are difficult to characterize: outcrops give limited view and seismic is limited by resolution and image quality. We propose an integrated approach to study fault zone evolution and its impact on seismic. We model fault zones using the discrete-element method (DEM). The finite strain of these models is used to condition seismic properties. Finally, seismic imaging of the DEM analogues is performed. An example is presented for a normal fault zone (60 m of fault displacement) in a 2D shale-sandstone layered model of size 1 x 0.5 km at 1 km depth. The fault zone has a complex distribution of shear and volumetric strain. Density, seismic velocities and reflectivities are conditioned by the volumetric strain of the DEM. Seismic imaging shows a response from the fault zone. Enhancing this part of the image is a challenge in acquisition and processing. Our approach can be extended to 3D. Future research will involve denser particles assemblages (smaller particles), and new techniques to pick much of the energy from the fault zone.
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The Value of Geomechanical Modeling of Fault Properties for Trap Analysis
Authors M. Welch, A. Frischbutter and R.J. KnipeGeomechanical and kinematic modelling techniques were applied to support the trap integrity risking process for a North Sea appraisal project in faulted Carboniferous clastic strata. A key issue here is whether clay smearing has the potential to provide a valid seal across reservoir compartmentalizing faults. Standard fault seal analysis workflows use the critical clay smear factor (CCSF) to assess the potential for fault sealing. However in a mechanically layered sequence, the CCSF will vary between layers depending on the position of the layer within the overall stratigraphy and the rate of fault propagation. We use a combination of mechanical and kinematic models to predict (a) where the faults will nucleate in a mechanically layered sequence and how fast they will propagate through it, and (b) the resultant fault propagation folding in shale layers and hence the CCSF for each shale layer. We show that the development of clay smears is dependent on both the surrounding stratigraphy, the depth of burial at the time of faulting, and the rate of fault propagation.
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Evaluation of Caprock Integrity in a Carbonate Field Undergoing Steamflood
Authors J. Ita, C. Schoofs, A. Bauer, S. Oates and F. al-KindyInjection projects usually include two significant field development uncertainties as goals of the operation: flood front conformance and maintenance of cap rock integrity. These become even more important during steam flood operations. Here, the contributions of geomechanics toward caprock integrity during steam flood development are outlined. Geomechanical simulation coupled with an understanding of the shear failure process has provided confidence that cap rock integrity should be preserved over the lifetime of the field. This was essential in the interpretation of microseismic surveillance data that at first glance might seem to indicate that the likelihood of caprock breach was increasing. A comparison of semi-analytical and finite-element simulators was also made to determine what the optimum usage of these tools is in a caprock integrity assessment workflow.
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Top Seal Integrity - New Advances in Geomechanical Modeling
Authors M. Lejri, L. Maerten, F. Maerten and R. SolivaTop seal integrity can be affected by many factors including the occurrence of natural fractures as well as the distribution of the present day heterogeneous stress field. While opened natural fractures are known to be capable of significantly promoting the flow of hydrocarbons through the top seal, present day stress distribution can lead to critical leakage in the same top seal during drilling and production when pressure is changed. Therefore, understanding and quantifying the spatial and temporal development of the natural fracture as well as the distribution of the heterogeneous present day stress can have great economic impact in the context of hydrocarbon exploration and production and thus significantly improve decision making and reduce production risks. In this contribution we present preliminary results of the new generation of paleo-stress and present day stress inversion and analysis (Maerten F, 2010) using iBem3D, a numerical tool based on innovative iterative 3D boundary element method (BEM), also called Poly3D (Thomas, 1992; Maerten F, 2010, Ch. 1). We described the method, its benefits and limitations and apply it to an example for both natural fracture and present day stress modelling that lead to a better quantification of top seal integrity.
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Membrane Seal Leakage in Non-fractured Caprocks by the Formation of Oil-wet Flow Paths
Authors G. Teige and C. HermanrudLeakage through non-fractured shaly caprock sequences is often envisioned as fluid percolation through water-wet pore networks. Leakage by this process requires that the buoyancy of the hydrocarbons or CO2 column overcomes the capillary entry pressure of the caprock pores. If it does not, then leakage through such caprocks depends on the extremely slow diffusion process. In the latter case, fluid residence time would be almost infinite. Experimental research that have addressed wettability alterations and pore-scale fluid flow in the presence of capillary sealing have triggered fresh thoughts that conclude with modifications to above mentioned leakage model. The main premise of this model is that residual water in hydrocarbon-filled reservoirs can be both continuous and mobile, and can also extend into overlying membrane seals whereas oil is retained in the reservoirs by capillary forces is. We suggest that oil-wet flow paths can be established in membrane seals due to wettability changes as a result of residual water movement that supplies acidic components to the seals. Such wettability alterations allow seals to leak by Darcy flow and small pore throats will promote leakage instead of inhibiting it. Capillary sealing is not so ever-lasting under such conditions.
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Leaky Top Seals - Evidence, Rates and Mechanisms
Authors K.D. Kurtev, U. Böker, M. Drews, S. Abrakasa and A.C. AplinA wide range of observations indicates that petroleum commonly migrates or leaks through thick sections of mud-rich sediments. In continental slope settings, for example, petroleum often migrates from mature source intervals to traps through several kilometres of mud dominated section. Seismic bright spots, waste zones and gas chimneys indicate the presence of petroleum in mud-rich seismic volumes and have been used to imply leakage pathways. However, whilst seismic data may indicate the presence of gas, it cannot indicate its origin. Results indicate that at a single porosity, the range of potential column heights between facies is a factor of two. Permeabilities of the facies vary by three orders of magnitude, depending on effective stress, and laminated and lenticular facies are highly anisotropic. Petroleum saturations required to breach the mudstone facies are also highly variable, ranging from < 1% to > 20%. All these numbers help us to quantify seal risk in a more quantitative way, related both to capillary breach or in the context of a permeability-constrained charge-leak model. An important conclusion is that total connected porosity for any facies is a function of lithologies ratio,while permeability and CEPc were functions of internal architecture/type of connectivity patthern typical for each facies.
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Towards a more Realistic Cap-rock Leakage Model
Authors O.I. Frette, A. Bakk, M. Daszinnies and H.M. HelsetIn this paper we present a model for capillary leakage that incorporates hysteresis in the modelling of hydrocarbon relative permeability in cap-rocks. The model captures the experimental behaviour observed in laboratory test on capillary leakage in synthetic trap models, Vassenden et al. (2003). In our model the hydrocarbon column height reaches the snap off height when the supply of hydrocarbons have stopped. We compare this model with another leakage model and concludes that the new model fits better with the experimental results. Thus the models differ in the imbibition process. A consequence is that hydrocarbons in place in the trap differs significantly between the two models; which will be of concern in basin modelling exploration.
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CO2 Wettability of Minerals Relevant to Sealing Efficiency of Caprocks for CO2 Storage in Saline Aquifer
Authors R. Farokhpoor, B.J.A. Bjørkvik, E. Lindeberg and O. TorsæterThe capillary-sealing efficiency of the caprock is one of the major factors that control the safety of geological CO2 storage. Possible changes in wettability due to geochemical processes could possibly decrease the capillary entry pressure and reduce the sealing integrity of the caprock. Changes in wettability have been measured by measuring the CO2 contact angle mineral substrates mica, quartz, calcite and feldspar in the presence of brine for pressures extending from atmospheric pressure up to 400 bars and for temperatures of 36 °C and 66 °C. The change in contact angle as function of pressure was non-significant for CO2 drop on the surface of quartz, feldspar and calcite. There was a significant change in contact angle on mica in which case the wettability changed from strongly water-wet to intermediate water-wet. The minimum in contact angle near the critical pressure was observed at 36 °C for three minerals (feldspar, calcite, quartz) and was maybe masked in the case of mica by the significant decrease in contact angle with increasing pressure. A relatively strong anomaly in CO2 compressibility is observed at 36 °C, less so at 66°C, indicating that the observed minimum may be related to the near-critical behavior of the CO2-phase.
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Fracture Corridors as Seal-bypass Systems - Case Studies from the Jurassic Entrada Formation Utah, USA
Authors K. Ogata, A. Braathen, K. Senger and E. PetrieWe discuss discrete zones of enhanced fracturing characterized by closely-spaced, sub-parallel fracture networks (i.e. fracture corridors) as preferential fluid flow pathways which serve to bypass sealing systems and to connect reservoirs at different stratigraphic levels. We identify 3 types of fracture corridors on the basis of their structural relationships with larger faults and folds, representing end-members of a continuum of possibly interrelated products. These fracture networks are: 1) fault damage zone, 2) fault tip process zone and 3) fold-related crestal zone fracture corridors. The tabular corridors show different orientations and patterns of fracturing, defining a local- to large-scale network of inferred high-permeability vertical and lateral conduits. In the study area (Utah, USA), analyzed fracture corridors share the occurrence of discolored (bleached) zones, testifying to the ancient circulation of reducing fluids. These observations can be used to support reservoir-caprock systems modeling and fluid flow simulation assessment, for instance in geological CO2 storage.
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Production-induced Capillary Fault Seal Failure – How Common Is It?
Authors T. Manzocchi and C.J. ChildsThere are widespread anecdotal reports that sealing faults sometimes break down during production, but little evidence, in the public domain at least, to support these reports. In this paper we review the theoretical basis for production-induced capillary (i.e. membrane) fault seal failure, investigate numerical models of the mechanism, and discuss the few published examples where observed production behaviour is attributed to fault seal breakdown in a production context due to pressure depletion on one side of a fault. We conclude that the case for fault seal failure is not, perhaps, as strong as many of us believe it to be.
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Fault and Top Seal Strength at Nansen-Boomvang Fields, East Breaks, Gulf of Mexico
A trap analysis of a highly faulted Pliocene-Pleistocene turbidite sequence is performed in order to understand the controls on column heights, which range from ~110-2400 ft. in >20 reservoir compartments. A kinematically viable structural and stratigraphic model is generated from an interpretation of 3D seismic data, stratigraphic picks from >20 wells, and systematic petrophysical analysis of well logs to develop a field-wide model of shale volumes. This model forms the framework for analyzing reservoir juxtapositions and estimating fault properties in five main reservoirs intervals that are distributed throughout four main fault blocks at Nansen. In the adjacent Boomvang field, two main reservoir units are evaluated in one fault block. Fault seal capacity functions derived from this work are currently being utilized for exploration risking in nearby fault-dependent prospects. These functions are significantly different than recently published relations, but consistent with core-derived data from the region, and highlight the requirement for local calibration.
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Integrity of Faulted Traps for CO2 Storage
By P.G. BretanEvaluating the structural integrity of fault-bounded traps for CO2 storage requires a thorough assessment of the likely sealing or non-sealing behavior of faults, in particular, i) will the increase in pressure generated by CO2 injection (or by a CO2 column) trigger fault instability and reactivation, thus leading to loss of CO2 from the trap, and ii) will the fault act as a capillary barrier, thus permitting CO2 to accumulate, and if so what might the likely height of the trapped column be before the fault leaks? The structural integrity of fault-bounded CO2 traps can be evaluated using workflows and predictive algorithms originally developed for the prediction of capillary seal of hydrocarbon, using appropriate CO2 fluid densities. Three-dimensional faulted-framework models are an essential first step in assessing the integrity of a fault-bounded CO2 trap. Fault-plane diagrams are used to investigate the juxtaposition geometry of CO2 bearing reservoir/non-reservoir intervals at the fault plane. Predictive algorithms for fault-sealing, such as Shale Gouge Ratio, and for stress-driven leakage enable a better understanding of the possible fault behavior to be derived.
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Effective Stress Constraints on Vertical Flow in Fault Zones: Learnings from Natural CO2 Reservoirs
Authors J.G. Solum, S.J. Naruk, J.P. Brandenburg, D.E. Wolf, P. Origo and D.L. KirschnerUnderstanding the factors that control vertical fault leakage is essential for predicting such leakage for both conventional reservoir development and subsurface CCS. This study compares leaking and non-leaking natural CO2 traps, providing such constraints. We constrain trap configurations, fluid pressures, and stress states for three natural CO2 accumulations from the Colorado Plateau. Surface geologic data are integrated with subsurface data from groundwater and hydrocarbon wells. Leakage of CO2 is documented by soil surveys and the occurrence of travertine deposits. Leakage occurs where the total fluid pressure reduces the effective stress to approximately zero, and the tensile strength of the rock is effectively zero due to the presence of pre-existing fractures within fault damage zones. These results appear to be consistent with induced fault zone leakage from some deepwater hydrocarbon reservoirs, where the leakage has been related to high water injection pressures and ceased when water injection stopped. The results imply that vertical leakage along fault zones occurs where the fault zones have fracture-based damage zones and the effective normal stress acting on the fault zone is low. These criteria can be used to evaluate the potential for vertical fault leakage regardless of the type of fluid contained within a reservoir.
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Discrete Element Modeling of Friction of Sand-clay Mixtures
More LessThis paper presents a preliminary study on the effect of the clay content on the mechanical properties of the fault gouge materials using the discrete element method (DEM). It is also a feasibility study to examine how numerical modeling based on the DEM can be used to predict the hydro-mechanical properties of the fault. 2-dimensional (2D) models were used. The sand and the clay particles were explicitly mimicked using mixed two groups of elements with different sizes and different mechanical properties. Simulations of biaxial compression tests were performed, and the friction coefficients of the models with different clay contents were calculated from the simulation results. The modeling results were compared with some published laboratory experimental results. They agreed qualitatively. This study therefore demonstrates that a simple 2D DEM model can qualitatively capture the effect of the clay contents on the mechanical properties of the sand-clay mixtures. However, further studies are expected to be done in order to quantitatively predict the hydro-mechanical parameters of the fault gouge materials using DEM modeling.
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Developing a New Algorithm for Calculating Fault Seals within the Structural Model
Authors P. Røe, H. Kjønsberg and T. BarkveTraditionally fault seal calculations take place directly within the simulation grid. This approach works well for grids where all the faults are aligned along the grid pillars, but implementing an algorithm that works with stair-stepped representation of the faults has proven to be very difficult. Especially the calculation of the displacement field used both indirectly in the fault seal parameter calculation and directly in the calculation of fault zone permeability is challenging. It is hard to find where the different grid layers intersect the fault trace, and the layers are not always completely represented on both sides of the fault. We present a novel algorithm where the calculation of the fault zone permeability is carried out on a 2D plane representing the fault surface. The input parameters needed for calculating the fault zone permeability are resampled from the simulation grid onto the 2D plane, while the resulting fault zone permeability is resampled back into the simulation grid, prior to calculation of the fault transmissibility. The new approach is shown to generate good results both for pillar-faulted grids, and for grids with stair-stepped faults, and also works well near complex truncations.
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Methods to Evaluate and Predict the Effects of Extreme Reservoir Compartmentalization, BNA Reservoir, Canada
Authors P.J. Vrolijk, R. Skelly, S.E. Bradley, A.C. Ostridge, N. MacCallum and J. WoodReservoir compartmentalization; fault compartments; production pressure histories; engineering data; subsurface flow; fault seal; fault network; fault baffle; fluid flow modeling Presentation will integrate geologic interpretation methods (including uncertainty) of a complexly faulted reservoir with subsurface fluid flow and pressure histories to constrain oil and water flow between fault block compartments
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Fault Core/damage Zone; an Unhelpful Description of Fault Zone Structure?
Authors C.J. Childs, T. Manzocchi, J.J. Walsh and M.P.J. SchopferThe standard outcrop description of fault zones currently in vogue is a high strain fault core containing fault rock surrounded by a low strain halo termed a damage zone. This description does not acknowledge the significance of fault segmentation or displacement partitioning within fault zones and therefore fails to capture features which are crucial for defining the flow charateristics of faults. This terminology derives from outcrop studies but it is limited in it's ability to describe faults in 3D. Outcrop studies can best contribute towards an understanding of fault zones if they are set in the context of an appropriate 3D appreciation of faults, including quantitative definition of internal displacements and strain. Fault terminology should be guided by those datasets where 3D fault zone structure can be deciphered rather than by what is convenient in outcrops where it cannot. We suggest that the damage zone/fault core description promotes not only a simplified view of faults, but also a misleading one which is an obstacle to understanding them.
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Permeability of Fault Rocks and Shale Caprocks: Where we Stand
Authors Q.J. Fisher, C. Grattoni, J. Haneef and M. ShahMuch published fault rock permeability data was measured using inappropriate laboratory conditions (low confining pressure and distilled water as the permeant). However, the results are reasonably consistent with theoretical predictions from simple clay-sand mixing models. This may suggest that the effects low confining pressures (increases permeability) and distilled water (reduces permeability) to a certain extent cancel each other out. The results from the mixing models suggest that the permeability of the sand end-member may have an important control on the permeability of fault rocks. This model helps explain the large scatter on Vclay-permeability cross plots but is not considered by existing fault seal algorithms. Techniques used in the shale gas industry for estimating permeability are not appropriate for use in measuring the permeability of shale caprocks. The lack of a rapid decline in permeability as a function of porosity of measurements made on low permeability shale core plugs as well as inconsistent relationships between pore-throat size and permeability, suggests perhaps that techniques using core plugs overestimate the permeability of low porosity (<15%) shales by several orders of magnitude. It is likely that this reflects the inability of current techniques to accurately measure permeabilities significantly below 1nD.
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Testing Fault Seal Prediction Algorithms Using Geomodels of Experimentally Produced Fault Zones
Authors B.N. Ciftci, S.B. Giger and M.B. ClennellGeometries of clay smears produced in a series of direct shear experiments were analyzed and related back to the monitored hydraulic response, the deformation conditions, the clay content and the strength contrast between the seal and reservoir rock. The sheared blocks were imaged by CT scanning that was interpreted to construct geomodels of the fault zones from which quantitative information was obtained. The distribution of smears varies according to the level of stress during the shearing and to the contrast in brittleness of layering. Brittle clay layers form more segmented seals while ductile soft clays form more diffuse smears.Fault drag and tapering of the smear is limited to the fault cut-offs contrasting the predictions of CSP model. Increased normal stress improved fault sealing by increasing fault zone width, which led to more clay involvement in the fault zone. The average clay proportion of the fault conforms to the prediction of the SGR model. However, the hydraulic seal performance doesn’t correlate to the SGR but the net clay volume in the fault zone.
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Diagenetic and Tectonic Evolution of Pore Networks in Carbonate Normal Fault Zones and their Effects on Permeability
Authors T.J. Haines, E.A.H. Michie, J. Neilson, D. Healy, G.I. Alsop, N. Timms and A. AplinThis study quantifies changes in carbonate fabrics and pore network characteristics in fault zones using field analogues with the ultimate aim of understanding fluid flow around carbonate hosted normal fault zones. The shallow water carbonate sequence in Malta, which is dissected by an array of normal faults of varying displacements, is the chosen field analogue. The study reveals a wide range in petrophysical properties on the core plug scale. Porosity ranges from less than 5 % to greater than 35 %, permeability varies by seven orders of magnitude from 0.001 mDs to 1000 mDs and ultrasonic p-wave velocity ranges between 2 and 6 km/s. The range in these petrophysical properties is in part due to primary depositional fabric. However, modifications of the primary fabric during subsequent diagenesis and deformation are important in shaping the petrophysical properties of the rock. Pore throat size and pore type are important characteristics of the pore network which control the permeability. The changes in the carbonate fabrics into fault zones results in pore throat size and pore type changes and hence modifies the permeability. Total porosity and rock fabric are important controls on the p-wave velocity and can allow for predictions of pore network characteristics.
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Cataclastic Deformation Band Formation
Authors A. Nicol, C. Childs and J.J. WalshArrays of closely spaced (1-5 mm) sub-parallel cataclastic deformation bands are a common feature of faults in high porosity sandstones. The distribution of strain onto many low displacement bands is generally thought to require strain hardening within the deformation bands. This strain hardening model does not however account for widespread coalescing deformation bands and appears to have little support from laboratory data. The geometries and locations of many zones of deformation bands point to the role of fault geometrical complexities, rather than mechanical properties, in the formation of clusters of deformation bands. The geometric model is principally one of slip localisation by removal of fault surface irregularities and fault-rock weakening, a mechanism which is generally accepted for fault-zone development during brittle deformation for rock types other than high porosity sandstones.
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Strain Localisation in Porous Sandstone as a Function of Tectonic Setting, Burial and Material Properties
Authors R. Soliva, G. Ballas, R.A. Schultz, A. Taboada, C. Wibberley, E. Saillet and A. BenedictoThe analysis of three cataclastic band sets from Provence (France) reveals that the band density, their conjugate angles, their ratio of shear displacement to compaction, and the amount of cataclasis are different and can be expressed as a function of tectonic setting and petrophysical properties. We identify (1) a dense and closely spaced network of shear enhanced (reverse) compaction bands, (2) a regularly spaced less dense network of reverse compactional shear bands, and (3) a localized network of normal shear bands. The field data show that the localization of strain is favored in an extensional setting and is characterized by shear bands with a large shear to compaction ratio and a low conjugate band angle. In contrast, distributed strain is favored in a contractional setting and is characterized by compactional bands with a low ratio of shear to compaction and a large conjugate band angle. To explain the mechanical origin of this strain localization, we quantify the yield strength and the stress evolution in extensional and contractional regimes. We propose a model of strain localisation in porous sandstone as a function of tectonic stresses, burial depth, material properties, strain hardening and fluid pressure.
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The Role of Cataclasis in Shear and Compaction Bands on Water Flow in Porous Sandstone, Provence, France
Authors G. Ballas, R. Soliva, J.P. Sizun and A. BenedictoDetermination of membrane seal capacity of deformation bands is a critical problem for the managing of the geologic reservoirs in porous sandstones. In this study, we describe the geometry, the microstructure and the petrophysical properties of two sets of deformation bands: (1) a set of shear enhanced compaction bands and (2) a set of shear bands. At the microscopic scale, the image analysis porosities and the grain size distributions allow us to define three different types of microstructural deformation: crush microbreccia, protocataclastic, and cataclastic. Cataclastic strands are characterized by a porosity reduction of 10% to 25% and permeability reduction of 3 to 5 orders of magnitude compared to the host rock. Field observations of iron hydroxide precipitations around the bands suggest that only cataclastic shear bands were membrane seals to water flow under vadose condition. This study therefore highlights the importance of the degree of cataclasis in deformation bands as membrane seals to subsurface fluid flows in sandstone reservoirs.
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Deformation Mechanisms in Uncemented Sandstone, Examples from Triaxial Tests
Authors E. Skurtveit, A. Torabi, R.H. Gabrielsen, R. Alikarami, G. Ballas, H. Fossen and R. SolivaDeformation mechanisms in porous sandstone and sand are important for understanding the distribution of faults and their associated properties. Controlling mechanism during localization depends on stress history, burial depth and fluid composition, but also lithological variations like porosity, grain size and mineralogy. An experimental program is design to investigate the pressure conditions and mechanical constrain on localization of deformation structures with varying confining pressure in uncemented sandstone. A natural sandstone material from the Boncavaï quarry in Provence, France, where deformation structures are observed, has been selected for the tests program. In the experiments shear enhanced compaction and cataclasis are identified whereas identification of deformation bands is difficult.
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Integrated Containment Analysis - Wiki-accessible Workflows and Tools for Exploration Top and Fault Seal Analysis
Authors P.S. D'Onfro, R.R.D. Treverton and O.A. JokanolaA seal analysis knowledge-base, accessible on the ConocoPhillips Wiki, was created to codify ConocoPhillips' knowledge of fault and top seal analysis practices in a form that has longevity, can be easily updated and that can be easily accessed by exploration geoscientists. The purpose of the knowledge-base,when combined with classroom and field training, is to help sustain in the exploration community a reasonable and realistic level of self-sufficiency with fault and top seal analysis. With the creation of the knowledge-base, ConocoPhillips has established a corporate "best practice" for seal analysis that includes this knowledge-base, a group of centralized experts, a network of distributed practitioners, globally-available vendor-provided analysis tools, and a portfolio of classroom and field training resources.
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The Use of Internally Consistent Models in Seal Analysis
Authors H.M. Nordgård Bolås, C. Hermanrud, P. Brockbank, J.R. Eide, R. Heggland and G.M.G. TeigeSeal presence is required for hydrocarbons to be accumulated, and the quality of the seals will often directly determine the fluid contact levels and hence the trapped hydrocarbon volumes. It is therefore crucial that a proper seal analysis always is included in prospect evaluation. A complete seal analysis includes evaluations of both cap rocks and faults. Sometimes also bottom and side seals need to be evaluated. Various tools and methods for evaluating several of these aspects of seal analysis exist. Despite this, an overall work procedure that describes how all the individual tasks of seal analysis should be integrated and prioritized is currently not readily available to the explorationist. Through recent research and case studies we have experienced that it is difficult to apply a standardized work flow for seal analysis. This is so because the relevance of the individual tasks commonly included in such analysis will vary significantly between different areas and between individual traps. However, we have found that the concept of “Internally Consistent Models” (ICM) can be very useful for such analyses regardless of the geological setting. The use of such models will aid integration and provide a basis for risking hydrocarbon volumes in prospect evaluation.
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Delineation of Petroleum System Using Faults and Associated Gas Chimneys
Authors M.A. Raza and W.I.W.B. YusoffUnlike other geological techniques and phenomenon for the identification of fault behavior and defining the petroleum system, this study deals with the delineation of faults and gas chimneys for understanding the petroleum system present. Both of these features are self sufficient for achieving the objective. Gas chimneys can provide great information about source, migration path and the accumulation area. Faults carry a vital importance; they can restrict the hydrocarbons in a place by their sealing behavior or act as a conduit for the passage of fluids. Understanding the faults behavior from the gas migration and associated features provides an extra dimension with accurate results. These features are worth studying as they provide information about migration path and helps to define the petroleum system.Faults may act as preferred pathways for fluid migration when permeability of the fault or fault segments becomes higher than the surrounding country rocks.
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High Resolution Fault and Seals Imaging from Geological Model - A North Sea Case Study
More LessA new workflow to enhance seals and faults from the seismic imaging is proposed. Whereas most of the proposed methods for the characterization of seals and faults are obtained from seismic attributes, we have used a novel method based on a geological modeling from the seismic. From this model, new kinds of attributes can be derived to enhance sequence stratigraphy, faults, seals and traps. By computing the vertical derivatives of the geological model, also called the "Thickness" cube, it reveals the instantaneous variations of the geological layers in the volume on each seismic voxel. It is sensitive to the convergence and divergence of the geological horizons and therefore appears to be particularly well adapted to reveal traps, seal, spatial distribution of reservoirs. The fault imaging is obtained from the computation of 3D derivatives of the geological model, which shows a detailed detection of faults even in zones with a low signal noise ratio. Applied on the block F03 in the North Sea, this method has proven to be fast and efficient to evaluate prospects for well placement application and also maximize chance of success in exploration.
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Improved Fault Imaging by Integration of Frequency Decomposition and Fault Attributes, Example from Mid Norwegian Sea
By B. AlaeiSeismic attributes have been utilized to enhance identification and mapping of faults on seismic volumes. Application of 3D seismic attributes together with advanced visualization techniques allow combination of different fault characteristics and result in a better understanding of complex fault systems. In this study fault attribute analysis is integrated with frequency decomposition process. A fault imaging workflow including structurally oriented semblance attribute volume and fault enhancement attribute volumes have been generated using two different sets of seismic input. First the fault imaging workflow was carried out using full band seismic volume. Then, seismic data has been decomposed into amplitude volumes representing individual frequencies. The same fault imaging was repeated using amplitude volume from high frequency. Amplitude volumes of higher frequencies correspond to higher resolution. The method has been tested on a 3D dataset from the Mid Norwegian Sea. The results show that using amplitude volumes of higher frequencies can image smaller scale faults that are not visible using full band seismic volumes as input for the fault imaging. Improved fault imaging achieved using this method can help derisk fault seal related issues in areas with complex structural setting such as the one presented in this study.
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Integrated Reservoir Connectivity Analysis, Zechstein Carbonate Reservoir, Northern German Basin
Authors F.V. Corona, J. Neble and K. RettenmaierAn integrated workflow is presented for evaluating static and dynamic connectivity of a complex Upper Permian Zechstein carbonate reservoir, mainly platform facies, in the Scholen Concession of the Northern German Gas Basin. Sour gas production in the concession involves drainage from multiple, tectonically emplaced and, in places, vertically stacked allochthonous Zechstein carbonate reservoir sections. These allochthons lie within the Permian Zechstein salt and overlie the in-place autochthonous Zechstein reservoir. The key issues in the Scholen Concession are the static connectivity and fill-and-spill scenarios among the multiple Zechstein sour gas accumulations and the dynamic connection between the autochthonous and allochthonous reservoir intervals. Understanding these issues is important in the evaluation of infill drilling opportunities and near-field appraisal wells, and whether or not these opportunities can be drilled safely and economically. The integrated reservoir connectivity analysis incorporating geoscience and reservoir engineering data revealed that the five main gas accumulations in the concession appear to be connected statically through a common aquifer and the in-place autochthon and most detached allochthons likely are connected dynamically with baffles. These learnings ultimately led to realistic and reliable evaluations of infill opportunities and near-field appraisals in the concession area, and changes to the drill-well inventory seriatim.
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Comparing Simulated Production Response Across Realistic and Seismically Forward Modelled Fault Geometries
Authors A.M. Wood, R.E.L. Collier and D.A. PatonOutcrop examples show us that reservoir scale faults often have complex geometries, both in cross section and in three dimensions. This complexity may take the form of multiple slip planes, entrained lenses of host material, drag structures and relay zones, however the limitations of seismic data prevent much of this detail from being resolved and incorporated into geological and reservoir simulation models. By forward modelling the predicted seismic response of detailed fault geometries we are able to compare realistic fault architectures to those which can be identified in seismic datasets. In turn, by constructing reservoir simulation models, we are able to assess the impact of these different geometries on predicted hydrocarbon recovery. In certain situations differences in modelled cross fault juxtapositions and calculated fault rock properties between the realistic and forward modelled fault geometries result in significant variations in the simulated production response.
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Quantifying the Impact of Fault Damage Zones on Reservoir Performance
Authors D.E. Wolf, J.G. Solum, J.P. Brandenburg and S.J. NarukThis study quantified the contribution of a damage zone for a range of observed host rock and damage zone characteristics, such as: reservoir Vshale; fault shale-gouge ratio (SGR); damage intensity; and permeability of reservoir, fault, and deformation bands. A hypothetical reservoir was utilized to show the impact of a damage zone on the overall FTM calculation. The relative impact of a damage zone on the total cross-fault flow was calculated empirically relative to the percentage of reservoir offset. Single variables were modified in each calculation to isolate the greatest controls on damage zone impact. The calculations were made for a single hypothetical reservoir unit, surrounded by shale above and below. Fault permeabilities were calculated using equations similar to those in Manzocchi et al. (1999). Effective permeabilities were calculated using a harmonic mean. The difference between cross-fault FTM from only the fault core versus the fault core acting with a damage zone was calculated, and used to assemble risking tables to be assessed for a range of Vshale and percentage of reservoir offset. The difference represents the correction factor to be applied to the currently calculated fault transmissibility multiplier (FTM).
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A Multi-Disciplinary Workflow for Characterising Shale Seals
Authors D.N. Dewhurst, M.B. Clennell, I. Burgar, M. Josh, J. Sarout, C. Delle Piane, L. Esteban, M. Pervukhina and M.D. RavenEvaluation of the various rock properties of shales has become more prevalent in recent years although our understanding of these properties and the links between them is still relatively embryonic. While thick shale sequences can form sealing units above hydrocarbon traps, intra-reservoir shales can form baffles to flow in both petroleum and groundwater contexts. High field and low field nuclear magnetic resonance were used to evaluate wettability of shales. Preserved shales show mineral dependent variations in surface affinity for oil versus water. Hydrophilic shales have a higher cation exchange capacity (e.g. shales rich in illitic and/or smectite), whereas kaolinitic mudrocks are potentially hydrophobic and can be wetted preferentially by oil, sometimes retaining oil on the mineral surfaces after further exposure to brines. Porosity and cation exchange capacity correlate well with strength properties and dielectric constant measurements on intact shales and pastes made from powdered shales show strong relationships between high frequency electrical properties, mineralogy, cation exchange capacity and mechanical strength. Calibrating wireline logs with laboratory measurements and the development of physics-based models allows the prediction of rock properties and extrapolation to the borehole scale.
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Structures and Mechanics of Shear Enhanced Compaction Bands, Provence, France
Authors G. Ballas, R. Soliva, A. Benedicto, E. Skurtveit and H. FossenDeformation bands are zones of strain localization in porous sandstone characterized by their microstructural deformation modes and by their amount of compaction and shear components from pure dilation, pure shear to pure compaction. The mechanism for their formation remains debated, especially the role of burial depth and host rock properties on the formation of compaction bands. In this study, we describe a set of shear enhanced compaction bands formed in the porous Montmout sandstone (Provence, France) under shallow burial conditions, i.e. less than 500 m. These bands are organized in conjugate and strongly distributed network. They are located within the coarse sandstone units, i.e. with a mean grain size around 0.6 mm, whereas few or no bands are observed in the fine sandstone units, i.e. with a mean grain size less than 0.3 mm. The mechanical approach using calculated stress path and cap envelopes in Q-P diagram confirms that these compaction bands can only form within the coarse sandstone units, i.e. only the cap envelopes of these units are reached by the stress path. This analysis underlines the importance of the grain size of the host sandstone for compaction band formation under shallow burial conditions.
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Fracture Permeability in Seals and Reservoirs - New Insights on Joint Structure and Formation Mechanisms from Laboratory
By J.P. PetitJoints are pervasive fractures strongly affecting the reservoir permeability and seals integrity. They are currently interpreted as mode I cracks forming normal to the minimal tensile effective stress S3. This view is challenged by the results of both axisymmetric and polyaxial (true 3-D) extension experiments carried on rock analogue materials where joint like S3-normal fracture sets were produced. They are of two types defined by the effective mean stress S. When S is very small, the fractures are mode I cracks with smooth fracture surfaces. At higher S, these surfaces have plumose morphology. In the latter case, both SEM observations and mechanical measurements show that joints are initiated as dilatancy deformation localization bands. They form under slightly tensile or even compressive normal stress and therefore cannot be mode I fractures. The similarity between the plumose fractography and the internal structure of experimental and natural joints observed in dolomicrite strongly suggests a similarity in the formation mechanism. It is therefore proposed that most of natural joints could form as dilatancy (dilation) bands at higher pressure (depth) than expected for the mode I mechanism. These “dilatancy joints” ca be seen as constitutive instabilities and can be modelled numerically.
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Impact of Tensile Strength Anisotropy on Fracturing Pressure of Svalbard Sandstone and Shale Cap Rocks
Authors B. Bohloli, L. Grande, E. Aker and E. SkurtveitThis paper presents results of the laboratory tests for determining the tensile strength of anisotropic shale samples cored from Aagardhfjell Formation in Longyearbyen, Svalbard. This formation is considered as seal for the suggested CO2 storage reservoir. Therefore, it is essential to characterize and investigate behavior of the shale under storage conditions. This study includes results and analysis of Brazilian indirect tensile strength of cores parallel and perpendicular to bedding. The results are analyzed to incorporate in the calculation of fracture pressure. Because of significant burial and uplift of the Barents Sea region, the shale exhibits very high tensile strength and strong anisotropy. Calculation and analysis of fracture pressure, based on the laboratory data and some assumptions, show that the orientation of possible fractures can be dictated by the tensile strength instead of in-situ stresses. An example of the impact of tensile strength anisotropy on the fracture pressure of shale formation has been assessed. The results of this study suggest that the tensile strength of rock has significant impact on fracture pressure and the orientation of fractures.
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Fault-related Advective and Diffusive Fluid Flow from Fe-oxide Concretions and Liesegang Bands in Poorly Lithified Sands
Authors F. Balsamo, F.H. Bezerra, M. Vieira and F. StortiIron oxide coloration and deposits in sandstone are significant indicators of the mobility of solutes (Fe2+ and O2) in groundwater, mainly controlled by host porosity and permeability. We describe the occurrence and geometry of different types of iron oxide deposits developed within the vadose zone along faults affecting poorly lithified, quartz-dominated, heterolithic sandy sediments in the Paraíba Basin, NE Brazil. Development of highly-permeable damage zones and low-permeability fault core-mixed zones, promote physical mixing of advective Fe2+-rich waters and oxygenated groundwater along soft-linked fault zones. This favors iron oxide precipitation as m-scale sand impregnations, cm- to dm-scale concretions, and well cemented dm- to m-thick mineral masses. The formation of hydraulically isolated compartments along hard-linked, strike-slip faults promotes the development of Liesegang bands, in a reaction zone dominated by pore-water molecular diffusion of O2 into Fe2+-rich stagnant water, and the precipitation of Fe-oxide impregnations and concretions in the fault core-mixed zone boundaries, due to O2 diffusion in advective Fe2+-rich waters. Fault zone architecture, permeability and geometry determine the dominant mode of solution interaction, leading to the formation of Fe-oxide Liesegang bands where O2 diffuse in stagnant Fe2+-rich water, and concretions when diffusion is complemented by Fe2+ advective flow.
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Fault Seal Prediction in Sandstone Reservoirs - A Quantitative and Calibrated Geo-mechanical Method
More LessSealing faults form a major element in trapping hydrocarbons. They can form crucial elements for trapping hydrocarbons and isolated compartments in reservoirs. Alternatively, faults can form leak zones and conduits for fluid flow. Prediction of fault seals is therefore essential for efficient hydrocarbon exploration, field development and underground storage of natural gas and CO2. Presented is a geo-mechanical method to predict the sealing potential of faults in sandstone reservoirs. The method is based on a field case with core samples, rock-mechanical tests and numerical calculations and was successfully applied and tested in two additional field cases. The result is a reliable quantitative, calibrated method and a newly developed tool, the reactivation circle, with which it is possible to predict fault the fault seal quality in sandstone reservoirs. The method is especially useful in clean sandstones in which abundant disperse clay or ductile clay layers present are not present. The key to predicting fault sealing in clean sandstone reservoirs requires predicting the formation of a sealing cataclastic gouge on a fault plane and the conditions that control its formation and sealing capacity. In predicting the formation of a sealing cataclastic gouge required the development of a technique for palaeo stress analysis.
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Fracture Patterns in Carbonate Fault Zones and their Influence on Petrophysical Properties
Authors E.A.H. Michie, T.J. Haines, D. Healy, J. Neilson, G.I. Alsop and N.E. TimmsMaltese field examples of variable carbonate lithofacies in fault displacements ranging from ~20cm up to 100m are used to examine how different carbonates deform, and to understand evolution of the fault zone architecture and ultimately their petrophysical signatures. The evolution of fault zone architecture in time and space, and the associated changes in deformation mechanisms, exert an important control over the sealing potential of faults. It has been suggested that faults in carbonates could form seals after as little as ~20 m displacement, especially when juxtaposed next to a different formation. However, as seen in the Maltese examples, the complete opposite may occur. An intense zone of deformation is formed, which enhances both the porosity and permeability on an outcrop scale. This also prevents the localisation of deformation onto one slip surface, stopping the formation of a continuous, impermeable fault core. Examination of how the rocks deform, through different deformation mechanisms, can potentially help unravel the relationship between fault zone architecture and petrophysical properties. It can also help to indicate the potential evolution of the petrophysics through understanding scaling of the damage zone relationships with displacement of different carbonate lithofacies.
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Permeability Anisotropy in Sandstone Hosted Normal Faults
Authors N.J.C. Farrell and D. HealyMeasurements of permeability (k) from over ninety oriented core plugs sampled around two extensional faults set in aeolian and fluvial sandstone formations show anisotropy in three orientations independent of the original sedimentary fabric permeability. The degree of permeability anisotropy and the orientations of kmax vary with distance from the fault plane and show asymmetry between the hanging wall and footwall. Analysis of current porosity from core plugs and image analysis implies that variations in pore geometry associated with faulting may control permeability anisotropy around these fault zones. Integration of this new petrophysical information can improve understanding of permeability and transmissivity for reservoir modelling around fault zones. Previous research shows that changes in pore pressure, associated with fluid flow, can induce changes in the stresses acting on the rock (Teufel et al., 1991). Quantitative study of complex pore geometries along a fault zone can also provide information about the mechanical behaviour of rocks during faulting and inform predictions for fault stability.
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The Fault Object - A Step Forward for a Generalised Fault Model
By C. GoutThe fault characterisation is a key for the hydrocarbon fields’ exploration and production where it provides trap boundary and fluid flow baffle at many scales. By its nature the fault is a complex, discrete and much localized component of the subsurface and its characterisation is a difficult task involving many techniques, expertise on poorly accurate data sources. A review of the fault’s fundamental concepts, operational context, source data, tools and methods leading and constraining its characterisation and representation is proposed. These extensive reviews on the yet blooming fault modelling tools, R&D activity and know-how by object-oriented analysis and design techniques inherited from the computer science domain allow proposing a paradigm, the Fault Object, a self descriptive, self operative tool. The “Fault Object”, a proposal for building a generalized fault model, is introduced and its attributes, representations, interfaces and methods are detailed. Finally, the added-value of the model is highlighted by its application to workflow design and adaptation, software toolbox developments, training set-up, uncertainty evaluation and parameters ranking or R&D projects design.
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Evaluating Controls on Cross-fault Flow within Reservoirs
Authors A.A. Brown, V.S. O'Connor-Wood and R.K. DaviesFlow across a series of simulated faults were modeled to evaluate how variation of fault properties (juxtaposition, complexity, and fault rock) affect upscaled fault flow resistance. . Simulations with cell size similar to that typical of reservoir simulators overestimate juxtaposition flow resistance. Juxtaposition flow resistance also deviates where upscaled reservoir cells near the faults contain a significant fraction of shale. Fault complexity in the absence of fault rock adds little flow resistance to that created by juxtaposition along a single shear plane. Continuous, tight fault rock causes the greatest flow resistance. However, the presence of even a small number of high-permeability pathways through the barrier greatly reduce its flow resistance. Overall, the major cross-fault flow control is continuity of tight fault rock. Where the fault rock is not continuous, flow resistance is low and controlled by the fault architecture. Where tight fault rock is continuous, flow resistance is high and controlled by fault rock properties. The abruptness of change between low flow resistance with discontinuous barriers to high resistance with continuous barriers is controlled by the ratio of reservoir to fault rock flow resistance.
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Quantifying Patterns of Deformation Bands - Examples from the Hopeman Sandstone, Moray, Scotland
Authors A. Awdal, D. Healy and G.I. AlsopSubsurface fractures control the flow of fluids through reservoirs containing economic resources. Fractures can act as conduits or barriers to fluid flow and understanding the geometry of the fracture pattern is a crucial step in quantifying the connectivity of fractures, which is the major factor governing bulk permeability. Natural fractures are not randomly distributed and exhibit clustering in various forms particularly in the damage zones around larger faults. We identify the shape of lozenges, which is refer to the area (volume) relatively undeformed sandstone situated between two strands of a composite deformation band. We also mapped and quantified geometry of the deformation band patterns and ladder fractures. The published scaling relationships for attributes across dimensions depend on the assumption of random spatial distributions, and yet fractures are rarely randomly distributed in space. We aim to quantify these relationships and this will guide the construction of testable expressions for multidimensional scaling relationships using empirically derived power law exponents.
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Palaeo-fluid Flow History of an Extensional Fault System from Carbonate Concretions in Shallow Marine Sediments, Italy
Authors F. Balsamo, D.R. Gröcke and F. StortiWe present a multidisciplinary study on the relationships between the structural architecture of an extensional fault system in poorly lithified shallow marine sands and the associated pattern of diagenetic carbonate concretions. Based on their shape, spatial distribution, cement texture and chemistry and isotopic signature, carbonate concretions are grouped into (i) tabular concretions developed within the fault zones, and (ii) elongate and coalescent strata-bound concretions formed adjacent and mostly parallel to fault zones. Tabular concretions formed during early diagenesis in the vadose mixing marine-meteoric zone, possibly during coseismic rupture propagation and, in the interseismic periods, due to slow capillary suction along the low-permeable fault zones. On the other hand, elongate concretions formed after regional uplift and during telodiagenesis by precipitation from meteoric, phreatic water flowing parallel to the fault zones which acted as hydraulic barrier. The concretion patterns record the evolution of fluid flow orientation and the changing chemistry of fault-zone fluids that were fundamentally driven by the the fault zones architecture during two major diagenetic stages.
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Mapping of Regional Potential Leak Paths
Authors K.A. Beintema and D.E. EdwardsThis study was aimed at derisking several Jurassic prospects situated in the Central North Sea. Prior to this study, reservoir presence was thought to be the main risk. The structural history of the area is very complex, with an interplay of extension, salt tectonics and sediment deposition. The stratigraphic and structural frameworks together determine the reservoir and seal risks of prospects within the Jurassic sequence. This study comprises of a combined stratigraphic-structural analysis on reprocessed seismic, and has revealed that the interaction between tectonics and sedimentation leads to seal risks not previously identified. This study shows that in the area of interest, potential leaks paths exist connecting Fulmar prospects and Kimmeridge Mass Flows by both top seal erosion and juxtaposition of thief sands across faults.
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Fluid Flow Effects of Faults in Carbonate Reservoirs, an Example from the Kharyaga Field, Russia
Authors O.P. Wennberg, J.I. Logstein and N. HashemiFaults in carbonate reservoirs may have a dramatic effect on fluid flow and represent a major uncertainty in hydrocarbon exploration and reservoir characterization since they are often associated with high permeability conduits as well as potential barriers. The fluid flow properties result from a combination of mechanical and diagenetic processes controlled by the brittle and reactive characteristics of carbonate rocks. The Late Devonian reservoir in the Kharyaga Field of the Pechora Basin is extremely heterogeneous due to fractures and karstification superimposed on the depositional heterogeneity. Faults and open fractures strike dominantly E-W, and the data indicate that fracture density tends to increase towards faults and is associated with decreasing porosity in the matrix rocks. Dissolution and karstification is commonly associated with the faults and fractures. The best and most realistic match of historical production data was achieved when the faults in the simulation models were represented as conduits for flow parallel to the fault and barriers for flow perpendicular to the fault. Geologically this is explained by: a cemented cataclastic fault core representing the barrier, and by a fault damage zone consisting of a connected open fracture network enhanced by dissolution producing high permeability parallel to the fault.
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An Improved Method for Generating Fault Zone Grid
Authors D. Qu, P. Roe and J. TverangerFaults are volumetric in nature and can cause complex fluid flow inside the fault zone because of its special fault zone architecture and different petrophysical properties from the host rock. Thus explicit fault zone modeling is important for capturing the fluid flow inside and through the fault zone precisely. Generation of a refined volumetric fault zone grid is the first step to perform explicit fault zone modeling. An algorithm for generating volumetric fault zones has already been implemented in Havana, however this algorithm failed to generate continuous top and bottom surfaces for the fault zone. This lead to internal discontinuities in the fault zone grid, and made it hard to run flow simulations on the grid. We now present an improved version of the algorithm that works well on complex faults and indicate the capability of explicit fault facies modelling of real field cases.
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Comparison of Mechanical and Fracture Stratigraphy between Failed Seal Analogues
Authors E.S. Petrie and J.P. EvansThe presence of discontinuities in a seal affects both their mechanical and hydrogeologic properties; migration of fluids or gas through mm- to cm-scale discontinuity networks can lead to the failure of gas or fluid traps. The presence of discontinuity networks increases the volume of rock matrix in contact with subsurface fluids creating preferential pathways for fluid flow and changing the mechanical properties of the seal. We examine the mechanical and fracture stratigraphy of failed seals analogues exposed in central and south-east Utah. We use outcrop surveys to identify relationships between occurrence of discontinuities and sedimentologic variability, and to understand the nature of alteration associated with fluid flow through these fractures. We use data from each locality to quantitatively define mechano-stratigraphic units, based on consistency in fracture distribution, bed thickness, lithologic stacking pattern, field-derived compressive strength and permeability. These data allow us to define mechano-stratigraphic units at each locality and to compare the changes in deformation behaviour between localities. Using this systematic methodology to quantify differences and similarities between field analogues we improve our understanding of the important role of micro- to meso-scale fracture networks play within sealing lithologies and how various seal lithologies respond the changes in stress.
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Statistical Analysis of Fault Attributes
Authors D. Kolyukhin and A. TorabiThe statistical analysis of fault attributes scaling relationships is discussed. Dependences of length, width of damage zone and thickness of fault core on displacement were studied assuming power-law relations. The approximation forms a piecewise-linear function with few slopes in log-log scale. The Bayesian Information Criterion (BIC) was used to find the best fit for an optimal number of parameters. Numerical tests show that the best fit was obtained when using power-law relations with two slopes. Bayesian analysis of model parameters’ probability distribution was performed. The second part of this work is devoted to statistical analysis of single faults' attributes. Truncated power-law (TPL) is considered in comparison with commonly used simple power-law (PL) (or Parreto) distribution. The maximal likelihood and the confidence interval of the exponent for both PL and TPL are estimated by appropriate statistical methods. Kolmogorov-Smirnov (KS) test and likelihood ratio test (LRT) with alternative non-nested hypothesis for exponential distribution are used to verify the statistical approximation. Our results suggest that a truncated power-law is more suitable for describing fault attributes and its condition is satisfied for a wide range of fault scales. Furthermore, advantage of TPL is proved by BIC.
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Deformation of Poorly Consolidated Sanstone in Aghione Formation, Corsica, France
By A. TorabiDeformation of poorly consolidated sandstone and its effect on petrophysical properties of the rock are investigated by structural fieldwork, in-situ measurements of permeability, microstructural analyses of thin sections and ultrasonic measurements of samples. The studied locality comprises a series of listric normal faults that cut through Aghione Formation (Miocene) in Corsica, France. Three types of deformation bands are reported. Surprisingly, intense cataclasis is observed in the phyllosilicate-rich (more than 20%) poorly consolidated sandstone. Permeability decreases up to two orders of magnitude in the damage zone (deformation bands), while it increases in the slip surfaces. The slip surfaces are open and make conduit to fluid flow, which is also confirmed by the field observation of fluid flow path.
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Cross Faults - A Major Control on Lateral Compartmentalization of Hydrocarbon Reservoirs of Kuwait
Authors N.M.A. Nada Al-Ammar, R. Husain, S.A. Azim, A. Prakash, R. Mulyono, A. Al-Khamis, P. Singh and R. AndrianySignificant variations in production behavior of the hydrocarbon reservoirs were observed in Kuwait. Understanding of these variations is of vital importance for exploration and production of hydrocarbons. In a synergistic approach, geological, gravity, magnetic, seismic, geochemical and production data were integrated to decipher the reservoir behavior by understanding the structural architecture and role of faults in reservoir compartmentalization. Significant variation in initial reservoir pressures and oil properties across the major cross faults are suggestive of their sealing nature. These faults have developed after the peak oil migration from the Jurassic and Cretaceous source rocks.
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Fault Sealing Behaviour in Three North Sea Reservoirs Estimated from Petrophysical and Production Data
By A.D. IrvingAlthough understanding their flow behaviour is imperative for successful reservoir development, seismically resolvable faults are rarely sampled by wells. Their properties must be extrapolated from imperfect analogues or inferred from dynamic data and numerical reservoir simulation. Published methods regard fault transmissibilities as unknown tuning parameters with consequently limited predictive capability; workflows have therefore been developed for uncertainty assessment and conditioning to dynamic data of geologically consistent fault properties. This paper focuses on their application to three North Sea cases in which simulation models were able to reproduce observed field behaviour using fault properties consistent with core data. In contrast to published correlations between depth and reduced fault permeability, the interpreted permeability in the shallowest field was approximately one order of magnitude lower than for the other two, deeper reservoirs. Within the calculated ranges for Shale Gouge Ratio, the interpreted fault permeability functions for all three fields are within the prior uncertainty range estimated from analogue core data and are consistent with published studies. These functions add significantly to the number of available curves calibrated to sub-surface pressure data and could therefore be of considerable use for prediction of fault behaviour in similar fields without production data.
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Patterns of Clay Mineral Transformations in Fault Gouges
By S.H. HainesNeoformed clay minerals in fault rocks in the brittle crust are increasingly recognized as being key to the sealing behaviour of faults . Academic literature has recognized the importance of neoformation of clay in fault gouge for a number of years, but the concept has not reached most industry seal analysis workflows. Clay-rich gouges that form as a consequence of new clay mineral growth are distinct from clay smears or cataclastic fault rocks that form as a result of mechanical incorporation of wall-rock phyllosilicates, in that they form by chemical and not physical processes. We report a comprehensive field study of clay mineralogy on fault rocks from sedimentary basins and low-angle normal faults in the American Cordillera. We then synthesize the field study with a literature survey to identify controlling conditions for neo-formed clay in fault gouge. Neoformed mineral in gouges are illite, illite/smectite, smectite, and chlorite/smectite phases. Chlorite and kaolinite do not form as neoformed clays in fault gouges. Controlling conditions are wallrock chemistry, temperatures of ~60-180 C and fluid availability.
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The Impact of Aquifer/Caprock Morphology on CO2 Storage in Saline Aquifers
Authors S.M. Shariatipour, G.E. Pickup and E.J. MackayThe CO2 storage project at Sleipner has shown that the topography of the aquifer/caprock interface can have a significant effect on the migration paths of CO2 in an aquifer (Singh et al 2010). Therefore when appraising any reservoir or aquifer for CO2 storage, it is important to characterise the interface and to ensure that it is modelled adequately. Often simulations assume a sharp boundary smooth between the aquifer and the caprock. However studies of outcrops show that a variety of types of interface may arise in nature, depending on the depositional setting. For example, Shariatipour et al (2012) have shown that there may be a gradual transition from sand-rich facies in the aquifer to mud-rich facies in the caprock. Syversveen et al, have investigated the impact of top-seal morphology on CO2 storage capacity and migration patterns, and concluded that it is important to model geological details in order to predict CO2 migration. We intend to investigate different types of caprock/aquifer interface to determine how these affect the pressure build-up under the caprock and the amount of CO2 which dissolves in brine. This will enable us to identify which type of aquifer/caprock has the greatest potential for storing CO2 securely.
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The Role of Vertical Mechanical Heterogeneity in Predicting Fault Zone Architecture
Authors R.K. Davies, R.J. Knipe and M.J. WelchThe modeling of flow across faults in reservoir simulation and in exploration is based often on the permeability distribution. Detailed observations of faults across a range of stratigraphic facies, however, show that this simplification is likely to miss some of the fault zone architectural complexity that can play a significant role in the modeled flow. With over 10 years mapping the details in fault zones from a range of different tectonic and stratigraphic environments, RDR has compiled a unique set of observations that show the importance of the vertical mechanical heterogeneity (VMH) at the time of deformation. In our model for fault evolution, we define an incipient shear zone across which the layers and multilayers are sheared by brittle faulting and ductile folding. This incipient shear zone has been defined as a premonitory shear zone (PSZ) that “forecasts” the position of a fault with discrete offset. The complexity of the fault zone increases with an increase in the VMH with both discrete fault offsets and smearing of the ductile layers. A simple fault zone develops with a lower VMH. These conceptual models, combined with mechanical modeling, describe the process, which improves the prediction of the fault zone architecture.
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Fault Seal and Trap Analysis, High Temperature High Pressure (HPHT) Central North Sea
Authors J.T. Summers, S.J. Naruk, S. Thackrey, J. Solum and D. WolfFault seals seen in the HPHT area of the CNS are generally very good, possibly because of overpressures that, globally, appear to enhance fault sealing, and also because of the nature of Triassic and Jurassic stratigraphy in the area. The various laterally extensive and coherent shales within the Skagerrak Formation as well as the Jurassic Formations are helpful in creating a competent seal along fault planes. Seven of the nine fields also have reservoir-reservoir juxtaposition along fault planes above the hydrocarbon water contacts and appear to be sealing due to SGR. The workflows described here should be applied to future studies with the understanding that the uncertainties in stratigraphy and structure need to be incorporated to receive the best assessment of accumulation controls.
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Field Example of Application of Dynamic Fault Seal Analysis Technology and Well Test Matching to Condition Full Field Reservoir Models
Authors K.O. Onyeagoro, A.B.J. Smout and T.J. WellsQuantification of cross-fault flow and fault transmissibility multipliers is important for the development decisions, since the nature of hydrocarbon and water flow across faults will influence well counts, well placement, and ultimate recovery. The Corrib gas field discovered in 1996 lies 70 km offshore west of Ireland in 360 meter water depth. It is a faulted 4-way dip closure (Figure 1). Within the Sherwood Sandstone, extensional faulting with cataclastic fault rocks is the dominant expression of deformation. Two dominant fault families are present; one set dipping to the east and the other to the west. These faults mainly strike in NE-SW direction. Second generation ESE-WSW striking faults developed at a later stage of deformation probably as a result of sudden change in curvature and stress/strain states at the northern and southern flanks of the structure. This paper discusses workflows to determine Corrib field fault properties, and well test matching workflows to determine reservoir properties and condition dynamic models. The integration of results from fault seal analysis and well test matching deliver significant business impact and demonstrate the value of Shell’s in-house fault seal dynamic modeling capabilities.
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34% of Newly formed Phyllosilicates within Faults of Deeply Buried Sandstones Reservoir
Authors T. Cavailhes, R. Soliva, P. Labaume, C. Wibberley, J.P. Sizun, C. Gout, D. Charpentier and M. BuatierIn this study, we describe normal fault zones cutting foreland arkosic turbidites in deep diagenesis conditions (200°C). The microstructural analysis of the fault cores reveals a large amount of phyllosilicates (up to 34%) in the fault rock, derived from an intense feldspar muscovitisation. This process is favored by mechanical disaggregation of the feldspar grains following cleavage planes during deformation. Point-counting on thin sections sampled in the studied fault zones allows to quantify the proportion of feldspar transformed in white micas between 60-80%. We propose a modified version of the predictive Shale Gouge Ratio algorithm in order to integrate the proportion of phyllosilicates resulting from feldspar muscovitisation. We discuss the importance of feldspar alteration within faults of the deep brittle crust and its role on fault strength and high temperature reservoir partitioning.
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North Oman Fault Geometries in Outcrops, Analogues and Sub-surface and their Implication on Fault Seal
Authors P.D. Richard, L. Bazalgette and M.H.N. Al-KindyNorth Oman offers an opportunity for making outcrop observations of faults developed in carbonate reservoirs. It is possible to examine faults and associated damage zones in great detail, both in map view and cross sections. From these observations, geometrical concepts and rules can be defined. These help to interpret faults on seismic as well as to build static models. It is also possible to demonstrate the potential impact of the simplifications inherent to the static modelling process and to decide whether they are acceptable or not to define the most suitable modelling strategy. The understanding and the modelling of the fault sealing properties are an important part of this process. The main objective of this paper is to share how we can take advantage of the North Oman outcrops, in combination with a database of sandbox analogue models and high quality seismic data, in order to develop conceptual geometrical models of faults. These can be used as a foundation to 3D fault frameworks and to address static and dynamic fault seal properties. The conceptual geometrical models are also used to develop interpretation and modelling strategies, as well as workflows for the elaboration of geo-cellular models designed for specific recovery mechanisms.
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Modelling of Relay Ramp Using Fault Facies and Truncated Gaussian Simulation
Authors M. Fachri, A. Rotevatn and J. TverangerRelay ramps may considerably control fluid flow across otherwise sealing fault zones and, therefore, they are subjected to fluid flow simulation studies in relation with groundwater extraction and petroleum production. However, these studies used mainly deterministic modelling approach based on field observations. In this paper, we extend this approach by incorporating a conditional simulation technique, i.e. truncated Gaussian simulation (TGS), to provide stochastic elements in the modeling work and to improve the representation of detailed relay ramp structure. TGS attributes (facies proportions and variogram ranges of the Gaussian field) are defined based on field observations of the Delicate Arches Ramp, Utah, which is characterized by the presence of three structural sets. Groups of facies defined based on the structural sets are modelled independently using TGS, and the final model is generated by merging the TGS realizations. The relay ramp facies model is used to generate upscaled models. The upscaling procedure use petrophysical values and coarse grid used in a recent flow simulation study of the Delicate Arches Ramp deterministic model. Flow simulation results show that the role of relay ramp as conduits for fluid flow is more pronounced in models built using TGS than that in deterministic models.
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The Quantification of Fracture Patterns
By D. HealyFractures in deformed rocks are rarely uniform or random. Fracture orientations, sizes, shapes and spatial distributions often exhibit some kind of order, or pattern. In detail, there may be relationships among the different fracture attributes e.g. small fractures dominated by one orientation, larger fractures by another. This is important because the mechanical (e.g. strength, anisotropy) and transport (e.g. fluids, heat) properties of rock depend on these fracture patterns and fracture attribute relationships. This presentation describes a methodology to quantify fracture patterns, including distributions in the fracture attributes and their spatial variation. There is a large body of published work on the quantification of relatively simple joint patterns, but fault zones present a bigger, and arguably more important, challenge. The method presented is inherently scale independent, and a key task will be to analyse, interpret and integrate quantitative fracture pattern data from micro- to macro-scales.
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