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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
21 - 40 of 78 results
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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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