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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
41 - 60 of 68 results
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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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