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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
1 - 20 of 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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