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Second EAGE Workshop on Naturally Fractured Reservoirs
- Conference date: 08 Dec 2013 - 11 Dec 2013
- Location: Muscat, Oman
- ISBN: 978-90-73834-69-9
- Published: 08 December 2013
39 results
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How Far Facies and Diagenesis Control Fractures in Carbonates
Authors J. Lamarche, A.P.C. Lavenu and B.D.M. GauthierA general statement relate fracturing to tectonic events. In carbonates, the strong influence of diagenetic processes through time is a clue in rock brittle properties acquisition. Indeed,in such host-rock, early cementation developed rapidly after deposition and turn the deposited sediment into a brittle rock, allowing fracture to develop through burial. What we want to show is how these mechanism have to be taken into account when characterizing the fracture pattern in carbonates.
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Mechanical Stratigraphy of Large-Scale Fractured Zones in Platform Carbonates (Maiella Mountains, Apennines, Italy)
Authors S. Torrieri, C. Volery, L. Bazalgette and C.E.G. StraussUnderstanding the main mechanisms controlling fault and fracture impedance across different lithologies is an important factor to estimate the flow properties and recovery of geo-fluids in frac-tured reservoirs. A multidisciplinary study of the Cretaceous carbonate platform of the Maiella Moun-tains allowed concluding on the occurrence and impeding mechanism of two main mechanical boundaries for large-scale fractured zones. Additionally, a workflow for applying the outcrop findings to the subsurface has been described and illustrated. The two identified mechanical boundary types are related to porous, well-sorted bioclastic packstone/grainstone units and dolomitized layers respectively. The first type occurs within the Upper Cretaceous succession in an open platform environment, whereas the second type was found within the Lower Cretaceous deposits of a more restricted environment. Depending on its sequence strati-graphic position at/above flooding surfaces, the packstone/grainstone related mechanical boundary occurs at the base of 50-80m thick cycles. A method to record both mechanical boundary types in the subsurface with a fracture impedance of 100% through the analysis of borehole image and other well logs is described. Subsequent upscaling of the generated fracture impedance logs, the interpolation of upscaled logs and the conversion of relative fracture impedance into vertical transmissibility multipli-er arrays is finally described.
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Fractured Crystalline Reservoir: a Field Analog Analysis on Tamariu’s Granite (Catalonian Coastal Ranges)
Authors L. Bertrand, E. Le Garzic, Y. Géraud and M. DiraisonTamariu’s granite has been studied with the aim to characterize the fracture network and the porosity through the analysis of cementation associated to paleo-fluids circulation. With the help of regional and outcrop studies (Le Garzic 2010, Place 2010), we could interest on the fracture network over 8 orders of magnitudes. We used 1D scan lines and 2D fracture mapping on outcrop pictures to precisely define the fracture network and the organization of the flow in this network. The data acquired on our maps show a power law distribution of the cumulative fractures numbers and faults lengths between the centimeter scale and the hundred kilometers scale. This homogenous length distribution over a large range of scales contrasts with the fracture orientation distribution that highly depends on the scale observation and on their location relative to the larger structures. The volume of paleo-fluids associated cementation has been quantified using calcimetry and mapping of different parts of the porous network: the veins, breccias cemented by carbonates, fractured granite and poorly fractured granite. The percentage of percolated rock volume reaches to 3% of the total granitic body, and the main part of them are localized in fractured corridors composed of mains veins and breccias.
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Fractures in Steep-Rimmed Carbonate Platforms: Comparison of Tengiz Reservoir, Kazakhstan, and Outcrops in Canning Basin, NW Australia
More LessNatural fractures bear significant influence on productivity in Tengiz field, which is one of several giant light-oil accumulations trapped in isolated carbonate platforms in the Pricaspian Basin of Kazakhstan. Outcrop analogs are particularly important for understanding reservoir fracture systems because many aspects of fracture character (e.g. height, length) are impossible to measure with subsurface data. The Devonian margin of the Canning Basin in NW Australia presents a well-exposed outcrop analog for steep margin and slope deposits of Tengiz field. Fracture data gathered from Tengiz core and image logs suggest affinity to fractures in the Canning outcrops in terms of origin, orientation, and range of density. Inclusion of additional information - gained through outcrop study - into reservoir fracture description leads to improved understanding of stratigraphic influence on their occurrence and character. Shallow-burial fractures – those formed in carbonate strata prior to significant burial, including neptunian fractures – are the most important for reservoir productivity at Tengiz field. These fractures dip steeply and strike dominantly parallel and/or normal to the local orientation of the depositional margin. They are most well-developed in brittle, boundstone-dominant facies of the outer-platform to upper slope environment. Dissolution by corrosive fluids following burial led to enlargement of fracture apertures, which range from small to cavernous. In Tengiz field, cavernous fractures pose both high lost-circulation risk, as well as the reward of highly productive wells. Outcrop data from the Canning Basin show fractures in the mid- to upper-slope facies and reef core are, on average, not limited by bedding and hence much taller than fractures in the reef flat and outer platform areas. Fracture size cumulative distributions are mainly exponential, and they differ between stratigraphic settings. We expect such size differences will have important effects on fracture connectivity and permeability in a reservoir. Fracture density, which is measured routinely in the Tengiz reservoir, was measured with long pseudowells (i.e. scanlines) “drilled” along Canning outcrops. Fracture density shows significantly less variation among facies than does fracture height. Thus, outcrop-based data can add substantially to our understanding of key fracture system characteristics that are unavailable from well data alone.
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A Geologically Consistent Permeability Model of Fractured Folded Carbonate Reservoirs: Lessons from Outcropping Analogue
Authors K. Bisdom, G. Bertotti, B.D.M. Gauthier and N.J. HardebolPresently adopted fracture-related permeability models of large folded reservoirs are simplistic and often unrelated to the geological setting and evolution of the considered structure. In order to improve predictions of fluid flow in more complex subsurface fractured reservoirs, we build a 3D fracture network model of an outcropping fold in Tunisia, and populate different structural domains with fracture data, collected from outcrops. Within the studied fold, we find large variations in deformation mechanisms between different formations, with the main mechanisms being Layer Parallel Shortening (LPS), resulting in regional deformation, and the more localized impact of fiber stresses and flexural slip. Within the steep flank of the anticline, we find that in one formation fracturing is mostly controlled by fiber stresses, whereas in the underlying formation flexural slip is the main deformation mechanism. These two formations are separated by a detachment surface. Using stress and strain fields, we aim at reconstructing the conditions at which these fractures have been formed. This can provide a better understanding of the relation between fracture patterns in different structural domains of a fold and the stress evolution that formed these fractures, and the subsequent impact of different fracture patterns on fluid flow in fractured folds.
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Keynote: Geomechanical Aspects of Fractured Reservoirs
By G.D. CouplesGeomechanical concepts and approaches are associated with many aspects of fractured reservoirs. It is convenient to consider two main contexts: those where the goal is to understand the formation of the trap and its fracture system, and those where the goal is to predict or explain production performance. The practical justification for efforts aimed at understanding the pre-production history of the reservoir is that some or most of the learnings from this are to be used to inform the production evaluation/prediction efforts. The geological evolution of the trap considers aspects such as the tectonic cause of the reservoir’s shape, and how deformation processes operated to create fracture networks. These considerations often lead to the recognition that fractures may terminate against other fractures, but often do so at bedding contacts, leading to a conclusion that the pre-deformation vertical arrangement of rock types and their properties is an important control, along with the deformation-caused curvature or distortion of the initially-planar rock succession. The use of fracture observations for predicting reservoir performance can take several forms: from their use to estimate spatial variations of a static equivalent permeability, to the creation of explicit fracture network models with the potential for production-related changes in fracture opening, to methods that aim to calculate fully-coupled interactions between the mechanical state and the porefluid changes. Natural fracture arrays, as seen in outcrop analogues, can range from simple patterns composed of one or two sets, to complex networks whose genesis is not simply explained. The common concepts used to explain fracture formation are based on stress states, but this conceptual framework leads to difficulties in explaining complex networks, which may show overlapping creation and movement on the discontinuities belonging to many fracture sets (Fig. 1). An alternative conceptual framework focuses on fracture arrays in terms of the strain states that they define, and relegates stress to being a local indicator of instantaneous load-carrying arrangements that evolve during fracture creation and trap growth. This framework, with its emphasis on strain accumulation by fractures, provides the understanding needed to inform reservoir prediction.
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Geomechanics and Geophysical Processes in Naturally Fractured Reservoirs
By N.R. BartonIn the case of domal reservoirs, production may cause down-dip shearing on conjugate dipping fractures. This mechanism was deduced a long time ago from rock mechanics modelling and from new slickensiding in the case of the Ekofisk reservoir in the North Sea (Barton et al., 1988). There may also be changes in the stretching over-burden in the case of the compacting reservoir, causing the over-burden to subside. This will cause temporal changes to the strength of shear-wave anisotropy and attenuation, due to intra-bed joint opening and shear. It is insufficient in each of the above cases to refer to ‘stress or strain’ effects, as if a continuum alone was reacting to the multiple effects of production in a multi-km3 fractured reservoir, with a multi-km3 overburden. The relatively ‘early’ consideration of discontinuous behaviour at Ekofisk is presented in this classic ‘carbonates region’ of the Middle East, in the hope of stimulating the modelling of fracture deformation, and coupled behaviour, which still seems to be rare, despite being needed for realistic 4D interpretation. The possibilities of making good use of geomechanics understanding has improved a lot since LOF monitoring/interrogation of reservoirs was slowly introduced in this last decade, starting in the North Sea.
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Role of Geomechanics on Natural Fractures in Different Reservoirs: A Middle Eastern Perspective
By S. PerumallaThe behavior of fractures during production makes fractured reservoirs unique in terms of flow behavior and reservoir’s response to in-situ stresses. While the behavior of other reservoirs is controlled by textural properties of matrix, the behavior of fractured reservoirs is driven by anisotropy depending upon the spatial distribution of fracture and their reaction to stress. While those natural fractures which are intersected by the well are detectable on image logs, fractures in the vicinity of wellbore also need to be incorporated while studying the hydraulic conductivity at field scale. Detection of such fractures is possible with application of certain deep shear sonic technology. Well-scale geomechanical models offer reasonable accuracy to determine the sensitivity of detected fractures in respect to stress to become critically stressed and therefore possibly hydraulically conductive. However, the translation of such well-scale analyses to field-scale requires improvement in methodology. Appropriate up-scaling of well-based fracture interpretation to field-scale is already in practice in the form of DFN modeling based on the integration of field-scale data and well-based data. However, the effective integration of these models can offer solutions to identify possible stress sensitive fracture dominated sweetspots for improved well planning reservoir development. This presentation covers case studies.
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Natural Fracture Interaction with Hydraulic Fractures in the Athel Formation, South Oman
Authors G.J. Stone, M. Healey, M. Busafi, A. Jahdhami, O. Quintero Sarmiento, L. Bazalgette, A. Santagati and S. ShualiThis paper focuses on the challenges in understanding the interaction between hydraulic fractures and natural fractures and the impact on the development of the unique, ultra-tight Athel silicilyte reservoir formation in a South Oman oil field. Significant in place volumes remain in Field A (recovery factor <4%) and substantial investment is planned to further develop these volumes both by flank drilling and crestal Miscible Gas Injection. To maximize return on future investments, further understanding of the interaction between the hydraulic fracturing, the rock matrix, and the natural fracture network is seen as key to optimizing development well spacing, well patterns and hydraulic fracture design and placement. Integrated evaluation of the existing data set is being performed using Petrel and SVS software,to visualise key data, and generate conceptual models of natural fracture networks and hydraulic fracture character. These are transferred to effective properties in the dynamic simulator to enable simulation of well productivity, reservoir drainage and in turn development optimization. In-well micro-seismic data is a critical part of this dataset, and will be presented as an invaluable calibration of the hydraulic fracture models.
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Applications of a Workflow to Establish the Viability of Shear Stimulation of Fractured Reservoirs
By D. MoosDue to the success of slick water stimulation in shale plays such as the Barnett, this approach is increasingly being proposed for use in other plays. However, there has been little discussion of how to determine its appropriateness. We present here four cases in which such a workflow has been applied – these include a fractured basement tight light oil play, a fractured basement geothermal field, a mud rock gas play, and a tight gas sand. Shear stimulation was determined to be appropriate in two of these cases. In a third, insufficient benefit accrued. The fourth case demonstrates the importance of the work flow; had data been obtained prior to stimulation an alternative stimulation method might have been proposed. Required data include natural fracture orientations and distribution, and stress orientations and magnitudes. A pre-stimulation test is essential to determine fracture strengths, flow properties, and pre- and post-stimulation connectivity. Microseismic monitoring provides significant benefits in evaluating the results of these tests. Where shear stimulation is revealed not to provide sufficient enhancement of reservoir permeability to make a prospect viable, the choices are either to use a different stimulation method or to search for a more appropriate development location.
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Innovative Natural Fracture Prediction using a Geomechanically-Based Solution: Application to the Malay Basin (Malaysia)
Authors J.P. Joonnekindt, X. Legrand, B.C. Lee, M. Lefranc, L. Maerten and L. AnisIn the framework of field development in Malay Basin, a geomechanically-based fracture modeling was carried out for a structural trapping. Detailed seismic interpretation, 1D Mechanical Earth Models computed from well data and 3D structural restorations were integrated in the workflow to reduce uncertainties in the Natural Fracture Prediction Model. The model reveals evidence of two main tectonic stress regime through time (σH N80 and N170) which led to a positive tectonic inversion. As a consequence, the understanding of the reservoir characterization was greatly improved and helped to better evaluate the plan of development.
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Hybrid Discrete Fracture Network Simulation Driven by Statistics, Tectonic History and Geomechanics
Authors F. Bonneau, G. Caumon, P. Renard and J. SausseStochastic approaches are often used to simulate discrete fracture networks that are consistent with statistics obtained from field observations. However, classical stochastic methods do not account for fracture interactions to draw the geometry and the position of fractures. In this work, the tectonic history is considered. The fractures related to the oldest tectonic events are simulated first and others are simulated by order until the youngest tectonic events is considered. We use geomechanical consideration to define both a repulsion zone (constraint release zone) and an attraction zone (constraint accumulation zone) around each fracture. The implantation and the growth of each fracture are optimized considering the effect of neighboring fracture already simulated. Simulated discrete fracture network may also be constrained by secondary data such as micro seismic events or connectivity data.
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Keynote Speaker: Fractured Reservoirs Modeling with Pressure Transient Test Interpretation
Authors F.J. Kuchuk, D. Biryukov, K. Morton, T. Fitzpatrick and R. BoothNaturally fractured reservoirs consist of fractures in igneous, metamorphic, and sedimentary rocks (matrix). Faults in many naturally fractured carbonate reservoirs often have high-permeability zones and are connected to numerous fractures with varying conductivities. In many naturally fractured reservoirs, faults and fractures can be discrete (rather than a connected network dual-porosity system). To accurately model pressure transient behavior of fractured reservoirs is important to gas and oil reservoirs. There are six important factors that dominate the pressure transient test flow regime behavior of fractured reservoirs those are fractures: 1) intersect the wellbore parallel to its axis; 2) intersect the wellbore with dipping angles from 0 to less than 90º; 3) are in the vicinity of the wellbore; 4) have extremely high- or low-fracture and fault conductivities; 5) have various lengths and distributions; and 6) have high and low matrix block permeabilities. All flow regimes associated with these factors will be demonstrated for continuously and discretely fractured reservoirs by using semi-analytical solutions. We employ principal component analysis to explore the model parameterization as pre-screening steps. Global sensitivity analysis methodology is employed to determine how the uncertainty of each parameter influences the uncertainty in the output from the reservoir model.
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Full Waveform Model Validation of Microseismic Shear-wave Splitting Fracture Parameter Inversion
Authors B. Yousef, D.A. Angus, M.W. Hildyard, J.P. Verdon and M. PerryAlthough the relationship between reservoir formation permeability and fractures is complex, it is recognized that fractures play an important role in reservoir fluid flow. Recent research has been using the integrated geomechanics and seismic modelling to characterize tight-gas reservoirs, which requires knowledge of the joint or fracture compliances both on the geo-mechanic and seismic scale. However, populating the geomechanical and/or seismic model with joint and fracture properties is based primarily on laboratory core data, which are on many times smaller length scales than observed in fractured reservoirs. As such, it would be ideal to measure and calibrate fracture compliance from field-scale measurements. The aim of this project is to explore whether observations of seismic anisotropy from P- and S-waves can constrain fracture compliance (normal and shear). In this study, we investigate the feasibility of using microseismic data to invert for fracture density, fracture strike and fracture compliance ratio from shear splitting results.
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Subtle Structure Detection Using Spectral Decomposition for Fracture Mapping
Authors B.E. Toelle, R. Nolen-Hoeksema and M. StellasThis paper describes the use of the iso-frequency volumes resulting from the spectral decomposition of surface seismic data for the identification of subtle structural features (shear faults) that control the formation, location, and distribution of open fracture trends within the Steckman Ridge Underground Gas Storage field in south-central Pennsylvania, USA. This gas field, originally developed with five vertical boreholes, was purchased by Spectra Energy for conversion to underground gas storage. After an initial examination it was determined that this field was controlled by an open fracture system which trended approximately 70° from the dominant structural trend. Various seismic attributes were investigated in an attempt to determine the fracture controlling structures. Subtle shear faults, not observed in the full frequency volume, were located on specific iso-frequency volumes and mapped. These structural features were found to be related to major cross strike discontinuities which could be observed on other geophysical data. These features were confirmed by the subsequent drilling of eight horizontal boreholes and the formation imaging logs which were tractored through these boreholes. The resulting injection/withdrawal rates for these wells have made Steckman Ridge gas storage field one of the most successful gas storage conversion Spectra Energy has performed to date.
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Characterization of Fracture Shearing for 4D Interpretation of Fractured Reservoirs
By N.R. BartonFractured reservoirs and their successful production my need to involve fracture shearing. This important mechanism may result in slight dilation of the fractures, and therefore substantial maintenance of conducting aperture, despite effective stress increase. Unless fractures are sealed with hard minerals, and channelized flow is occurring, closure would be likely with the standard geophysics model of one set of stress-parallel fractures. The minimum principal stress would close the fractures unless they were very rough and in hard rock, such as limestone. These scenarios suggest the need for fracture characterization, with a view to geomechanical coupled modeling, so that 4D reservoir monitoring results can be interpreted better than with continuum ‘stress and strain’ arguments, which have little relation to the detailed reality of continued fracture flow during production of petroleum.
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How Use Electrical Image Log for Fractured Basement Reservoir Characterization. Example of the Kharir Field (Yemen)
Authors E. Le Garzic, Y. Géraud, M. Diraison, P. Bourges, B. Gauthier and P. Le NenNo standard workflow, nor rules or common practices exist in the industry to describe reservoir properties of fractured basement. The purpose of this work, focused on a highly documented field case in Yemen (Kharir, block 10), is to develop a wide integrated analysis of structural data from well scale to reservoir scale to extract any possible driver which could be relevant for reservoir modeling. A new concept of interpretation based on field observations was developed for analyzing and upscalling wellbore electrical image log. This method aims to identify and characterize the first order structural domains: fault zones (thickness, main orientation, internal architecture…) and intercalated structural blocks (degree of fracturing, main fracture trends, dyke occurrence…). Particular attention has been given to fault ranking. When seismic and image log interpretations are compared, a good correlation is observed in terms of fault zones orientations, fault zones ranking (i.e. thickness versus throw), spatial distribution of deformation and structural inheritance. This approach permits to describe a first order structural network which can be considered as the reservoir backbone. In addition, detailed analyses on image log provide relevant input for reservoir modeling.
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Direct Imaging of Natural Fracture Flow Networks at the Reservoir Scale
Authors A. Lacazette, J. Vermilye, C. Sicking and P. GeiserUntil recently direct imaging of reservoir scale fracture-flow networks has been impossible. High-resolution reflection seismic data provides images of larger fractures, but does not show which fractures are likely to be hydraulically transmissive. To date, reservoir-scale fracture networks have been characterized primarily by indirect methods such as: stochastic and/or deterministic mechanical modeling to upscale borehole data or downscale reflection seismic data; attributes derived from seismic reflection data; and well or reservoir tests that measure bulk flow properties. This presentation reports on a new passive seismic imaging method (Tomographic Fracture Imaging™) that directly images reservoir-scale fracture networks either during hydraulic fracture treatments, during production or injection, or using only ambient seismic emissions. Ambient imaging can be performed during the course of standard 3D reflection surveys by recording continuously during quiet periods, for example at night when there is no activity on the grid. The presentation will focus on well-documented case-studies where independent validation of the results is available. Case studies presented will include examples from hydraulic fracture treatments, ambient surveys, and a water flood. The results are validated with independent data including chemical and radioactive tracer studies, pressure monitoring, production logs, reflection seismic data, and well results.
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Characterization and Simulation of a Complex Sub Salt Fractured Carbonate Field Offshore Brazil
Authors A. Ouenes, F. Laferrierre, R.O. Mota and A. BernerThe considered oilfield was recently discovered as part of Petrobras effort in developing subsalt assets. As with many carbonate fractured reservoirs it has proved difficult to describe and model the fracture distribution and their consequence on the reservoir effective permeability especially in its current exploration and delineation phase where the wells are limited to three or four at most. This integrated study used geophysical, geologic, and engineering data simultaneously to improve the reservoir description. The characterization of the fractures was mostly due to the extensive use of narrow azimuth 3D post and pre-stack seismic data. The derived 3D seismic attributes and 3D geologic and fracture models were used to better understand the factors affecting the distribution of the key rock properties including the fractures and their effect on the permeability. The resulting porosity, water saturation and permeability models were input into a reservoir simulator and allowed the reasonable match of the individual well performances without the need for any extensive history matching thus validating the input reservoir models. This achievement was made possible by the use of key high resolution post and pre-stack seismic attributes as drivers in the CFM technology that provided the resulting rock properties.
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Keynote Speaker - Fracture Characterization and Modeling Over the Last Few Decades, Current Progress and Future Outlook
More LessThe first part of this talk focuses on a short review on the major efforts made in the past few decades to understand fractures and fractured formations both in the industry (oil and gas, hydrogeology, nuclear waste management) and academic community. The second part of the talk focuses on a discussion of typical workflow for detection, characterization, parameterization, prediction, modeling, validation and verification of fractures from limited direct subsurface data, and how static and dynamic data can be integrated to get a more complete picture of the reservoir. The final part of the talk points out some general future research directions.
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Recent Advances in 3D and 4D Fracture Characterisation and Modeling of Fractured Reservoirs
Authors P.D. Richard, L. Bazalgette, K. Rawnsley and P. SwabyA significant proportion of the world future hydrocarbon production (conventional and unconventional) resides in fractured reservoirs. The success of the recovery methods relies on a good understanding and appropriate modeling of the fracture network. In order to support field development strategies, the Specialist Reservoir Modelling group carried out numerous fracture characterization and modelling studies over the past decade. In Shell E&P, one key enabler for these studies is a unique integrated software technology, SVS Fracture Solutions (Rawnsley et al, 2004) developed by the Carbonate Expertise and Research Teams, thanks to a collaboration of experts from all subsurface disciplines. The SVS workflow is iterative (Figure 1) and focuses on the understanding that can be extracted from the thorough analysis of all available static and dynamic data, in combination with the knowledge from numerous analogues. This paper illustrates some of the recent advances made in detailed fracture characterisation using examples of case studies around the globe. Particular emphasis will be made on demonstrating the impact of integrating core and Bore Hole Image (BHI) data to help constraining the structural understanding of the fields and build subsurface realizations.
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3D Deterministic Multi-Level Fracture and Matrix Flow Model of a Layered Platform Carbonate Reservoir Analog (Morocco)
Authors C. Milliotte, S.K. Matthai, S. Agar and G.S. BensonThe cross-disciplinary, multi-tool modelling and simulation approach we pursue on the Amellago Canyon platform carbonate outcrop (High Atlas, Morocco) aims at a rigorous characterization of stratigraphy, petrology, structure and flow, trying to capture a maximum of geological features of layered carbonate NFRs so that DFM simulation can reveal what level of detail has to be resolved for predictive simulation of reservoir behaviour / determination of equivalent properties by means of flow-based upscaling. This question of model convergence towards a sufficient level of realism is addressed by comparison of results obtained from a hierarchy of thin vertical slab models: Model (1) a coarse model with faults and fracture corridors only; Model (2) an intermediate model adding multi-layer fractures including the WE striking set with stylolitic overprint, and Model (3) a fine model also including different sets of bed-confined fractures. These DFM models are unique because they are largely deterministic: fractures are interpreted from the LiDAR survey data, including texture-mapped high-resolution photographic images. Bed confined fractures seem to contribute significantly to the equivalent permeability only when they intersect more continuous fractures, like in the fracture corridors or multi-layer fractures spanning low permeability layers.
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Simulation of Gas Oil Gravity Drainage: Comparison of the Dual Continuum with the Discrete-Fracture and Matrix Approach
Authors S. Matthai and S. BazrafkanGas-oil gravity drainage (GOGD) is perhaps the most effective recovery mechanism for those naturally fractured reservoirs that are lithologically stratified, the oil is stored in rock matrix, and vertical fractures cut through (multiple) low-permeability layers, providing the necessary permeability in the direction of gravity-driven flow (eg., Gilman 2003, 2011). The conceptual process model for GOGD is that fracture saturation with gas that is much less dens than the oil creates a pressure differential that drains the oil into the fractures, where it flows downward and can be produced from wells placed directly above the oil water contact. Prerequisites are that saturated conditions can be obtained, the fractures are sufficiently tall, and the oil is mobile enough (e.g., Kazemi et al., 1976). Since Gilman and Kazemi (1983) and Saidi (1983) GOGD has predominantly been simulated using the oil industry’s adaptation of the dual continuum (DC) approach (e.g., Warren and Root, 1963),using separate fracture and matrix grids coupled via source terms for fracture-matrix transfer (FMT),see Abushaika and Gosselin (2009). However, it is challenging to derive DC input parameters such as matrix-block size, shape factor or fracture permeability from field data because they tend to obey power-law or multi-modal distributions (e.g., Bonnet et al., 2001). Recently, new numerical fracturematrix upscaling methods have been proposed to overcome this DC model parameterization problem (Sarda et al., 1997, Karimi-Fard et al., 2006), however, they only address aspects of pseudo steadystate FMT and entail other restrictive assumptions. In many cases, history matching of DC models is only possible through over-calibration, raising doubts about the predictive capabilities of such models (e.g., Gilman, 2003). Since fractures have no discrete representations in DC models, these cannot be used to test long-standing hypotheses about the details of FMT (cross flow, capillary continuity, reimbibition of oil, see Fung, 1991; Saidi, 1993), or evaluate the balance between gravitational – viscous and capillary forces and the mutual influence of respective flow processes within the reservoir. Assumptions about how saturation changes in matrix blocks and the pathways that the oil takes on its way to the production wells cannot be tested. Discrete-Fracture and Matrix models (DFM) provide an alternative for such purposes (e.g., Kim and Deo 2000). Using a spatially adaptive poly-element type unstructured grid, complex fracture geometries can be represented in the model explicitly, permitting the modelling of realistic fracture geometries and property statistics (e.g., Matthai et al., 2007). In 2011, Bazrafkan and Matthai,presented a new hybrid FEM node-centered FVM methods where finite elements also serve as control volumes, permitting the correct representation of saturation jump discontinuities at material interfaces and also the realistic simulation of capillary FMT. Here, we apply this novel DFM approach to simulate GOGD using an outcrop analog model that has a fracture geometry that is simple and regular enough to construct an equivalent dual permeability model. The latter is run using a commercial simulator and here we present, for the first time, a comparison of DFM and DCM results, using the same saturation functions and parameters. The DFM results further allow an investigation of the flow patterns in individual fractures and matrix blocks so that the validity of the assumptions that underpin the DC approach can be evaluated.
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Pre-Caspian Basin Carbonate Reservoir - Modelling Fractures of the Platform Interior
Authors A. Francesconi, V. Tarantini, C. Albertini, R. Marino, L. Bado and A. CorraoLarge Carboniferous Fields in the PreCapisn Basin, like Aktote, Karachaganak and Kashagan, consist of carbonate build-up characterized by the presence of biohermal and breccias facies that result to be affected by high productivity sometime largely exceeding the permeability estimated from core samples. This fact has been generally related to the presence of both fractures and, in some cases, karst. These facies generally surround platform interior environment of deposition which is characterized by grainy facies which shows lower productivity, more close to that inferable from the core samples permeability. For this reason the platform interior environment of deposition is considered more homogeneous and targeted as the best reservoir where implementing the gas injection. However some clues suggest that also this environment of deposition could be affected, locally, by the presence of enhanced permeability related to fractures. The target of this paper is the description of the general workflow tested in some studies in order to model the presence of this fracture network and investigate the possible impact on the gas injection.
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Using Embedded Discrete Fracture Models (EDFMs) to Simulate Realistic Fluid Flow Problems
Authors P. Panfili, A. Cominelli and A. ScottiA large amount of world’s hydrocarbon reserves lies in reservoirs where one of the key features is the presence of a system of fractures spanning different length scales. Fractures may provide ways to drain the matrix, but they also drive gas and water towards wells. Fractures intensity, geometry and conductivity are characterised in conjunction with matrix properties using all possible data, such as production logging, mud-losses, image log, well test, geophysics and outcrops. This may lead to a geometrical characterisation in terms of a discrete fracture network (DFN), with the definition of a set of geometrical 2D objects embedded in the matrix domain. A DFN is a computational challenge for conventional dual-porosity simulators, where a dual medium formulation is cast in a corner point geometry grid (CPG). In this context small, with respect to CPG grid spacing, fractures can be easily incorporated using some practical recipes, see Dershowitz et al. (2000), in single porosity or in dual-porosity models. On the other hand, long range, interconnected fractures cannot be integrated in the simulation model unless some crude approximation is implemented in order to fit the fracture network geometry into the Warren and Root (1963) dual media framework, at least as it is available in conventional commercial simulators, see e.g, Eclipse (2011) . A more accurate solution is the implementation of unstructured gridding where the matrix is discretized by tetrahedral or more general polyhedra and the fractures are discretized using 2D polygons. These grids can be used with finite volume, connectivity based reservoir simulators (see Karimi-fard et al. (2004)), but the approach is computationally inefficient for most of the commercial simulators, and this motivated the development of the EDFM by Li and Lee (2008). In this approach the intersections between fractures and matrix blocks define the degrees of freedom (DOFs) for the high connectivity medium. Then, 2D flow between fracture DOFs and 1D flow between fractures and matrix can be integrated with 3D flow in the CPG grid representing the matrix. In a nut-shell, an unstructured grid for the fracture network is comined with a structured grid for the matrix. Differently from Li and Lee (2008), our implementation is not based on the customisation of the reservoir simulator. Rather, we exploit the capability of most commercial simulators to define non neighbouring connections across grid cells to implement EDFM in a non-invasive manner. Our results confirm that EDFM can be as effective as fully unstructured gridding but much more computationally efficient.
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Discrete Fracture Network Upscaling Workflows and Tools in SVS Fracture Solutions
Authors C. Tueckmantel, S. Lamine, B. Huisman and P. SwabyFor dynamic simulations on a fractured reservoir it is often necessary to translate a discrete fracture network (DFN) model into a grid-based set of effective properties. During this upscaling step, the critical characteristics of the original fracture network must be preserved to ensure that the previously performed fracture characterisation and modelling work is not compromised. The ultimate goal is to represent geologically sound fracture scenarios in simulation models so that the impact on fluid flow, well rates and development decisions can be tested. Different methodologies have been proposed and were incorporated into commercial fracture modelling packages to perform this translation. However, each method comes with its own set of advantages and disadvantages and none reliably preserves the connectivity of a given fracture network on the scale of a full field model. We will present tools developed in the Shell in-house fracture modelling software SVS Fracture Solutions to upscale a DFN and to quality check the resulting effective property grid. We will outline a workflow that ensures the preservation of the most important characteristic of a DFN, i.e., its connectivity, and which reliably works on full field models.
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The Relative Importance of Fracture Parameters During Fractured Reservoir Development
Authors S. Price Projects and Technology and L. WeiWhen developing a fractured reservoir, it is beneficial to learn from analogue fields about the fracture parameters which significantly impact fluid flow and hydrocarbon recovery under various recovery schemes. The lack of an industry standard workflow for the modelling of naturally fractured hydrocarbon reservoirs and the numerous ways in which naturally fractured rock volumes can be parameterized for input to modelling precludes a robust comparison of results from published studies. This paper presents a retrospective analysis of the results of a number of fracture modelling studies sharing a common workflow for parameterization, modelling and uncertainty handling. The objective is simple: to better understand which parameters of a natural fracture system are of key importance in estimating recovery for different development options and how parameter sensitivity varies over asset lifecycle.
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Fracture Modelling & Reservoir Management of the First Full Field TAGOGD Project with Implications for Future Developments
Authors K. Rawnsley, M. Gharbi, M. Breiki, S. Bettembourg, P. Putra, A. Sheheimi, H. Maamari, F. Yahyaai, I. Ismaili and T. Ma‘maryIn Oman the world’s first full field Thermally Assisted Gas Oil Gravity Drainage project in fractured carbonate heavy oil field has been producing for 2 years. Unlike a normal steam flood, steam is used as a heating agent to enhance the existing gas oil gravity drainage mechanisms [1]. The project has started up successfully.
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Fractured Basement – DFN Modelling in an Integrated Geologically Consistent History Matching Approach
Authors I. Farea, Y. Yanze Ntchao, P. Neff and I. ZabalzaThe geomodel of a fractured basement reservoir model is realized in order to integrate all available data, particularly 3D seismic cubes and attributes, and flowmeter logs. Considering the geological context of this field, a particular attention has been given to capture its flow behaviour. To do so, it has been necessary to use a dual media approach, thus to characterize and model the matrix and the fracture network. We intend to describe the implemented approach. Starting with the description of the generated Discrete Fracture Network DFN calibrated towards dynamic data which allows reducing the uncertainty on key production characteristics of the field and production forecasts for the field development. Focus will be set on the synergies arising while combining leading edge fracture modelling techniques and uncertainties management through history matching.
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Engineered Approach to Isolate Intervals Invaded by Water or Gas in Naturally Fractured Carbonate Formations
Authors G. Rivas, J. López, A. Martínez Ballesteros, R. Miranda, C. García, R. Zepeda, B. Vidick, R. Girón, A. Millán, A. López and E. MiquilenaAn excessive water cut or high gas/oil ratio in a production interval presents a major concern in sustaining oil production, requiring fast and efficient workover solutions to enhance the oil recovery process. Wells in the Cantarell field, a mature depleted field in the Bay of Campeche in the Gulf of Mexico, are facing drastic decreases in their production and an increase in either water cut or gas/oil ratio. Other developed fields in Mexico’s Region Marina, such as the Ku-Maloob-Zaap, have increased their hydrocarbon production through the years with an incipient presence of water and gas. The high water cut and gas increments have had a strong impact on the production strategy, opening the opportunity for application of non-conventional solutions to isolate or abandon production intervals invaded by gas or water and continue production from upper or deeper zones. The pay zones consist of naturally fractured, vugular carbonates with permeability as high as 5 Darcies that involves the following challenges to be overcome in order to succesfully achieve the required isolation: Loss of fluid circulation, high gas presence and lack of primary zonal isolation that results in migration of water or gas to the productive intervals.
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An Oman Carbonate Mixed Fractured/Matrix Oil and Gas Field: Water Flood or GOGD Decision Case Study.
Authors A.B.S. Clark, N. Al Wahabi, R. Kawar, H. Hillgartner, K. Subhi, M.B. Ferrero, R. Spiteri and H.S. MohammedA case study of a stacked, multi reservoir carbonate oil field in Central Onshore Oman is presented here to highlight the need for ongoing appraisal and integrated field development studies even after 25 years of production. Subsurface parameters such as abundance of high-k layers, and connectivity of faults, fault related fractures, distributed back ground fractures, and layer bound fractures are the key geological uncertainties that impact the selection of the field development options. Data integration is the key to a better understanding of such a complex fractured field. The proper and timely integration of reprocessed seismic (fault imaging, attribute interpretation), interference testing (cooperation with asset and field operations team) and iterations with dynamic reservoir simulation resulted in further improved history matches and reducing uncertainties, which enabled taking a development concept decision. This presentation is demonstrating the value of applying a fully integrated decision-driven static and experimental design dynamic modeling workflow to progress the ongoing FDP study.
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Detection and Modeling of Natural Fractures: Real Life Detection Examples and Workflows for Implementing Fractures in Simulation Models
Authors S. Lyngra, N. Najjar and C. Tsingasr of separate quantified data sets having a great variety of scales, ranging from core samples to seismic scale, are required to calibrate all the accessible information. The main challenge is the need to integrate all the available data sets, static and dynamic, in a consistent way, to allow construction of appropriate detailed geologic models and upscaled simulation models, with sufficient accuracy to allow history matching of all the available field dynamic data. The history matched simulation models are utilized to generate prediction scenarios of future oil and water production. Many hydrocarbon bearing reservoirs are faulted, and often little information is available about the actual physical characteristics of the faults. Some faults are known to be sealing and others are non-sealing to migration of hydrocarbons. While sealing faults block fluid and pressure communication with other regions of a reservoir, infinite-conductivity faults act as pressure support sources and allow fluid transfer across and along the fault planes. Finite-conductivity fault falls between these two limiting cases of sealing and totally non-sealing faults, and are believed to include the majority of faulted systems in many reservoirs of several prolific fields. Understanding of fluid flow in fractured reservoirs is important since a large percentage of the world’s oil reserves are situated in geological formations dominated by natural fractures. A quantitative description of fractures and their distribution is essential for the development and reservoir management of a reservoir. Preferential occurrence of the fractures is systematically dictated by key controlling geologic factors. The giant field described in this study is located at in the Eastern Province of the Kingdom of Saudi Arabia. The field production is primarily from two Jurassic-age fractured carbonate reservoirs, the Upper reservoir and the Lower reservoir, which are separated by a 500 ft (150m) thick, non-reservoir limestone formation (Fig. 1). The field has been in production since 1946. The Upper reservoir is prolific with excellent reservoir properties throughout the whole field. The Lower reservoir has low matrix permeability (1-2 mD) and is only hydrocarbon bearing in a high relief structural dome located toward the southern part of the field. The Lower reservoir well productivity is controlled predominantly by near wellbore fractures. Extensive stimulation treatments are required after drilling or workover operations to attain satisfactory well performance from Lower reservoir oil producers and water injectors. Vertical communication between the two reservoirs is evident from reservoir pressure data, and is believed to be caused by faults and extensive fracture corridors (fairways) that cut through the non-reservoir formation (Fig. 1).
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Multiscale Fracture Patterns in Flat-Top Carbonate Platforms: the Mt. Latemar Analog (Dolomites, Italy)
Authors G. Bertotti, N.J. Hardebol and H. BoroMultiscale fracture networks frequently affect carbonate reservoirs and play a crucial role in controlling permeability. Network geometries are here derived from the Latemar platform outcropping analog (Dolomites, N Italy) integrating satellite images, LiDAR airborne enriched Digital Elevation Models and outcrops. Our observations cover lengths from kms to cm. Two sets of fractures are recognised in the Latemar, both formed by mode I features and displaying traces insensitive to facies boundaries. The older set is visible in all length classes. Fractures heights and spacing change from one platform domain to the other. Fractures of the second set are generally confined between those of the first set and are, therefore, <50-80m long. Their heights and spacing distances are larger than those of the first, older fracture set, as a result of increased lithification between the two episodes of deformation. The existence of two fracture sets with significantly different geometries impacts platform-scale permeability patterns.
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Insight of Syn-Tectonic Crystalline Reservoirs. Effect of Inherited Structures on Fracture Networks (e.g. Elba Island)
Authors B. Walter, Y. Géraud, M. Diraison, L. Bertrand, E. Le Garzic, E. Oliot and G. PerryBecause of the potential reservoir that may represent fractured basement rocks, notably the igneous rocks, knowledge about the development of brittle structures and fracturation networks in such rocks must be enhanced. Formation of igneous intrusive complexes is often associated to large-scale tectonic structures, which may develop structural inheritance within the rocks, this latter controlling strongly the geometry of the brittle structures. This study aims to characterize in an extensional setting the complex relationship between major structures, fracturation density and matrix porosity, taking into account the structural inheritance of the studied complexes. The two Late-Miocene intrusive plutons of Elba Island (Italy) are each a syn-tectonic granitic pluton set up in the footwall of a low-angle normal fault, and are good examples of field analogue of a basement reservoir linked to a major tectonic structure. The main observations are that the potential fractured reservoir is located mainly on the edge of the pluton, and its geometry is controlled by the dynamic of flow of the magmatic supplies and by the induced structures: shear planes due to the contact effect of the host rock on the granite during its emplacement and in the roof of the pluton, structures associated to the detachment fault.
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Fractures and Diagenesis, their Occurrence and Impact on Paleocene Dolomite Reservoir
Authors A. Najem and H. El Werfalliin Carbonate reservoirs. These two natural phenomena can be acting as positive reservoir contributors, in terms of reservoir efficiency and production, and also can be acting as negative contributors. In the last years, the carbonate reservoir remains the most challengeable reservoirs in exploration and developments where their reserves estimation and production amounts remains undefined precisely. This is directly related to the nature of the Carbonate reservoirs depositional processes formation syn-depositional and post-depositional. Carbonate reservoirs account for 40% of today’s hydrocarbon production, and because of several giant fields in the Middle East they are expected to dominate production through the next century. In Libya, carbonate reservoirs are mainly concentrated in Sirte Basin and offshore where the first exploration in 50’s where in reef carbonate reservoir. Therefore, understanding carbonate reservoirs and producing them efficiently have become industry priorities and are likely to remain so.
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The Experimental Study of Hydraulic Fracturing Propagation in Multi-Layer Formations
Authors M. Behnia, K. Goshtasbi, G. Zhang, A. Golshani and M. FatehiThe study deals with the interaction between hydraulic fractures with the layer interfaces, fracture propagation pattern, and termination in multi-layered media. A true tri-axial cell was utilized to conduct experimental tests on cubic multi-layered samples with discontinuities. The tests were also aimed to investigate propagation of fractures from soft to stiff, stiff to soft media through changing the elastic properties of rocks. Results showed the condition of discontinuities (bonded, unbonded, and filled) and elastic properties of layers influence the geometry and propagation pattern of hydraulic fractures. In the block with bonded interfaces, the fracture penetrates in the adjacent layers. However, in the block with unbonded interfaces the fracture propagates toward the interface, then after the injected fluid fills the interface, a new fracture will propagate in the adjacent blocks. In the sample with infilling material, the fracture propagation is arrested, subsequent to which the fluid begins to seep through the interface. In addition, the results reveal that immediately after the fracture reaches the interface, the pressure increases, and more pressure is needed for fracture to propagate across the interface. The study displays that the fracture width and its penetration length in soft layers are greater than those in stiff layers.
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Predicting Multiscale Fracture Patterns in Buried Reservoirs: the Importance of Outcrop Data in a Coherent Workflow
Authors G. Bertotti, N.J. Hardebol and K. BisdomFracture data from outcropping analogs are often acquired but rarely used to improve permeability predictions in buried reservoirs. To improve this we present a systematic workflow which includes i) the building of a geometric model of the investigated structure, ii) the execution of mechanical simulations to determine state of stress and strain field, iii) the use of outcrop data to populate the model and iv) develop a reservoir-scale permeability model To make full use of outcrop data, we present new acquisition and processing tools allowing able to provide a full characterization of the fracture field even when fractures are not bed-confined. A few case studies are presented to discuss this workflow in sedimentologically and structurally heterogeneous settings.
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Can Mechanical Balancing Explain Untypical Fractures When Geometrical Balancing Fails?
Authors J. Lamarche, B. Gauthier, Y. Leroy, P. Souloumiac and A. LavenuThe motivation of the study is to demonstrate that geometrical constructions of fault‐related folds can be mechanically balanced according to the theory of limit analysis assuming that materials are cohesive and frictional. The advantage of the proposed methodology is to remain simple in terms of geometry and to introduce explicitly the various discontinuities such as the ramps which compose the geological structure. The main outcome of such mechanically balanced cross section construction is the availability of the stress field history which is fundamental to estimate the fracture pattern. These fracture predictions could be compared and benchmarked with data from outcrops. The proposed methodology is presented with the help of the field example of the Nerthe duplex, South‐Est France, with two horses, each well described with the classical fault‐bend fold geometry. The results highlight the role of erosion in the tectonic history of the folds and try to answer why early, inherited, fracture networks have not been reactivated during folding
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Enhanced Dynamic Simulation Through Continuous Fracture Modelling of Carbonate Reservoir, Oman
Authors S. Al-Azri, M. Al Raisi, A. Al-Harthy, I. Al-Simani and I. Al-RashediThe studied block is the largest and most faulted within the carbonate field which is currently under water-flood FDP. The study kicked off with extensive borehole image interpretation review of existing and new fracture picks. In parallel, several high resolution seismic inversions and spectral imaging attributes were generated as drivers to geological and fracture modelling. High resolution seismic was used to highlight subtle faults. Facies changes were also visible from seismic as seen in cored wells. Sequential geological modelling of GR, density, porosity and SW was constrained by seismic attributes. The derived fracture frequency logs were compared against geological, structural and seismic drivers in a process called driver ranking. The results confirmed the role of faults as well as facies being primary causes of fracturing. Subsequently, the screened and cross-correlated potential drivers were carried forward to constrain the fracture models. Multiple stochastic realizations were derived through NNT training and testing and an average model was kept. Final models were validated against hidden BHI data. New BHI was used to confirm model prediction. Different types of dynamic data in non-BHI wells were also used to validate the fracture models as specific production/injection related issues could be directly linked to presence of fractures. These data include PLT, PTA and tracer tests from which injectivity issues and short circuiting were explained by higher fracture densities and corridors. Through dynamic calibration, the fracture model was converted to fracture permeability. The fracture permeability is the product of fracture density and a scaling factor derived from history matching. Subsequently, the addition of matrix permeability and fracture permeability will determine the effective permeability. This Keffective was directly used in Daleel’s simulator without upscaling since it was part of the same grid hosting the fracture models. The results were encouraging as the simulation was smooth and error-free.
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