ISSN 0263-5046 FIRST BREAK May 2008 Volume 26 4D Seismic Special Topic Technical Articles Building velocity models for depth imaging: a North Sea case study Passive seismic and surface deformation monitoring of steam injection Technology Features Deep water pre-processing: East Coast India Mapping fracture corridors in naturally fractured reservoirs of the Middle East
first break volume 26, May 2008 technology feature Mapping fracture corridors in naturally fractured reservoirs: an example from Middle East carbonates Sunil K. Singh, Hanan Abu-Habbiel and Badruzzaman Khan (Kuwait Oil Company) and Mahmood Akbar, Arnaud Etchecopar and Bernard Montaron (Schlumberger) point to the importance of mapping fracture corridors in optimizing reservoir production and show how high resolution seismic can contribute in Middle East carbonate environments. In many oil and gas reservoirs, natural fracture networks help drain hydrocarbons and other fluids. The role of fractures is particularly important in reservoirs having a tight matrix. In carbonate formations, for example, it is quite common to observe a permeability contrast of 1000 or more between the rock matrix and surrounding fractures. Natural fractures are a recording of the reservoir stress history. They tend to be organized in different families oriented according to particular directions. Fracture corridors (FCs) are an extraordinary cluster of a huge number of quasi-parallel fractures. FCs vary in Figure 1 Sabriyah Field in North Kuwait used as a key area in the study. X-5 and X-6 are the new wells that were being drilled at the time of the study. The fracture clusters predicted by FCM for these wells were validated by the cores and borehole images obtained from them. size and extension (vertical and lateral). Their dimensions can vary over a wide range. For instance, some of them have been found to be 10 m wide, 100 m high, and 1000 m long. Such FCs can contain more than hundreds to ten thousand fractures and have a permeability well above 10 Darcy. Other types of clusters can also be observed. Individual fractures or those that do not appear to be a part of FCs are called diffuse fractures. Conductive FCs are major highways for fluids flow in the reservoir and their exact positions must be known and accurately mapped in the reservoir model in order to obtain realistic dynamic reservoir simulations. This information is essential in order to select injector and producer well locations that maximize the reservoir sweep efficiency. A workflow to map all major FCs in a reservoir is described here. Results are presented for five Middle East carbonate fields in Kuwait. Fracture cluster mapping workflow and results The workflow is based on the assumption that, when natural fractures exist in the form of clusters of larger dimensions (i.e. 10-30 m or more in width, vertical and horizontal lengths), they should be expressed in some way in the 3D seismic data. The workflow mainly involves integration of borehole data with the 3D seismic to optimize the extraction process achieved through the discontinuity extraction software (DES) processing. The 3D seismic data must exhibit optimal spatial/temporal bandwidth and signal-to-noise ratio to ensure that the subsequent attributes input to the DES processing contain meaningful information to map fracture clusters. This may require bespoke acquisition design and data processing workflows using single-sensor data. Seismic attributes sensitive to fracture clusters are identified and input to the DES. The directional (azimuthal) and inclination (dip) filters used in the DES processing are designed based on the analysis of cores, borehole images, sonic logs, and VSPs (e.g., offset, walkaround, walkaway, and 3D VSPs). Moreover, the structural and tectonic history of the study area is also employed in the process of parameter optimization and assessment of the results obtained. The general DES processing tends to overlook quite a significant percentage of fracture clusters of various orientations and dimensions when the directional filter is kept open to all 360 of azimuth with a fixed range of features inclination. In such a situation, the DES processing tends to follow the strongest lateral discontinuities in the vertical plane caused by larger fracture clusters, and it skips over the less strong and weaker discontinuities or signatures of fracture clusters that are either of the same orientation or different orientations. To capture such discontinuities, the directional filter is divided into a number of windows/ranges and the inclination filter is set at one or more than one dip inclination range. DES processing is run separately for each set of directional and inclination filters. Each run of 2008 EAGE www.firstbreak.org 109
technology feature first break volume 26, May 2008 Figure 2 Schematic showing the effect of azimuth filter on the extracted fracture clusters from the fracture sensitive 3D seismic attribute. The 3D cube of fracture clusters obtained through multiple azimuth filters gives more realistic picture fracture clusters (Model 2) than the one obtained through a single 360 azimuth filter (Model 1). Figure 3 A time slice from Middle Marrat showing fracture clusters orienting dominantly in NNE-SSW direction when the azimuthal filter is not constrained. DES gives a 3D volume cube of fracture cluster lineaments. Subsequently, these individual 3D cubes are merged into a single 3D volume cube (Figure 1) of fracture clusters that can be converted from time index to depth index. The workflow was applied to the sequence of Jurassic carbonates in five fields (NW Raudhatain, Raudhatain, Umm Niqqa, Sabriyah, and Bahra) located in the northern part of Kuwait. The Sabriyah field was selected as the key area for the study because of the maximum number of wells (four) drilled at the time of the study, new drilling, and a challenging structural setting (popped up structure caused by transpression along the east and west bounding strike-slip faults). In addition to the fracture evidence at the existing wells (X-3 and X-4, Figure 2), the newly drilled wells (X-5 and X-6, Figure 2) were used for the validation of the fracture clusters located by the DES on the seismic volume. Figure 3 shows mainly NNE-SSW trending fracture clusters at a certain horizon in the Middle Marrat carbonate reservoir, extracted by DES from the seismic volume. Fracture clusters of exactly the same orientation and inclination were observed in the 3D cube throughout the Marrat section. On the contrary, the borehole data at Well X-3 showed a large dominance of ENE- WSW striking fractures (more than 400 open fractures) within Marrat, in particular. When the DES process was applied to the same seismic attribute volume but with two different azimuthal filters (315-045, 135-225 and 045-135, 225-315), fracture clusters with NNE-SSW, ENE-WSE, NE-SW, NW-SE, and WNW-ESE strike got highlighted (Figure 4). The NNE-SSW striking fracture clusters most probably are fold-related being parallel to the axis of the Sabriyah anticline, and ENE- WSW and WNW-ESE striking fracture clusters, which are more concentrated within the Sabriyah anticline, are possible Riedel shears (Figure 4). The results were validated at the locations of existing wells and also at the new wells X5 and X-6. Figure 5 110 www.firstbreak.org 2008 EAGE
first break volume 26, May 2008 technology feature Figure 4 As observed in the borehole images, nearly all orientations of fracture clusters were detected from the 3D seismic by running DES for two different sets of parameters. Possibly fold related longitudinal fractures and strike-slip fault related riedel shear types of fracture clusters were detected with this technique. shows fracture clusters along a section through Wells X-2 and X-3 extracted by FCM technique using a 050-080 and 230-260 azimuth filter to enhance fracture clusters having strike orientations within that specific range of azimuth. It is clear from the plot that the Well X-3 does not intersect any fracture clusters over the interval from top Najmah to top Middle Marrat. It intersects a major fracture cluster in the interval from Middle Marrat to top Minjur (Figure 5). A similar observation was made in the wellbore using cores and borehole images as shown by the stick plot and fracture density (number of fractures per foot) curve for open fractures. In addition to the factors mentioned under the FCM workflow, resolution of the input seismic data is very important to highlight fracture clusters of larger to smaller dimensions. The FCM workflow was applied to the Q-Land seismic data from the NW- Raudhatain Field to determine how much improvement could be made in the details of fracture clusters. Figure 7 shows a comparison of FCM results from the conventional 3D surface seismic data with those derived from Q-Technology single-sensor data. A good correlation was observed between well productivity and the proximity of fracture clusters predicted by the FCM workflow. This shows that the acquisition of a fracture corridor map can be an essential element for the placement of injectors and producers to maximize recovery from a field. Possible origin of fracture corridors Whatever the real mechanism at microscopic scale, brittle failure of a rock occurs with respect to two modes at mesoscopic scale: the tensile and shear modes (Figure 7). The tensile mode (Mode 1) is a failure that occurs perpendicularly to the minimum principal stress without shear at the fracture plane. On the contrary, the shear failure 2008 EAGE www.firstbreak.org 111
technology feature first break volume 26, May 2008 Figure 6 Comparison of FCM results between those derived using conventional 3D seismic and those obtained by using Q-Land seismic data at the NW-Raudhatain Field. Figure 5 A section through wells X-2 and X-3 showing fracture clusters extracted by the FCM technique using 050-080 and 230-260 filter to enhance ENE-WSW trending lineaments. The well X-3 does not intersect any fracture cluster over the interval from top Najmah to top Middle Marrat. While it intersects a major fracture cluster in the interval from Middle Marrat to top Minjur. A similar observation was made in the wellbore using cores and borehole images as shown by the stick plot and fracture density curve for open fractures. Figure 7 Mohr s representation in which limits between elastic and brittle domain and between tensile and shear failure are intrinsic property of the rock. (Mode 2) induces a plane oblique to the main stress. This plane necessarily suffers a displacement of one side of the fracture relative to the other. This displacement is very small at failure time. Every brittle rock can react both ways but not for the same state of stress. The limit between the tensile and shear modes is an intrinsic property of the rock, like the Mohr s envelop that limits the elastic and brittle domains. In many vertical wells, induced fractures due to perturbation of the present-day stresses by drilling illustrate this dual behaviour. They are either tensile or en echelon depending, on the lithology. Similarly, during a tectonic phase, the failure may occur tensile for one particular lithology or shear for another one. During the initiation phase of a standard fault, the failure in a shear layer develops along a single plane (Figure 8). The movement along this plane is resolved into a vertical throw and a horizontal elongation. This horizontal elongation necessarily induces a reduction of the minimum stress magnitude in the layers above and below. If these layers are tensile type, a number of vertical fractures, i.e. a fracture corridor, will eventually occur at the vertical of the fault zone. The sum of the apertures of the fractures will compensate for the elongation observed in the shear layer when the vertical throw is usually accommodated by a vertical displacement on a few of these 112 www.firstbreak.org 2008 EAGE
first break volume 26, May 2008 technology feature fractures. These vertical displacements are very difficult to observe on borehole images as the great number of fractures makes it difficult to follow a marker. Many corridors of this type have been observed in the Paleozoic series of Algeria (Figure 9). It is certainly a common situation but other origins may exist. Conclusion We have presented a workflow that allows us to map in 3D all major fracture corridors in a field. The workflow was successfully applied to the NW Raudhatain, Sabriyah, Umm Niqqa, and Bahra carbonate fields in Kuwait. Due to the limited seismic signature of some of the fracture corridors, best results are obtained when using high-resolution seismic technology. The presence of fracture corridors has often been put forward as a possible explanation for water breakthroughs that occur much earlier than initially anticipated. With 3D maps of fracture corridors becoming available, these can now be integrated in reservoir models to determine the optimum well locations using more realistic reservoir simulations. It is hoped that such a methodology (Montaron et al., 2007) will avoid water breakthrough surprises and, Figure 8 Fracture corridor development. Left: Fault (f) initiation in a shear layer (S) inducing a lateral elongation (E) in this layer and extensional stress zones (ESZ) in tensile layers where fractures are initiated. Right: fault propagation inducing a corridor of fractures whose aperture compensates for the fault elongation (E) and vertical displacement on few of these fracture to compensate the vertical throw on the fault (vt). more importantly, will help significantly increase hydrocarbon recovery factors in carbonate reservoirs and other naturally fractured formations. Acknowledgements The authors thank Kuwait Company for permission to publish this work and Donatella Astratti and Robert Godfrey for their invaluable contributions. References Akbar, M., Singh, S.K., Khan, B., Abu-Habbiel, H., Maizeret, P.D., Astratti, D., Sonneland, L., Pedersen, L.S., Bakiler, C. and Godfrey, R. [2008] An Innovative Approach to Characterizing Fractures for a Large Carbonate Field of Kuwait by Integrating Borehole Data with the 3D Surface Seismic. 118994. GEO2008, Bahrain, Abstract Montaron, B.A.. Bradley, D., Cooke, A., Prouvost, L., Raffin, A., Vidal, A. and Wilt, M. Shapes of Flood Fronts in Heterogeneous Reservoirs and Oil Recovery Strategies. SPE/EAGE Reservoir Characterization & Simulation Conference, Paper 11147. Figure 9 Fracture corridor in quartzite developed on top of a small fault in shaly layers. 2008 EAGE www.firstbreak.org 113