--> Modelling of a Sub-Andean Fractured Reservoir

AAPG Latin America & Caribbean Region, Optimizing Exploration and Development in Thrust Belts and Foreland Basins.

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Modelling of a Sub-Andean Fractured Reservoir


The gas-condensate field, object of this study, is located in the Sub-Andean compressive zone. The structure is a typical faulted asymmetric anticline, 60km long, 5km across. The Devonian sandstones constitute the reservoir. Tests have shown the existence of a developed fracture network. The reservoir is a classical type 2 fractured reservoir: the fractures provide the permeability (15-70mD) while the matrix provides the porosity (2-6%). These characteristics make the fracture study a key element to understand the field behavior in order to maximize the production and anticipate possible issue like water-breakthrough. The applied workflow is mainly based on borehole imagery (BHI) analysis and implies the following phases: The BHI of 5 wells were analyzed and interpreted. Depending on the quality of the images, fractures crossing the wellbores can be picked. As induced fractures or artefacts of various origins can be easily confused with natural fractures, a quick analysis of core pictures was performed to minimize uncertainties. The output of BHI analysis is a log of fractures for each well. While the previous static data bring information regarding fractures at well, seismic data bring inter-well information allowing the detection of the major thrust-faults of the field. Faults and fracture logs along with other logs were then imported in the dedicated software FracaFlowTM for analysis and modeling. From the BHI analysis output, the number of fracture sets can be determined as well as their density and their possible relationships with facies, porosity, volumes of shale… This allows understanding the spatial organization of the fracture network. In this case, a clear relationship between the fracture density and the distance to faults was highlighted. However, the understanding of the contribution of fractures to the fluid flow requires dynamic well data: mud-losses, PLT, well-test and production data were thus carefully analyzed. By computing locally the equivalent permeability of the fracture network, FracaFlowTM calibration algorithm found the optimal fracture conductivity to match the KH interpreted from well-tests. All the equivalent properties (permeability along x/y/z, porosity and shape factor) were finally computed with a dual-media hypothesis. This study allowed building a fracture network, consistent with the geological context and matching the dynamic data. It is now ready to be used for the history matching and simulation/forecasts.