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The AAPG/Datapages Combined Publications Database

Four Corners Geological Society


Natural Fracture Systems in the Southern Rockies, 1999
Pages 165-171

Drainage Patterns in Naturally Fractured Tight Gas Reservoirs

Hugo Harstad, Lawrence W. Teufel, John C. Lorenz, William H. Babcock


From a reservoir engineering perspective the primary objective for characterizing natural fractures is to provide representative fracture permeabilities for the reservoir. Unfortunately, predicting fluid-flow response of fractured reservoirs is difficult because of the variability of the three dimensional fracture networks, permeability anisotropy, and interaction of fracture and matrix permeability. Integration of geological data from outcrops, logs and core with production, well testing and pressure interference testing provide the most reliable approach to fracture characterization and fluid flow simulation.

Fractures not only enhance the overall porosity and permeability of these reservoirs, but fractures also create significant permeability anisotropy. Permeability anisotropy causes the drainage area around the wells to be elliptical. The aspect ratio of the elliptical drainage area in these reservoirs is determined by the magnitude of the permeability anisotropy. Results demonstrate that well spacing of 320 acre squares based on radial drainage is not relevant when fractured reservoirs have permeability anisotropy greater than 10 to 1. Economic optimization of infill drilling using fewer wells can be accomplished if field development is based on elliptical drainage areas that lead to a reduction in drainage overlap of adjacent wells and prevent leaving undrained areas.

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