Impact of natural fractures on the shape and location of drained rock volumes in unconventional reservoirs: Case studies from the permian basin

Unconventional reservoirs with prolific production may contain a significant number of complex natural fracture networks. Partially conductive or partially mineralized natural fracture networks could further open due to the stress induced during hydraulic fracturing, and then provide additional pathways for the flow of reservoir fluids in the matrix near the wellbore and its hydraulic fractures. This study investigates the still insufficiently understood complex interaction of natural fracture networks with hydraulic fractures, which impacts the estimation of the drained rock volume (DRV), and fracture spacing for optimal production. Flow in natural fractures is modeled at high resolution using recently developed algorithms, which enable fast, grid-less, Eulerian particle tracking based on Complex Analysis Methods (CAM). Publicly available production data from the Permian Basin was used to visualize the DRV with time-of-flight contours and particle paths, initially assuming a homogeneous reservoir without any natural fractures. Next, the distortion of the DRV, by including natural fractures with different conductivity in the proximity of the hydraulic fractures, is visualized and compared to the homogeneous reservoir without any natural fractures. The shape and location of the DRV in shale wells will be profoundly impacted by the overall location, density, and hydraulic conductivity (strength) of the natural fractures. High-resolution contour plots of (1) drained rock volume, (2) pressure depletion, and (3) spatial velocity variations are presented to compare the fluid migration paths near hydraulically fractured wells with and without natural fractures. Detailed case studies of several wells completed in Wolfcamp landing zones from the Permian Basin (i.e. Midland Basin and Delaware Basin wells) are included. The impact of the natural fracture networks, which are assumed to occur in clusters at various distances from the hydraulic fractures, on wells with different production characteristics, is modeled. Wells in reservoir sections with numerous natural fractures develop DRV, pressure and fluid velocity patterns that are more complex as compared to wells in reservoir sections without natural fractures. The results highlight that the impact of natural fractures on fluid withdrawal patterns (DRV) needs to be considered to make better completion decisions and optimize fracture spacing in naturally fractured reservoirs.