Investigation of Hydraulic Fracture Networks in Shale Gas Reservoirs with Random Fractures

As shown in hydraulic fracture monitoring, the hydraulic fracture path in naturally fractured shale reservoirs is complex, and the method for describing the propagation of fractures in homogeneous sandstone cannot achieve sufficient accuracy when used in shale reservoirs. In this study, a discrete fracture network model is proposed by performing a Monte Carlo simulation on the fracture characteristics of downhole shale cores and shale outcrops in the Longmaxi formation in the Sichuan basin of China. Given the fracture fluid pressure drop and intersections between hydraulic and natural fractures, a 2D numerical model is developed to examine the hydraulic fracture propagation in a randomly fractured shale reservoir using the displacement discontinuity method. Numerical simulations show that (1) the density and length of natural fractures are positively correlated with the complexity of natural fractures when the orientation of such fractures is similarly distributed, (2) the influence of small natural fractures on a hydraulic fracture is localized and cannot change the overall direction of the hydraulic fracture, and (3) hydraulic fractures can cross the natural fracture when the intersection angle and minimum horizontal stress are small enough. The rationality of the numerical model results is supported by the hydraulic fracturing tests on fractured shale outcrops with a true triaxial fracturing system and microseismic monitoring from a hydraulic fracture test well in the Longmaxi shale formation.