Numerical Simulation on the Influence of Natural Fractures on Hydraulic Fracture Propagation in Fractured Coal Reservoirs
Fan Zhang, Hanjin Wan, Bin Liu, Mengjie Zhu, Xudong Lv, Xing Xu
Abstract
Natural fractures with different angles, different combinations, and different sizes in mid-deep fractured coal reservoirs lead to complex fracture propagation paths, which will affect the coalbed methane exploitation effect. In this study, the influencing factors of the intersection of hydraulic fractures and natural fractures were analyzed, the effects of different approaching angles, natural fracture spacings, and natural fracture combinations on the initiation and propagation of hydraulic fractures were studied, and the coupling relationship between the surface area of the fracture surface and propagation characteristics of hydraulic fractures in different stages was discussed. The results are as follows: (1) When the fluid pressure in the hydraulic fracture is greater than the horizontal principal stress difference, the natural fracture will open and produce new branch fractures. (2) When the fluid pressure in the hydraulic fracture is greater than σ t + T 0, the hydraulic fracture will pass through the natural fracture and continue to expand along the direction of the maximum horizontal principal stress. (3) With the increase of approaching angle, fracture time and fracture pressure increase. Multiple branch fractures are formed on the left side of the fracturing hole, and hydraulic fractures on the right side of the fracturing hole propagate from the lower end to the two ends. When hydraulic fracture communicates with natural fracture, the surface area of the fracture surface increases greatly. (4) With the increase of natural fracture spacing, the initiation time and initiation pressure decrease, and fracture time and fracture pressure increase gradually. On the left side of the fracturing hole, several branch fractures are formed in the direction of the maximum horizontal principal stress. On the right side of the fracturing hole, the hydraulic fracture at the upper end of the natural fracture gradually deflects from the direction parallel to the maximum horizontal principal stress to the minimum horizontal principal stress. (5) With the increase of natural fracture number, fracture time and fracture pressure gradually increase. On the left side of the fracturing hole, several branch fractures are formed in the direction of the maximum horizontal principal stress. On the right side of the fracturing hole, hydraulic fractures continue to expand upward and downward after connecting natural fractures, gradually turn to the direction of the maximum horizontal principal stress after connecting the second and third natural fractures, and form multiple branch fractures. The research results are helpful in studying the mechanism of natural fractures in fractured coal reservoirs during hydraulic fracturing and provide a reference for analyzing the hydraulic fracturing effect and coalbed methane mining in mid-deep coal seams.