Analysis of stress interference among multiple hydraulic fractures using a fully three-dimensional displacement discontinuity method

Abstract

Due to the extremely low permeability of shale matrix, a great number of hydraulic fractures are required to enhance the flow capacity of shale reservoirs to obtain economic productivity. The mechanical interactions among closely spaced hydraulic fractures could result in fracture curving, intersection and unbalanced growth. Current hydraulic fracturing models are mainly based on the assumption of 2D or Pseudo-3D, in which the fractures are vertical with constant or equilibrium height. To better analyze and understand the stress interference among multiple hydraulic fractures, a fully three-dimensional (3D) model based on the displacement discontinuity method (DDM) is developed in this paper. The challenges (eq. grid twisting, hyper-singular integrals) in implementing 3D DDM for fracture growth problems are systematically discussed and solved from different aspects. Optimization strategies for the 3D model, including adaptive mesh growth, non-crowding Gaussian points and distance-dependent integrals, are proposed. Using this model, we first compare the strength of stress interactions with different fracture geometries. The simulation results show that the length of the shorter edge of the hydraulic fracture dominates the strength of stress shadowing effect. The stress redistribution due to fracture interference in 3D space is also calculated which delivers a much more complex shape of potentially stress re-orientation region than the 2D results. Then, the simultaneous propagation of multiple hydraulic fractures in different in-situ stress fields is analyzed, highlighting the importance of in-situ stress distribution on fracture geometries and interactions. The limitation on fracture height growth can reduce the mechanical interactions under the constant net pressure assumption. Besides, the detailed introduction of the present stable and efficient 3D DDM-based fracture propagation model can be used as a basis for other investigation purposes.