Modeling of Complex Hydraulic Fracture Propagation in Shales Through Dual-Lattice Discrete Element Method

Abstract

Successful shale gas and tight oil production is enabled by the engineering innovation of horizontal well and hydraulic fracturing. In the reservoirs with ultra-low permeability, closely spaced hydraulic fractures and multilateral wells are required to improve production. Thus understanding the stress evolution and potential fracture interaction is critical in optimizing fracture/well design and completion strategy in multi-stage horizontal wells. In this paper, a new fully coupled flow and geomechanics model based on the dual-lattice system is developed to simulate multiple non-planar fractures propagation. The numerical model from Discrete Element Method (DEM) is used to simulate the mechanics of fracture propagations and interactions, while a conjugate irregular lattice network is generated to represent fluid flow in both fractures and formation. Initiation, growth and coalescence of the microcracks will lead to the generation of macroscopic fractures, which is explicitly mimicked by failure and removal of bonds between particles from the discrete element network. Based on above model, a sensitivity study is performed to investigate the effects of fracture spacing, in-situ stress anisotropy and fluid viscosity on hydraulic fracture propagation. The results show that fracture from one perforation cluster may inhibit the growth of neighboring perforation clusters due to the stress shadow effect. However, a larger initial in-situ anisotropy helps overcome this phenomenon. Reducing the viscosity of injection fluid was found to increase the possibility of hydraulic fracture branching, resulting in the formation of a more complex fracture network. This effect is not captured in traditional fracturing simulators. Multiple fractures creation both simultaneously and sequentially from horizontal wells is examined next. In simultaneous fracturing, fractures from the same horizontal well tend to repel each other while fractures from two different horizontal wells appear to attract each other. The curvature of fractures is controlled by the magnitude of stress shadow and far-field stress contrast. In sequential fracturing, the subsequent fracture trajectory is significantly affected by both fracture spacing and the fracture treatment in previous fractures. Finally, a case study on a shale reservoir in Eagle Ford is presented and this dual lattice DEM-Flow model is shown to capture realistic growth pattern of hydraulic fractures in heterogeneous reservoirs.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015