Fracture Initiation of Multilithology Combination With Respect to Hydraulic Fracturing Stimulation in the Late Permian Coal Measure Strata From the Songzao Coalfield, SW China

Abstract

Pushed by the increasing demand regarding the energy supply, coal measure gas has a tendency to get more and more attention. Therein, greater gas production is usually supported by an effective hydraulic fracturing in this unconventional reservoir—an overlapping multilithology system. However, the fracturing operation is difficult because of the complicated multilithology combination. In this work, the finite element modeling method ensures a series of numerical simulations to investigate the fracture initiation behavior when the hydraulic fracturing is applied, under the real geological background of the Late Permian coal measure strata in the Songzao Coalfield. Herein, two operation conditions are simulated, that is, the perforations are located in adjacent cataclastic coal and powdered coal (Case 1) and are set in adjacent cataclastic coal and stratified sandstone (Case 2). Therein, Case 2 has two situations—the stratified sandstone is embedded with joint plane (Case 2.1) and is free of joint plane (Case 2.2). As for Case 1, so the initiation location is mainly distributed in the cataclastic coal seam section. In Case 2.1, the stress of the lower cataclastic coal preferentially changes from compressive stress to tensile stress, making the fracture initiation occur. When the joint plane is involved, in Case 2.2, reservoir damage may start preferentially near the borehole of joint plane. Besides, the perforation density, perforation length, and perforation diameter influence fracture initiation pressure a lot. In addition, the horizontal stress difference and pore pressure also affect the fracture initiation pressure in different ways.