A Numerical Gas Fracturing Model of Coupled Thermal, Flowing and Mechanical Effects

Abstract

Gas fracturing, which overcomes the limitation of hydraulic fracturing, is a potential alternative technology for the development of unconventional gas and oil resources. However, the mechanical principle of gas fracturing has not been learned comprehensively when the fluid is injected into the borehole. In this paper, a damagebased model of coupled thermal-flowing-mechanical effects was adopted to illustrate the mechanical principle of gas fracturing. Numerical simulation tools Comsol Multiphysics and Matlab were integrated to simulate the coupled process during the gas fracturing. Besides, the damage evolution of drilling areas under several conditions was fully analyzed. Simulation results indicate that the maximum tensile stress, which occurs in the upper and lower of the injection hole, decreases with the increase of the tectonic stress coefficient (TSC). As the TSC increases, shear fractures increase, a crushed area is gradually formed and the seepage area increases rapidly. The influence of TSC on fracture expansion is concluded as follows: with the decrease of TSC, the relative width of fractures decreases whilst the depth increases. It indicates that thermal stress and pore pressure promote the expansion of tensile fractures but restrain the expansion of shear fractures. Therefore, a relatively lower injection gas pressure is required to obtain the same degree of fracturing with a coupled thermal gradient.