Simulation of coupled hydro-mechanical-chemical phenomena in hydraulically fractured gas shale during fracturing-fluid flowback

Fracturing-fluid flowback in hydraulically fractured gas shale is a complicated transport behavior involving hydrodynamic (H), mechanical (M) and chemical (C) processes. Although, many flowback models have been published, none of the models flly coupled the transient fluid flow modeling with chemical-potential equilibrium and fracture closure phenomena. In this paper, a coupled hydro-mechanical-chemical (HMC) model based on hydrodynamics, linear-elastic fracture mechanics, and chemical-potential equations is presented to simulate the fracturing-fluid flowback behavior in hydraulically fractured gas shale. The HMC model takes into account a gas-liquid two-phase flow and a triple-porosity medium, which includes primary hydraulic fractures, secondary induced fractures and shale matrix. The flowback simulation with the HMC model accounts for all the important processes in fractured shale system, including (1) water transport driven by hydraulic, capillary and osmotic convections, (2) gas transport induced by both hydraulic pressure driven convection and adsorption, and (3) fracture closure considered as an elastic deformation. The fluid transport, coupled with rock deformation, are described by a set of partial differential equations. The semi-implicit finite-difference method is used to solve these equations. The evolution of pressure, saturation, salinity and aperture profiles of hydraulic fractures, induced fractures and matrix is calculated, revealing the multi-field coupled flowback behavior in fractured gas shale. The sensitivity analysis is also performed to investigate the physiochemical properties of shale on the water load recovery and gas production rate during the fracturing-fluid flowback. The results indicate that the capillarity has the most effect followed by chemical osmosis and induced fracture closure. The sub-irreducible water saturation has the weakest effect. Moreover, the matrix-related physiochemical parameters cause opposite influences on water load recovery and gas production rate. While, the influence of induced fracture-related parameters on both water load recovery and gas production rate is consistent. Results from this study are expected to explain which is the predominant mechanism for fracturing-fluid retention and the inherent relationship between fracturing-fluid flowback and gas production for different initial physiochemical properties of shale.