Coupled Thermo-Hydro-Mechanical-Chemical Modeling of Water Leak-Off Process during Hydraulic Fracturing in Shale Gas Reservoirs

• Published in: Energies (Basel) 2017-12, Vol.10 (12), p.1960
• Main Authors: Wang, Fei, Li, Baoman, Zhang, Yichi, Zhang, Shicheng
• Format: Article
• Language: English
• Subjects: Computational fluid dynamics; Computer simulation; Conduction; Convection; Deformation; Deformation mechanisms; Differential equations; Dilation; Energy conservation; Evolutionary algorithms; Finite difference method; Fluid flow; Fractures; Free convection; Gas transport; Heat transfer; Heat transport; Hydraulic fracturing; Hydraulic pressure; Hydrodynamics; Mathematical models; Modelling; multi-field coupling; numerical simulation; Osmosis; Partial differential equations; Porosity; Pressure; Reservoirs; Rock mechanics; Shale; Shale gas; thermo-hydro-mechanical-chemical model; water leak-off; Water transport
• ISSN: 1996-1073; 1996-1073
• DOI: 10.3390/en10121960

The water leak-off during hydraulic fracturing in shale gas reservoirs is a complicated transport behavior involving thermal (T), hydrodynamic (H), mechanical (M) and chemical (C) processes. Although many leak-off models have been published, none of the models fully coupled the transient fluid flow modeling with heat transfer, chemical-potential equilibrium and natural-fracture dilation phenomena. In this paper, a coupled thermo-hydro-mechanical-chemical (THMC) model based on non-equilibrium thermodynamics, hydrodynamics, thermo-poroelastic rock mechanics, and non-isothermal chemical-potential equations is presented to simulate the water leak-off process in shale gas reservoirs. The THMC model takes into account a triple-porosity medium, which includes hydraulic fractures, natural fractures and shale matrix. The leak-off simulation with the THMC model involves all the important processes in this triple-porosity medium, including: (1) water transport driven by hydraulic, capillary, chemical and thermal osmotic convections; (2) gas transport induced by both hydraulic pressure driven convection and adsorption; (3) heat transport driven by thermal convection and conduction; and (4) natural-fracture dilation considered as a thermo-poroelastic rock deformation. The fluid and heat transport, coupled with rock deformation, are described by a set of partial differential equations resulting from the conservation of mass, momentum, and energy. The semi-implicit finite-difference algorithm is proposed to solve these equations. The evolution of pressure, temperature, saturation and salinity profiles of hydraulic fractures, natural fractures and matrix is calculated, revealing the multi-field coupled water leak-off process in shale gas reservoirs. The influences of hydraulic pressure, natural-fracture dilation, chemical osmosis and thermal osmosis on water leak-off are investigated. Results from this study are expected to provide a better understanding of the predominant leak-off mechanisms for slickwater fracturing-fluids in hydraulically fractured shale gas reservoirs.