Experimental and numerical simulation of water adsorption and diffusion in shale gas reservoir rocks

Abstract

Despite the success of deep horizontal drilling and hydraulic fracturing in yielding large production increases from unconventional shale gas reservoirs, uncertainties associated with basic transport processes require understanding in order to improve efficiency and minimize environmental impacts. The hydraulic fracturing process introduces large volumes of water into shale gas reservoirs, most of which remains unrecoverable and interferes with gas production. In this study, the water adsorption and diffusion measurements of the Longmaxi Formation shale were conducted at 30 • C and 50 • C for relative humidities from 11.1% to 97.0%. Based on the experiment, a computational model based on the Maxwell-Stefan diffusion equation was constructed to analyze water adsorption and diffusion in shale rocks, and the Guggenheim-Anderson-de Boer (GAB) isotherm for gas adsorption was included in the model. The results show that water adsorption isotherms of shales belong to type II curve, including the monolayer, multilayer adsorption and capillary condensation, and the GAB model can be used to describe the water adsorption process in shale rocks. With the increasing of relative pressure, the water adsorption of shale increases, and the organic carbon content and temperature strengthen the water adsorption in shale. The capillary pressure can reach the order of several hundreds of MPa after the hydraulic fracturing process, and it results in a large amount of fracturing fluid retained in shale gas reservoirs. Furthermore, the simulation of water adsorption and diffusion in shale rocks is less than the experimental value, which further indicates that capillary condensation occurs in shale rocks.

An Open-Source Code for Fluid Flow Simulations in Unconventional Fractured Reservoirs

“…With the development of unconventional tight reservoirs in recent years, enormous effort has been spent to simulate gas adsorption and transport in shale (Figure 1). The related mechanisms and corresponding models [36][37][38][39][40][41] are summarized in Table 2, with indications of the relative domain of application (matrix or fracture).…”

Review of Formation and Gas Characteristics in Shale Gas Reservoirs

“…Currently, the commercial development of shale gas resources in North America and China mainly benefits from advanced engineering technology. The theoretical understanding of shale gas storage capacity, gas transporting mechanism in nanopores or micropores, and pore structure characterization is still not clear, being far behind engineering practice [1].…”

Flow simulation considering adsorption boundary layer based on digital rock and finite element method

“…1), which is one of the main reasons for the occurrence of nonlinear flow (Huang 1998;Song et al 2010;Huang et al 2013;Yang et al 2020c). The mechanism of pseudo-threshold pressure gradient can be interpreted by the molecular interaction theory and ABL theory (Xiong et al 2009;Li et al 2015b;Shen et al 2019). In porous media, fluid viscosity is influenced by the ABL (Song et al 2016).…”