Fracture-Matrix Modeling Technique Unlocks CO2 Enhanced Shale Gas Recovery
Authors: Carpenter Chris 
Published: February 2022

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 202222, “Fracture-Matrix Modeling of CO2 Enhanced Shale Gas Recovery in Compressible Shale,” by Dhruvit Satishchandra Berawala, SPE, Equinor, and Pål Østebø Andersen, University of Stavanger. The paper has not been peer reviewed.
With technology available at the time of writing, only 3–10% of gas from tight shale is recovered economically through natural depletion, demonstrating a signiﬁcant potential for enhanced shale gas recovery (ESGR). Experimental studies have demonstrated that shale kerogen/organic matter has a higher adsorption aﬃnity for carbon dioxide (CO2) than methane (CH4). CO2 is preferentially adsorbed over CH4 with a ratio of as much as 5:1. The complete paper examines CO2 ESGR in compressible shale during huﬀ ’n’ puﬀ injection to understand better the parameters controlling its feasibility and eﬀectiveness. The authors present a mathematical model in the complete paper in which the CO2/CH4 substitution mechanism is implemented in an injection/production setting representative of ﬁeld implementation.

Introduction
Modeling of CO2 injection and the interplay between CO2 and CH4 sorption has been extremely challenging for scientists and engineers. The presence of CO2 with methane during the CO2 ESGR process makes gas-desorption behavior and measurement more diﬃcult. Few researchers have evaluated the eﬃciency of CO2 ESGR in compressible shale.
To improve the understanding of this technique, the authors present a numerical modeling approach using a 1D+1D fracture-matrix model in order to study the feasibility of CO2 injection in shale formations. The model consists of a high-permeability fracture extending from a well perforation, symmetrically surrounded by a shale matrix as shown in Fig. 1 of the complete paper. The fracture is assumed to have ﬁxed width for simplicity. The system is assumed to consist of free gas in the pores as well as adsorbed gas in the matrix. When pressure in the well is reduced, free and adsorbed gas from the matrix ﬂows toward the fracture and then to the well. The pressure-based advective forces, along with concentration-based diﬀusive forces, are considered the main transport mechanisms in this work. After the CH4 production cycle, CO2 is injected from the same well into the fracture and to the matrix. Injection of CO2 leads to an increase in total gas pressure in the system. CH4/CO2 adsorption kinetics are modeled using a multicomponent adsorption isotherm presented in earlier works by the authors.
Apparent permeability is used to account for gas slippage eﬀect, eﬀective stress, adsorption, and other ﬂow regimes relevant to the nanopore structure of the shale formation. The effect that compressible rock has on the porosity and apparent permeability changes with CH4 production. CO2 injection also is considered. The resulting model is composed of nonlinear partial differential equations that are solved numerically using an operator splitting approach.
The geometry, mole conservation, pressure-dependent parameters, and initial and boundary conditions of the mathematical model are detailed in the complete paper, including mathematical definitions and solution procedures in its appendix.