Geomechanics of CO₂ enhanced shale gas recovery

Shale gas has become an increasingly important source of natural gas (CH₄) in the United States over the last decade. Due to its unconventional characteristics, injecting carbondioxide (CO₂) to enhance shale gas recovery (ESGR) is a potentially feasible method to increase gas-yield while both affording a sink for CO₂ and in reducing the potential for induced seismicity. However, understanding of this issue is limited with few pilot field studies proposed. This study examines CO₂-ESGR to better understand its feasibility and effectiveness. We explore the roles of important coupled phenomena activated during gas substitution especially vigorous feedbacks between sorptive behavior and permeability evolution. Permeability and porosity evolution models developed for sorptive fractured coal are adapted to the component characteristics of gas shales. These adapted models are used to probe the optimization of CO₂-ESGR for injection of CO₂ at overpressures of 0 MPa, 4 MPa and 8 MPa to investigate magnitudes of elevated CH₄ production, CO₂ storage rate and capacity, and of CO₂ early-breakthrough and permeability evolution in the reservoir. For the injection pressures selected, CH₄ production was enhanced by 2.3%, 14.3%, 28.5%, respectively, over the case where CO₂ is not injected. Distinctly different evolutions are noted for permeability in both fractures and matrix due to different dominating mechanisms. Fracture permeability increased by ∼1/3 for the injection scenarios due to the dominant influence of CH₄ de-sorption over CO₂ sorption. CO₂ sequestration capacity was only of the order of 10⁴ m³ when supercritical for a net recovery of CH₄ of 10⁸ m³. We investigated the potential of optimal CO₂-pulsed injection to enhance CH₄ production (absolute mass recovered)-without the undesirable effects of CO₂ early-breakthrough and also minimum cost on CO₂ injection. This utilizes the competitive sorptive behavior between CH₄ and CO₂, can also reduce the potential for induced seismicity hence the entire system can be near net neutrality in terms of its carbon and seismic footprint.