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.
All Science Journal Classification (ASJC) codes
- Energy Engineering and Power Technology