Assessment of the Potential for CO₂-Enhanced Shale Gas Recovery: Studies on CO₂/CH₄ Exchange in Shales

The article presents the results of tests on shales collected from the Lewino-1G2 well (Poland) at a depth of 3400 m. The shale was classified as meso and macroporous with a specific surface area in the range of 4.0–9.2 m2/g and pore volume in the range of 7.4–16.3 cm3/g. The highest measured sorption capacities were 2.4 cm3/g for CO2 and 1.3 cm3/g for CH4. The permeability coefficients to helium were determined under conditions of confining pressure in the range from 2 to 35 MPa, and at 35 MPa─corresponding to a burial depth of 1000 m, it was impermeable in the direction perpendicular to the bedding, and in the direction parallel to the bedding it had very low permeability of 1.4 × 10–3 mD. The CO2/CH4 exchange experiments were performed at a confining pressure of 5 MPa. The CO2/CH4 selectivity of the exchange process was 1.1, and the sweep efficiency was 81%. The results of the obtained tests were compared with the results of CO2/CH4 exchange on hard coal.In shale formations, the CO2/CH4 exchange occurs predominantly through gas-phase mechanisms, with competitive sorption/desorption processes playing a negligible role. One of the key factors contributing to this unfavorable behavior is the low total organic carbon (TOC) content commonly observed in Polish shales, often not exceeding 1.5%–3.0%, which is considerably lower than in many prolific shale gas formations worldwide. As a result, the CO2/CH4 exchange in shale exhibits relatively low efficiency and selectivity. The limited development of the pore network, along with low sorption capacity and extremely low permeability under in situ conditions, significantly limits the feasibility of CO2-ESGR in Polish shale formations. These geological and petrophysical limitations highlight the necessity of hydraulic fracturing to enhance permeability and pore connectivity in shales before the implementation of CO2-ESGR technology can be considered viable.