Experimental study of permeability changes and its influencing factors with CO2 injection in coal

Abstract

Injecting CO2 into coal seams is a significant CO2 utilization approach when considering the dual environmental and energy benefits. However, the interaction of the coal matrix with CO2 is closely related to the permeability of a coal reservoir, which is simultaneously influenced by various factors, e.g., the moisture, temperature, coal rank and effective stress. To investigate the impact mechanism and the contribution of each influencing factor for the reservoir permeability and provide helps for the implementation schemes during the geological CO2 sequestration, a series of corresponding permeability tests were conducted under different adsorption pressures (2–10 MPa), moisture states (dry and wet), temperatures (35–65 °C), coal ranks (Ro,max = 0.68%–3.33%), effective stresses (3–10 MPa). The results show that coal matrix swelling induced by CO2 adsorption narrows the cleat width and reduces the permeability of a coal reservoir, which phenomenon is more serious for supercritical CO2. Injecting CO2 into the wet coal causes a higher PALR than the dry coal, which can be owned to the synthesized influences of the CO2 adsorption swelling, moisture-induced swelling and water blocking effect. The temperature first reduces the permeability of CO2 bearing coal incipiently by the swelling strains (thermal swelling and CO2 adsorption swelling) and then alleviates the permeability loss due to the remarkable gas slippage effect. Although the medium rank coal possesses the highest PALR, its permeability is also larger than the low rank coal and the high rank coal after CO2 adsorption. A cyclic loading and unloading leads to an irreversible permeability loss of a coal, which is more obviously reflected on the CO2 bearing coals. The analysis of the permeability contribution ratio indicates that the coal rank, CO2 adsorption and effective stress variety are the main controlling factors for the reservoir permeability, followed by the moisture, and the influence of the temperature on the permeability drop is quite weak. In summary, taking into account the various influencing factors and optimizing the injection arrangement are conducive to the successful implementation of the geological CO2 sequestration.