A small-scale experimental study of CO2 enhanced injectivity methods of the high-rank coal

The attenuation of CO2 injectivity has become the biggest technical barrier for the application of CO2 enhanced coalbed methane recovery (CO2-ECBM). Commonly, the intermittent CO2 injection, N2 displacing CO2 and pre-fracturing are the potential CO2 enhanced injectivity methods for coal reservoirs, but their mechanism and effectiveness remain to be clarified. This paper thus conducted small-scale experiments to simulate the working process of these engineering measures by an independently developed experimental device. Results show that the CO2 injectivity of coal is remarkably improved by the intermittent injection mode since the CO2 injection time is increased by folds and the loss of reservoir pressure can be complemented in time. The N2 displacing CO2 method promotes the desorption of CO2 and reduces the swelling strain, with the result that the permeability of coal is improved by 74.82% and 64.95% compared with the methods of the primary subcritical CO2 (SubCO2) and supercritical CO2 (ScCO2) injection. However, the permeability reduces again with the secondary CO2 injection. The permeability of the coal sample after pre-fracturing is averagely improved by 1–2 orders of magnitude, the irreversible permeability loss rate, average stress sensitivity coefficient and the permeability loss rate due to adsorption are averagely reduced by 95.885%, 61.538% and 96.297%, respectively. This indicates that the permeability of coal after pre-fracturing is no longer sensitive to both the effective stress and ScCO2 adsorption, the injectivity is thus improved and stable. The CO2 enhanced injectivity effects of the intermittent CO2 injection, the N2 displacing CO2 and the pre-fracturing are various, which thus can be selected individually or jointly to improve the CO2 injectivity according to the reservoir physical properties and geological conditions. This research deepens the understanding of the functional mechanism of CO2 enhanced injectivity methods and provides some guidance for their selection and application in engineering practices.