A small-scale experimental study of CO_(2) enhanced injectivity methods of the high-rank coal

The attenuation of CO_(2)injectivity has become the biggest technical barrier for the application of CO_(2)enhanced coalbed methane recovery(CO_(2)-ECBM).Commonly,the intermittent CO_(2)injection,N2 displacing CO_(2)and pre-fracturing are the potential CO_(2)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 CO_(2)injectivity of coal is remarkably improved by the intermittent injection mode since the CO_(2)injection time is increased by folds and the loss of reservoir pressure can be complemented in time.The N_(2)displacing CO_(2)method promotes the desorption of CO_(2)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 CO_(2)(Sub CO_(2))and supercritical CO_(2)(Sc CO_(2))injection.However,the permeability reduces again with the secondary CO_(2)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 Sc CO_(2)adsorption,the injectivity is thus improved and stable.The CO_(2)enhanced injectivity effects of the intermittent CO_(2)injection,the N_(2)displacing CO_(2)and the pre-fracturing are various,which thus can be selected individually or jointly to improve the CO_(2)injectivity according to the reservoir physical properties and geological conditions.This research deepens the understanding of the functional mechanism of CO_(2)enhanced injectivity methods and provides some guidance for their selection and application in engineering practices.