Experimental studies of the course of CO2/CH4 exchange sorption on hard coal subjected to confining pressure.

Studies were carried out on exchange sorption of CO2/CH4 on four samples of hard coal differing in the degree of coalification. The samples were subjected to various confining pressures in the range from 1.5 MPa to 30 MPa. The author's conception of the measuring apparatus made it possible to use cylindrical samples of considerable length. This made it possible to register the phenomena as a function of time and distance. Measurements at each of the investigated pressures were carried out according to the following sequence: (1) pumping out the coal sample, (2) injecting CH4 at the pressure of 0.8 MPa until sorption equilibrium was reached, (3) exchange of gas at the sample inlet from CH4 to CO2, (4) decrease of pressure at the sample outlet, (5) observation of sorption exchange: amount of injected CO2, CH4 emission, gas pressure at the inlet, outlet and inside the sample, confining pressure exerting axial stresses, change of sample volume as a result of swelling. The conducted research allowed us to determine the influence of confining pressure on the key parameter - sweep effeciency of coals. This parameter is the ratio of CH4 recovered from coal to the total sorbed CH4. It is most often presented as a function of displaced volume, which is the ratio of injected CO2 to total sorbed CH4. Sweep effeciency was about 90% in the coals studied. The effect of confining pressure on this parameter was insignificant, but in all cases the sweep effeciency value was highest for the lowest confining pressure and decreased with increasing pressure. The strongest effect of confining pressure was observed with respect to the kinetics of the exchange sorption process. The sorption exchange process for the lowest confining pressure (1.5MPa) occurred about ten times faster than for the highest confining pressure (30MPa). During CH4 sorption, the coal material swelled, and the volume change at CH4 pressure at 0.8MPa ranged from 0.45% to 0.8% of the initial volume for 1.5 MPa. A high confining pressure of 30MPa resulted in a swelling reduction of 0.1% to 0.2%. Further swelling was recorded during exchange sorption, from about 0.6% to 1.2% for the lowest confining pressure and from about 0.2% to 0.6% for the highest confining pressure.