Parameters influencing the flow performance of natural cleat systems in deep coal seams experiencing carbon dioxide injection and sequestration

Abstract

Carbon dioxide (CO2) sequestration in deep, unmineable coal seams may provide an immediate and economically feasible solution for mitigation of anthropogenic CO2 emissions. Coal contains natural cleats, which largely control fluid movement in coal seams. This study uses experimental and numerical methods to investigate the variables (i.e. injection pressure, injection depth, and coal temperature) that will influence cleat performance in the CO2 sequestration process for black coal. The steady-state downstream pressure (and pressure differential from the injection value) that develops during undrained triaxial tests performed on fractured black coal, without deviatoric load, is taken as a measure of cleat performance. The results of tests show that increasing injection pressure has a significant detrimental effect on cleat performance, in accord with greater adsorption-induced swelling (and cleat closure) with higher injection pressure. Testing also shows that increasing injection depth has a moderate (negative) effect on cleat performance and that temperature has a minor (positive) effect on cleat performance. The experimental results were used to validate a numerical model produced using the COMET 3 numerical simulator, which was used to further explore coupling between the variables that influence cleat performance in coal-seam CO2 sequestration. The numerical results show that the influence of injection pressure on cleat performance is dependent on injection depth, whereby a given increase in injection pressure will have more detrimental effects on cleat performance at greater depths than it would at more shallow depths. The influence of injection depth on cleat performance is similarly dependent on the absolute depth range considered. Coupling was observed in the influence of temperature on cleat performance and both injection pressure and depth. Modeling shows that increasing temperature has a negative influence on cleat performance for low injection pressures (i.e. 6 MPa), but a positive influence on cleat performance for higher injection pressures (i.e. 12 MPa). Modeling shows that temperature will have an influence on the manner by which depth effects cleat performance if injection pressures are high (i.e. 12 MPa), but no such influence if injection pressures are low (i.e. 6 MPa).