IMPLICATIONS OF VOLUMETRIC SWELLING/SHRINKAGE OF COAL IN SEQUESTRATION OF ACID GASES

A series of volumetric swelling and shrinkage experiments using N2, CH4, CO2, and H2S and subsequent numerical modelling was carried out on coals from the Western Canadian Sedimentary Basin in order to assess potential permeability damage or enhancement due to injection of various gases for enhanced coalbed methane production or for sequestration of acid gases. Investigated coals vary in rank from subbituminous ‘C’ (Ror 0.46%) to medium volatile (Ror 0.62%) and experiments were carried out on samples on an as received basis. Adsorption of N2, CH4, CO2 and H2S resulted in marked swelling of the coal matrix. The order of volumetric swelling at 0.6 MPa in decreasing order is H2S (1.4%-9.3%) > CO2 (0.26%-0.66%) > CH4 (0.09%0.30%) > N2 (0.004% 0.026%). Volumetric strain is strongly and positively correlated with quantity of adsorbed gas. Overall the sorption capacity and volumetric strain increases with rank however the comparative sorption capacity and volumetric strain vary markedly with coal rank and type of gas. Desorption of CH4 results in shrinkage of the coal matrix from 0.4% to 0.9% depending on coal rank. The volumetric shrinkage during CH4 desorption (per cc basis) is significantly greater (1.4 – 4.4 times) than the volumetric swelling caused by N2 adsorption, therefore N2 injection will enhance CH4 production from coalbeds due to increase in N2 partial pressure coupled with increased permeability of coal beds because of net shrinkage. Injection of pure H2S or CO2 will significantly reduce the permeability of coal seams as injection of H2S causes swelling up to 5 times and CO2 causes swelling more than 2.2 times the shrinkage created by CH4 desorption. Since volumetric strain varies with volume and type of fluid sorbed, drilling and completion fluids may result in swelling or shrinkage of the coal matrix and thus enhance or reduce permeability. Our numerical modeling results indicate that the variation in cleat permeability during acid gas injection into coal seams is strongly affected by the initial cleat porosity and the mechanical properties of coals. Injection of CO2 can reduce the permeability by 50% for coals with a permeability of 10 md and cleat spacing of 0.5 cm and by two or more orders of magnitude for coals with a lower initial permeability (1 md) and large cleat spacing (1 cm). Injection of pure H2S results in such marked swelling of the coal that the in-situ coal permeability will be rapidly reduced to such low values that significant sequestration will be impossible. Up to a 5 times increase in permeability is predicted for injection of N2. Therefore, the injection of a mixture of N2 and CO2 or N2 and H2S may facilitate sequestration of acid gas. Based on the results of our studies we have developed a new permeability model for acid gas sequestration taking into account coal matrix shrinkage and variable swelling property of coals with different gases during sequestration. Our model suggests that an optimum coal seam for sequestration of acid gases would have a high initial permeability and, most importantly, a large initial cleat porosity to reduce the effects of permeability reduction induced by coal swelling on the efficiency of the acid gas sequestration.