CO2 Sequestration into Coalbeds: Insights from Laboratory Experiments and Numerical Modeling

Abstract

Sorption, strain, and flow-related laboratory experiments combined with numerical modeling have been conducted with CO2 and other gases, including N2, CH4, H2, H2S, and SO2 on a variety of coal cores and coal powders to investigate the interplay of the parameters controlling storage, migration, and permeability changes during sequestration. The experiments include sorption isotherms, volumetric swelling or shrinkage of coal matrix during sorption (with a variety of gases on solid coal cores), N2 flow-through experiments on CH4-saturated coal cores, and N2 effects on permeability. The order of adsorption capacity for a given coal in ascending order was H2 N2 CH4 CO2 H2S SO2. The ratio of adsorptive capacity of the gases to various coals is rank dependent, which our experiments show is mainly attributable to declining moisture content with increasing coal rank. In low volatile bituminous rank coals, the ratio of CO2 to CH4 adsorption capacity at a given pressure is about 2:1, but this is about 10:1 in subbituminous coals. Moisture content in the coal reduces the adsorption capacity of CH4 whereas increased adsorption capacity was observed with CO2, H2S, and SO2 with increasing moisture content. Because these gases have high Henry's solubility coefficient, moisture from the coal micropore surface is stripped off to react with gases making moisture-occupied sorption sites available for the gases to adsorb resulting high adsorption capacity with high-moisture coals. Sorption-related strain experiments with N2, CH4, CO2, and H2S show that adsorption of gases on coal causes swelling of the coal matrix, which is directly proportional to the amount of gas adsorbed onto the coal and hence increases with rank. The average volumetric strain of the samples tested in decreasing order is H2S (2.5 103 g/cm3) CO2 (9.9 104 g/cm3) CH4 (6.9 104 g/cm3) N2 (3.1 104 g/cm3). Adsorption of CO2 relative to CH4 causes a relatively higher volumetric strain of the coal matrix and in turn reduces cleat permeability causing significant reduction in the sequestration capacity into coalbeds. Injection of N2 into coalbed significantly improves the permeability while displacing the CH4 because of its lower adsorption and associated swelling. Our experiments with associated analytical and numerical modeling using real data clearly indicate that sequestering pure CO2 into most coal seams results in volumetric strain and associated loss of permeability that quickly inhibit further or significant sequestration. Hence, it is very unlikely that in-situ sequestration of significant amounts of pure CO2 will be possible in any but the most permeable coals such as those of the Powder River Basin. However, mixing of N2 with CO2 significantly enhances the sequestration potential into coalbeds. Based on our results, a new numerical model was developed, which takes into consideration the shrinkage coefficients derived from experimental results with various gases coupled with mechanical properties of rocks, which closely predict the behavior of CO2 sequestration in coalbeds. The N2 flow-through experiments on CH4-saturated coal cores confirm the modeling results that N2 displaces the methane while inhibiting the permeability reduction because of its low sorption property. However, this process requires a minimum permeability to start with and has to be coupled with the drawdown of CH4; otherwise N2 sorbs into coalbeds because of increased pressure in the overall system without an associated decrease in the partial desorption of CH4 pressure. Based on our experimental and modeling experience, analytical and numerical solutions provide a good approximation of the behavior of the multicomponent, multiphase flow of gases in coalbeds. However, much more work is required in understanding the sorption behavior of multicomponent gases and their effects on volumetric strain vis-a-vis their sensitivity to permeability on a variety of coals.