Effects of Water Salinity, CO2 Solubility, and Gas Composition on Coal Wettability

Abstract Geological sequestration of CO2 in coal seams has become an attractive carbon sequestration technology for two reasons. First, the injection of CO2 or mixtures of CO2 and N2 enhances methane production from coalbeds (ECBM). Second, it reduces the effect of global warming by storing CO2. The sequestration of CO2 in Tiffany and Allison fields in the San Juan Basin is an example of this technology. The efficiency of this process is strongly dependent on the wetting behavior of the coal–water–CO2 system and the CO2 diffusion rate from the cleat network, through the micro-cleats, and to the matrix surface, which is improved if the coal is CO2 wet. This study aims to investigate the effects of the salt type and concentration of coal seam water, the solubility of CO2 in water, and the injected gas composition on the wettability of a highly volatile bitumen coal with 82 wt% carbon content from Bull Hill, Oklahoma. Contact angles in the coal–water–CO2 system were measured using a captive bubble method at pressures up to 1000 psi. Adsorption isotherm measurements were conducted to confirm the effect of salt concentration on the CO2/coal surface adsorption behavior. The zeta potential was measured to examine the effects of the salt concentration on coal hydrophobicity. The results showed that the coal tended to be CO2 wet as the pressure increased where the contact angle changed from 80° at atmospheric pressure to 125° at high pressure (1000 psi). It was found that the salt concentration increased the coal wettability to the CO2. NaCl increased the contact angle values from 125° to 140° with a NaCl concentration of 15 g/L. It was found that MgCl2 and CaCl2 slightly increased the contact angles. These results were confirmed by measuring the adsorption isotherm and the zeta potential at different water salinities. The CO2 adsorption was improved as the salt concentration increased and the absolute zeta potential value decreased, especially in NaCl cases. It was found that the contact angles of flue gases were generally smaller than those of CO2 and the contact angles did not exceed 90° even at high pressures and high N2 contents (81° at 1000 psi for 100% N2). The solubility of CO2 in water did not affect the stabilized contact angle values but it minimize the stabilization time as the solubility increased. Based on these observations, for CO2 storage and ECBM purposes, injection of CO2 into highly volatile bitumen coal seams is more efficient as the salt concentration content increase. However, other types of coals might show different behaviors due to their varying rank and mineral compositions.