Effect of Water Salinity on Coal Wettability During CO2 Sequestration in Coal Seams

One of the most effective ways to reduce carbon dioxide (CO2) emissions is sequestration in geological reservoirs. CO2 sequestration in coal formations enhances the methane production from coalbeds (ECBM) in addition to storing CO2. CO2 can be stored in coalbeds in three ways: free gas phase in the pore space or the cleat network system, adsorbed molecules onto the organic surface of the coal, and dissolved in the groundwater within the coal. The performance of this process is dependent on the wettability behavior of the coal-water-CO2 system. This paper presents an investigation of the effect of the formation water salinity on the wettability behavior of highly volatile bitumen coal. The captive bubble method was used to measure the contact angle in coal-water-CO2 systems at pressures up to 2000 psi. The contact angle (CA) was measured at different NaCl concentrations (0-20 g/L). The CO, adsorption isotherm on the coal surface was examined at different water salinities (0-20 g/L NaCl). Zeta-potential measurements were conducted to understand the effect of salt concentration on coal hydrophobicity. The contact-angle measurements showed that as the pressure increased, the coal wettability inverted from water wet to gas-wet. The contact angle increased from 61 degrees at atmospheric pressure to 123 at 2000 psi, and the wettability altered from water-wet to CO2-wet at pressures around 400 psi. The coal became more CO2-wet as the NaCl concentration increased. Furthermore, the adsorption isotherm and the zeta-potential measurements confirmed that the coal became more hydrophobic at high NaCI concentrations. The CO2 adsorption onto the coal surface increased as the salt concentration increased, whereas the absolute zeta potential value decreased. On the basis of these observations, the injection of CO2 into highly volatile bitumen coal seams for CO2 sequestration and ECBM purposes is more efficient as the salt concentration increases.