A Molecular Dynamics Study on Natural Gas Solubility Enhancement in Water Confined to Small Pores
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AbstractUnconventional natural gas reservoirs are a major source for the global energy market. Unconventional gas reserves generally are considered to consist of a volumetric component, hydrocarbons stored in the pore space as free gas, and a surface component, gas adsorbed on the large surface area of these microporous systems. A second volumetric component, gas dissolved in the formation water, is generally not considered. For such reservoirs, free gas is quantified by modifications of standard reservoir evaluation methods. Physically adsorbed gas has generally been quantified from laboratory studies to establish the equilibrium adsorption isotherms. Based on bulk solubility calculations, the amount of dissolved gas in water, however, has not been considered important. Our group has recently performed a theoretical study which, for reasonable values of its parameters, predicts that methane solubility is enhanced in microporous media. The implication of this is that the dissolved gas in the formation water may represent a significant portion of the total gas reserves in unconventional reservoirs. Best quantification of this result and insight into this solution chemistry problem should ideally come from theoretical studies at the pore-scale, particularly from molecular-level simulations and the subsequent free energy change calculations accompanying dissolution. In this paper, we examine natural gas solubility enhancement in water residing in a graphene slit-pore with a length-scale on the order of ten nanometers, namely a micropore, using an analytical approach and molecular dynamics simulations. The solubility of methane in water is estimated under controlled temperature condition using the test particle insertion method with the excluded volume map sampling. The results indicate that the methane solubility in the pores is improved significantly (typically, one order of magnitude larger), compared with that in the bulk phase, i.e., in the absence of pore walls. The enhancement is further investigated under varying force-field conditions due to pore-wall surface potential, namely hydrophobicity and found that the solubility is extremely sensitive to small pore wall wettability. The work is a fundamental approach to better understand the unconventional gas reservoirs, and important for the estimation of gas reserves.
