Effect of Rough Surface and Moisture Content on the Competitive Adsorption off CO2/CH4 Mixtures in Orthoclase Nanopores with the Insight for Carbon Storage

Abstract Injecting CO2 into shale reservoirs can enhance shale gas recovery and store CO2 to help mitigate global warming. Orthoclase is a key mineral in shale reservoirs. To understand the competitive adsorption mechanism of CO2/CH4 mixtures in orthoclase nanopores under various conditions, we employed molecular simulations. Using three different orthoclase nanopore models—dry, water-containing, and rough—we studied the adsorption behavior of CO2, CH4 single component gas and their binary mixtures in orthoclase pores with the Grand Canonical Monte Carlo (GCMC) method. We comprehensively considered the effects of temperature (303.15-393.15 K), pressure (0-30 MPa), gas molar volume fraction, water content, and surface roughness on the adsorption characteristics of gas molecules in 3nm orthoclase pores. In dry orthoclase nanopores, the adsorption of CO2 decreases while CH4 adsorption increases with rising temperature and pressure, with CO2 being adsorbed more than CH4. CO2's interaction energy is primarily electrostatic, whereas CH4's is mainly van der Waals, leading to different patterns in their adsorption heat curves. The gas mole fraction has a much greater effect on CO2 in binary mixtures compared to CH4. In water-containing pores, the adsorption capacity of both gas components introduced into the pores of orthoclase is inhibited, with water having a significantly greater inhibitory effect on CO2 than on CH4. At 10 MPa, the adsorption heat of CO2 is 12.684 kcal/mol when the water content is 0, and it decreases to 3.273 kcal/mol when the water content increases to 6%, a decrease of about 74%. Surface roughening increases the retention of CO2 and CH4 in surface "depressions," significantly influencing CO2 storage and CH4 recovery. This study elucidates the mechanism of competitive gas adsorption in orthoclase-rich shale reservoirs, offering theoretical support for the advancement of CO2 storage and natural gas enhanced recovery (CSEGR) technologies.