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Abstract. Shales are abundant source rocks for natural gas in sedimentary basins, and gas recovery from shales has gained increased attention given the global rise in energy demand. Despite advancements in horizontal drilling, hydraulic fracturing, and depressurization to atmospheric conditions, recovery rates for natural gas from shales remain limited to 20-60%. This is primarily due to the strong adsorption of methane CH4 on shale surfaces, highlighting the need for additional stimulation techniques. Enhanced CH4 recovery via carbon dioxide CO2 injection into shales is an appealing method due to the ability of CO2 to displace adsorbed CH4, facilitating its desorption. Experiments under high-pressure and high-temperature conditions have shown that shales adsorb more CO2 isothermally than CH4. Moreover, CO2 adsorption presents the dual benefit of enabling gas recovery while contributing to CO2 emission mitigation. This study employs low-field Nuclear Magnetic Resonance NMR measurements to investigate competitive CO2-CH4 adsorption in high-specific-surface-area shales, supplemented by sodium bentonite, illite-smectite and kaolinite to simulate key shale components. Low-field NMR offers a distinct advantage over gravimetric and volumetric methods by differentiating between relaxations of adsorbed and free gas phases. Furthermore, in CH4-CO2 mixtures, only CH4 molecules are detectable via 1H NMR, which enables precise analysis of CH4 desorption following CO2 injection. The research is conducted in two stages. First, we analyze the NMR relaxation and adsorption effects of single-phase CH4 on geomaterials at high pressure and high temperature. Second, we study the time-dependent CO2-CH4 competitive adsorption through NMR spectral changes in selected geomaterials. These experiments are complemented by Grand Canonical Monte Carlo GCMC simulations of adsorption on clay mineral surfaces resembling the tested geomaterials, incorporating single-phase CH4, CO2-CH4 mixtures, and He-CH4 mixtures to analyze competitive adsorption dynamics. Results reveal that the transverse T2 relaxation response of CH4 gas in geomaterials exhibits two relaxation peaks at low pressure (P<~1 MPa) and three relaxation peaks at higher pressures (P=2-to-10 MPa), reflecting Langmuir adsorption and the behavior of bulk CH4 within pores and external to grains. CH4 desorption triggered by CO2 injection is governed by two mechanisms: (1) partial pressure reduction of CH4, a universal response to gas injection, and (2) preferential adsorption of CO2 on clay minerals due to higher CO2 selectivity. A modified kinematic desorption equation is proposed to account for the residual adsorbed CH4 fraction following CO2 injection. The combined insights from low-field NMR measurements and molecular simulations provide a detailed understanding of competitive gas adsorption at the pore scale, advancing the knowledge needed to optimize enhanced CH4 recovery techniques.
| Country | United States |
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