Study on the occurrence mechanism of natural gas in sour gas reservoirs based on molecular simulation

Abstract

Sour natural gas, as a significant part of natural gas resources, has vast reserves in China. Its development and utilization are crucial for national energy security and sustainable development. Current research on sour natural gas primarily centers on the competitive adsorption of binary components, which falls short of addressing the evolving needs of multi-component natural gas and the complexities of reservoir physical properties, especially in the study of sour gas reservoirs. This study uses equilibrium molecular dynamics (EMD) simulation to investigate the occurrence mechanisms of sour natural gas containing CO₂, H₂S, and elemental sulfur(S₈) within slit-pore models. The effects of pore width, gas composition, sulfur content and reservoir water saturation on gas occurrence mechanisms were explored. The study reveals that in the calcite slit, the overall adsorption capacity of gas components follows the order CO₂ ≈ H₂S > CH₄, and the competitive adsorption mechanism will lead to the increase of sour gases such as CO₂ and H₂S during the pressure relief mining process (When the pressure is < 20 MPa, i.e., a decrease of more than 75 %), so on-site monitoring should be strengthened. As temperature increases, CH₄ partially replaces CO₂ at adsorption sites, and adsorption first increases with pressure before decreasing, peaking at 5–10 MPa. Lowering the abandonment pressure is recommended to enhance CH₄ recovery. The variation in sour gas composition has a minimal impact on CH₄ adsorption in the pores. However, with increasing sulfur content and water saturation, sulfur clusters displace CH₄ from adsorption sites, and water molecules preferentially occupy adsorption sites, forming a water film. Regarding adsorbents, CH₄ is more easily extracted from pores > 3 nm, and for larger pores, abandonment pressure should be <10 MPa. This research holds significant implications for optimizing extraction strategies and enhancing recovery in sour gas reservoirs.