He Ruiyu1, Liu Xiaoqiang1*, Li Meijun2, Luo Qingyong2, You Bing2
(1 College of Earth Sciences and Engineering, China University of Petroleum‑Beijing at Karamay, Xinjiang 834000, China; College of Geosciences, China University of Petroleum (Beijing), Beijing 102249, China)
Abstract: Helium is a critical strategic resource in China, and dissolution and exsolution in formation water play important roles in helium migration, preservation, and enrichment. He and CH4 coexist in natural gas, and their competitive dissolution and differential occurrence in formation water are jointly controlled by gas composition and burial-related temperature-pressure conditions. High-salinity CaCl2-type formation water is widely developed in the Dengying Formation of the Weiyuan Gas Field, Sichuan Basin, yet the molecular mechanisms governing He-CH4 competitive dissolution remain poorly understood. In this study, a CaCl2-type formation water-CH₄-He molecular model was constructed based on measured formation-water compositions. Molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) simulations were conducted over burial depths of 1000-7000 m using a geothermal gradient of 2.5 °C/100 m and a pressure gradient of 10 MPa/km to investigate gas solubility, adsorption heat, and competitive adsorption selectivity under different He/CH4 ratios. The results show that He solubility generally increases with burial depth but approaches a dissolution capacity limit beyond approximately 5500 m, whereas CH4 solubility exhibits pronounced fluctuations under coupled temperature–pressure conditions. The adsorption heat of He generally increases with burial depth, while that of CH4 varies nonlinearly. Although CH4 interacts more strongly with formation water than He, the smaller atomic size of He allows it to access microscopic free-volume spaces that are less accessible to CH4, preventing the stronger CH4-water interaction from producing effective He displacement. At He concentrations of 0.5%-50%, the CH4/He adsorption selectivity (SCH4/He) is predominantly below unity, indicating that CH4 cannoteffectively displace He from CaCl2-type formation water. At an extremely low He concentration of 0.1%, SCH4/He increases toward unity and occasionally exceeds 1, indicating enhanced CH4 competition. The selectivity reaches relatively high values near 5500 m, coinciding with the minimum water density and maximum system volume. These results demonstrate that deep CaCl2-type formation water can act as an important medium for He dissolution, transport, and preservation in CH4-rich systems. Subsequent exsolution caused by changes in geological conditions may release dissolved He into the free gas phase and promote secondary helium enrichment. This study provides a molecular-scale basis for understanding helium migration, preservation, and enrichment in the Weiyuan Gas Field.
Keywords: water-dissolved helium; methane-helium fractionation; molecular simulation; temperature-pressure coupling; Weiyuan Gas Field
Author Profile:
First author: He Ruiyu (2003‑), female, master candidate, mainly engaged in molecular simulation of petroleum geochemistry. China University of Petroleum‑Beijing at Karamay, Address: No.355 Anding Road, Karamay District, Karamay City, Xinjiang Uygur Autonomous Region, P.R.China, Postcode: 834000. Tel: 15729957966; E‑mail: 15729957966@163.com
*Corresponding author 1: Liu Xiaoqiang (1987‑), male, Ph.D., Associate Professor, Master’s supervisor. His research mainly focuses on molecular simulation of petroleum geochemistry.
E‑mail: xqliu@cupk.edu.cn
