Diffusion mechanisms of coal-measure helium and their implications for helium accumulation: A case study of the Taiyuan Formation, northern Qinshui Basin, China

Abstract

ObjectiveThe global helium-rich gas reservoirs are dominated by conventional natural gas reservoirs. In recent years, helium-bearing natural gas has been discovered in the Carboniferous-Permian coal measures in the Ordos Basin and surrounding regions, revealing the resource potential of coal-measure helium. However, there is almost a total lack of studies on mechanisms underlying the diffusion and migration of coal-measure helium. This severely restricts further research on the accumulation patterns of coal-measure helium.MethodsThis study investigated a coal seam in the Taiyuan Formation in the northern Qinshui Basin and mudstones in the coal seam roof. Using physical diffusion experiments on coal-measure helium under different water saturations and gas concentrations, as well as the simulation of the regional sedimentary, burial, and hydrocarbon generation histories, this study calculated the helium diffusion fluxes in the coals and mudstones. Accordingly, the mechanisms behind the enrichment of coal-measure helium gas were determined.Results and ConclusionsUnder experimental temperature (20 ℃) and pressure (1 MPa), helium diffusion was dominated by Knudsen diffusion in pores with sizes less than or equal to 13.5 nm, with similar diffusion rates observed in the coals and mudstones. In contrast, helium diffusion shifted to Fick diffusion in pores with sizes greater than 13.5 nm. The diffusion of coal-measure helium gas was primarily affected by pore structure, water content, and gas concentration. Poorer pore connectivity, higher water saturation, and a lower helium concentration corresponded to slower helium diffusion and a higher sealing capacity. The coals exhibited a developed cleavage-fracture system, which contributed to high connectivity. As a result, the coals had a higher helium diffusion coefficient (1.1 × 10−8 m2/s) than the mudstones (5.8 × 10−9 m2/s). High water content reduced the helium diffusion rate by blocking pore pathways and changing the behavior of the gas-liquid interface. The influence of the gas concentration on the helium diffusion rate was principally related to both the partial pressures of gases and differences in physical properties between helium and methane. In combination with the physical and numerical simulation results, this study established a prediction model for the helium diffusion flux throughout geological history. The prediction results indicate that the helium diffusion fluxes in the coals and mudstones were 13.24 cm3/m2 and 5.05 cm3/m2, respectively under standard temperature and pressure conditions. Furthermore, the helium diffusion occurred predominantly in the early hydrocarbon generation stage with high gas concentrations, while the amount of helium escaping in the later stage can be ignored. Given the relatively weak helium generation capacity of the coal measures, the enrichment and accumulation of coal-measure helium resulted from the dynamic balance between exogenetic recharge and preservation. Deep faults facilitate the upward migration of deep helium. Meanwhile, the low gas concentration gradient, high water saturation, and low connectivity enhance the helium storage capacity by suppressing diffusion. Therefore, it is necessary to highlight the connectivity of deep fault systems and the sealing performance of cap rocks in the exploration of coal-measure helium.

Keywords

• coal-measure helium
• diffusion mechanism
• diffusion flux
• enrichment mechanism
• prediction model