Geochemical characterization of gas-water output from deep coalrock methane wells in the Ordos Basin and its geological responses
), Abstract
Investigating the geochemical characteristics of gas and water produced from deep coalrock methane wells is significant for understanding the enrichment mechanisms and development patterns of coal rock mathane. Focusing on nine deep coal rock mathane horizontal wells along the eastern margin of the Yishan slope in the Ordos Basin, we examine the components and carbon isotopes in the produced gas, as well as the conventional ions and hydrogen and oxygen isotopes in the formation water to identify the genetic types of coal rock mathane, the gas fractionation effect in the process of coal rock mathane production, and the sources of the produced water. The results show that the coal rock mathane in the study area is of middle- to late-stage thermogenic gas with methane (CH4) as the main component while heavy hydrocarbons and non-hydrocarbon gases as the minor components, exhibiting positive carbon isotope sequence. The methane carbon isotope value (δ13C1) of the produced gas during coal rock mathane production can be used to assist in the calibration and determination of the production stages of free and adsorbed gases and presents a relatively large lightening trend as the coal rock mathane production goes on. Specifically, the produced gas is dominated by free gas initially and then exhibits both desorption and fractionation of adsorbed gas in the late stage. This finding aligns with the change pattern in the δ13C1 of the pressure-retaining free gas and initially desorbed gas releasing stages of the gas content test. A longer period of high-to-low transition of δ13C1 is associated with a higher possibility of high coal rock mathane yield. The water produced from a typical coal rock mathane well exhibits a total dissolved solids (TDS) of up to 193.08 g/L, suggesting a calcium-chloride (Ca-Cl) type water. Such water consists primarily of connate water trapped in sediments. Its low Na/Cl ratio, desulfurization coefficient, and Mg/Ca ratio, along with high metamorphism and salinization coefficients, indicate excellent stratigraphic sealing performance and favorable coal rock mathane preservation conditions. The hydrogen and oxygen isotope values of the produced water deviate from the local meteoric water line (LMWL), largely exhibiting significant oxygen (18O) shifts. This is inferred to be influenced by recharge with water from the roof and floor strata of coal seams in a high-temperature stratigraphic environment.
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References
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