鄂尔多斯盆地深部煤岩气井产出气-水地球化学特征及其地质响应

Geochemical characterization of gas-water output from deep coalrock methane wells in the Ordos Basin and its geological responses

黄道军 周国晓 杨兆彪 顾俊雨 荆雪媛 王嘉楠

Daojun HUANG Guoxiao ZHOU Zhaobiao YANG Junyu GU Xueyuan JING Jianan WANG

研究深部煤岩层气井产出气-水的地球化学特征，对认识煤岩气富集机理和开发规律具有重要意义。以鄂尔多斯盆地伊陕斜坡东缘9口深部煤岩气水平井为研究对象，开展了产出气气体组分、碳同位素及地层水常规离子以及氢、氧同位素特征研究，查明了煤岩气的成因类型、排采过程中的气体分馏效应以及产出水的来源。结果表明：① 研究区煤岩气为中-晚期热成因气，组分构成以甲烷（CH）为主，重烃及非烃气体含量较低，碳同位素表现为正碳同位素组成序列。② 排采过程产出气中的甲烷碳同位素值（δC）可辅助标定并划分游离气和吸附气产出阶段，甲烷碳同位素随着排采天数的增加而较大幅度地变轻，具体表现为初期以游离气为主，后期吸附气解吸、分馏叠加，与含气量测试中保压游离气-初始解吸气释放阶段的δC变化规律吻合，甲烷碳同位素由重变轻过程持续时间越久，煤岩气高产可能性越大。③ 典型煤岩气井产出水矿化度（）高达193.08 g/L，为钙-氯型（Ca-Cl型）水，以原始沉积水为主，低钠氯系数、脱硫系数、镁钙系数和高变质系数、盐化系数反映地层封闭性和煤岩气保存条件较好。④产出水氢、氧同位素值偏离大气降水线，大部分表现为明显的氧漂移（O漂移），推测受到了高温地层环境下煤层顶、底板岩层水补给的影响。

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 （CH） 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 （δC） 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 δC 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 δC 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 （） 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 （O） 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.

10.11743/ogg20240609