贵州西部地区煤层气井产出气地球化学特征

Geochemical characteristics of produced gas by CBM wells in western Guizhou

基于贵州西部17口煤层气井不同排采时间的产出气,通过对气体组分、甲烷碳氢同位素测试以及产能数据的收集,结合研究区水动力条件以及地质特征,研究了该区域煤层气地球化学特征、地质控制因素及与煤层气产能的关系。结果显示:研究区煤层气组分以甲烷为主,属于干气~特别干的气体,甲烷组分体积分数介于91.504%~99.508%,其次为N2和CO2,不同地方煤层气井重烃含量变化大。甲烷稳定碳同位素δ13C1值介于-44.1‰^-27.8‰,δD值介于-196.5‰^-120.8‰,属于热成因气,松河GP井组和大河边Z-1井接近于原生煤层气的特征,研究区东北部织金ZJ井组及其余各井明显受到运移-扩散次生作用的影响。贵州西部煤层气井产出气的甲烷碳氢同位素值呈现东北高西南低的分布趋势,煤变质程度对产出气的地球化学特征具有主要的控制作用,其中甲烷氢同位素值的分布亦受到沉积环境的变化控制。在同一煤层气井组内部,甲烷的碳氢同位素值大小受井组内水动力条件的变化以及开发煤层埋深的影响较大。甲烷碳氢同位素值与日产气量呈现负相关关系,其本质在于甲烷的碳氢同位素受控于埋深及埋深主导的水动力条件的变化,而埋深的差异性很好的控制了产气能力。煤层气井产出气随着排采时间的变化,煤层气气体组分和甲烷碳氢同位素值呈现一定规律的变化,其中碳氢同位素值随着排采时间变化总体有波动性增大变重趋势,这与组分分馏和同位素分馏有关。

Based on the gas components,methane carbon and hydrogen isotopic tests and productivity data of produced gas samples from 17 coalbed methane(CBM) wells in Western Guizhou,the hydrodynamic conditions and geological characteristics of the study area are combined,the geochemical characteristics of CBM,the geological control factors and their relationship with CBM productivity are studied in the study area.Research shows that methane is the main component of CBM in the study area,which belongs to dry gas^extremely dry gas.The volume fraction of methane is between 91.504% and 99.508%,followed by N2 and CO2.The heavy hydrocarbon content of produced gas by different CBM wells varies greatly.The δ13C1-CH4 values are between-44.1‰ and-27.8‰,and the δD-CH4 values are between-196.5‰ and-120.8‰,indicating that methane is the thermogenic gas.The GP well group and Z-1 well have the characteristics of primary CBM,and ZJ well group and other wells in the eastern part of the study area are obviously affected by the secondary action of migration and diffusion.The δ13C1-CH4 and δD-CH4 values of produced gas by CBM wells in western Guizhou tend to be high in the northeast and low in the southwest.The metamorphic degree of coal plays a major role in controlling the geochemical characteristics of the produced gas.The distribution of δD-CH4 values is also controlled by the change of sedi-mentary environment.In the same coalbed methane well group,the δ13C1-CH4 and δD-CH4 values are greatly affected by the change of hydrodynamic conditions in the well group and the buried depth of coal seam.Meanwhile,the δ13C1-CH4 and δD-CH4 values are negatively correlated with the daily gas production,the essence of which is that the methane carbon and hydrogen isotopic are controlled by the diversity of buried depth and hydrodynamic conditions dominated by depth.With the change of production time,the composition of coalbed methane and the δ13C1-CH4 and δD-CH4 values show a certain regular trend,while the δ13C1-CH4 and δD-CH4 values fluctuate and become heavier,which is related to component fractionation and isotope fractionation.