煤层气井产出水溶解无机碳特征及其地质意义

Characteristics of dissolved inorganic carbon in produced water from coalbed methane wells and its geological significance

对贵州西部20口煤层气井产出水溶解无机碳(DIC)稳定碳同位素(δ13CDIC)进行了长期动态跟踪,重点分析了多层合采煤层气GP井组δ13CDIC的时空动态,探讨了δ13CDIC值与煤层气产能的关系;选取典型井产出水进行了微生物16S rDNA扩增测序分析,提出了煤层气多层合采产出水δ13CDIC地质响应模式。研究表明:中煤阶煤层气井产出水δ13CDIC正异常普遍,产出水中包含15种以上的甲烷菌属,Methanobacterium为优势属;产出水中优势甲烷菌属序列数目与δ13CDIC值显著正相关,δ13CDIC值正异常由产甲烷菌还原作用造成,且主要为氢营养型甲烷菌还原作用;多煤层煤系沉积相及岩性的分段性会造成渗透性和富水性的分段性,从而控制产出水δ13CDIC值和古菌群落的分段性,渗透性和富水性较好的层段,产出水δ13CDIC值正异常显著,古菌属主要为Methanobacterium;渗透性和富水性较弱的层段,产出水δ13CDIC值较小,微生物作用较弱;接近煤层露头的较浅部位,容易受到大气降水的补给,产出水δ13CDIC值较小;中煤阶多煤层煤层气井产出水δ13CDIC地质响应模式揭示了多层合采煤层气井产出水中δ13CDIC差异的地质机理和微生物作用机理,为沉积相控制的叠置流体系统提供了有效的地球化学证据,也可用于多层合采煤层气井产层气、水贡献分析。

Based on long-term dynamic tracing of dissolved inorganic carbon(DIC) and stable carbon isotope(δ13 CDIC) in produced water from 20 coalbed methane(CBM) wells in western Guizhou, the spatial-temporal dynamic variations of δ13 CDIC of the GP well group produced in multi-layer commingled manner were analyzed, and the relationship between the value of δ13 CDIC and CBM productivity was examined. The produced water samples of typical wells in the GP well group were amplified and sequenced using 16 S r DNA, and a geological response model of δ13 CDIC in produced water from CBM wells with multi-coal seams was put forward. The research shows that: δ13 CDIC in produced water from medium-rank coal seams commonly show positive anomalies, the produced water contains more than 15 species of methanogens, and Methanobacterium is the dominant genus. The dominant methanogens sequence numbers in the produced water are positively correlated with δ13 CDIC, and the positive anomaly of δ13 CDIC is caused by reduction of methanogens, and especially hydrogenotrophic methanogens. Vertical segmentation of sedimentary facies and lithology in stratum with multi-coal seams will result in permeability and water cut segmentation, which will lead to the segmentation of δ13 CDIC and archaea community in produced water, so in the strata with better permeability and high water cut, the δ13 CDIC of the produced water is abnormally enriched, and the dominant archaea is mainly Methanobacterium. In the strata with weak permeability and low water cut, the δ13 CDIC of the produced water is small, and the microbial action is weak. The shallow layer close to the coal seam outcrop is likely to be affected by meteoric precipitation, so the δ13 CDIC of the produced water is smaller. The geological response model of δ13 CDIC in produced water from multi-coal seams CBM wells in the medium-rank coal reveals the geological mechanism and microbial action mechanism of the δ13 CDIC difference in the produced water from the multi-coal seams CBM wells. It also provides effective geochemical evidence for the superimposed fluid system controlled by sedimentary facies, and can also be used for the contribution analysis of the produced gas and water by the multi-layer CBM wells.