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

摘要

Based on long-term dynamic tracing of dissolved inorganic carbon (DIC) and stable carbon isotope (δ13CDIC) in produced water from 20 coalbed methane (CBM) wells in western Guizhou, the spatial-temporal dynamic variations ofδ13CDICof the GP well group produced in multi-layer commingled manner were analyzed, and the relationship between the value ofδ13CDICand CBM productivity was examined. The produced water samples of typical wells in the GP well group were amplified and sequenced using 16S rDNA, and a geological response model ofδ13CDICin produced water from CBM wells with multi-coal seams was put forward. The research shows that:δ13CDICin produced water from medium-rank coal seams commonly show positive anomalies, the produced water contains more than 15 species of methanogens, andMethanobacteriumis the dominant genus. The dominant methanogens sequence numbers in the produced water are positively correlated withδ13CDIC, and the positive anomaly ofδ13CDICis 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 ofδ13CDICand archaea community in produced water, so in the strata with better permeability and high water cut, theδ13CDICof the produced water is abnormally enriched, and the dominant archaea is mainlyMethanobacterium. In the strata with weak permeability and low water cut, theδ13CDICof 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 theδ13CDICof the produced water is smaller. The geological response model ofδ13CDICin produced water from multi-coal seams CBM wells in the medium-rank coal reveals the geological mechanism and microbial action mechanism of theδ13CDICdifference 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.