Microbial nutrient stimulation enhances in situ methanogenesis ~ 900 m depth in coalbeds: evidence from long-term field trial in Hancheng Block
摘要
To enhance in-situ bioconversion of coalbed methane by microbial nutrient solution, two long-term operating coalbed methane wells in the Hancheng block of Shaanxi Province, China (up to 2010) were employed as the research objects. The thickness of the main coal seam is approximately 26–115 m, the salinity is appropriate (3,000–12,000 mg/L), and the permeability is low (0.001–3.503 md), which is suitable for large-scale coalbed methane exploration. After nutrient solution (660 m3) was injected, the pH value gradually increased from 6.5 to 7.5 (100th day) and then slowly decreased to 7.0 (300th day) as the accumulated VFAs were rapidly utilized by the inoculated bacteria. The bottom hole pressure recovery rate after injection was 0.37–0.95 MPa (15th day), which was better than 0.7–0.8 MPa (40th day) before injection. The microbial community structure indicated that the bacteria in both wells were more dominant, while the archaea in the test well increased to 6.3%. The relative abundance of Methanobacterium formicicum increased from 0.25% to 4.14%. At the species level, the dominant microorganisms in the control well were Ralstonia pickettii and Variovorax sp., while those in the test well were Stutzerimonas stutzeri and Variovorax paradoxus. The typical denitrifying microorganisms Stutzerimonas stutzeri and Comamonas sp. were significantly increased by 14.29% and 5.53%. The abundance of Ralstonia pickettii decreased by 18.41%, and the methanooxidizing bacteria genus accounted for 15.89%. Gene function enrichment analysis indicated that compared with the phenylpropionate degradation mode of the control group, the metabolic mode after microbial intervention shifted to pyruvate metabolism.