Gas and water performance from the full-cycle of coalbed methane enrichment-drainage-output: A case study of Daning-jixian area in the eastern margin of Ordos Basin

Abstract

Coalbed water (CBW), as a stratigraphic fluid, is stored and transported together with coalbed methane (CBM), and has an important influence on the generation, transport, enrichment and production patterns of CBM. In order to study the interaction mechanism between CBM and CBW in the full cycle of CBM enrichment-drainage-output, we collected CBW samples from typical wells at different production times and development data of CBM wells in Daning-jixian area in the eastern margin of Ordos Basin. According to the geochemical test results of CBW samples and the main parameters of 5# coal seam, the basic characteristics of CBW in the study area were analyzed. Then, the control of hydrochemical field on CBM enrichment, the influence of hydrodynamic field on CBM drainage, and the connection between CBW evolution and gas production were discussed, respectively. The results show that in the low burial depth area of the 5# coal seam in the monoclinic structure, the ion concentration of CBW is higher, and the Cl–Na coefficient, desulfurization coefficient and carbonate equilibrium coefficient are relatively small, making it a relatively enriched area for CBM. CBM production in high hydrodynamic areas has a greater need for drainage and depressurization, and the average daily gas production is relatively small. The evolution in daily gas production and the hydrochemical characteristics of CBW during CBM drainage show a good correlation. Every increase in gas production is accompanied by an increase in TDS and a decrease in Cl–Na​ coefficient, desulfurization coefficient and carbonate equilibrium coefficient in the extracted water.