Coupling effect of water-rock in the process of coalbed methane extraction and its response to production capacity-Fanzhuang block in Qinshui basin as an example

WANG Bo; XU Fengyin; ZHANG Aoxiang; WANG Lilong; WANG Wenlong; LI Zhi; QU Zhenghui; WANG Shuo; SHI Mingjian

doi:10.13225/j.cnki.jccs.DM25.0251

Abstract：To clarify the water-rock coupling mechanism and its influence on productivity during Coalbed Methane(CBM)extraction,a series of hydraulic-flushing-based simulation experiments were conducted on different types of pro-duction wells following similarity principles.The variations in ionic composition of produced water before and after the experiments were systematically analyzed.By integrating field production data,the response relationship between the geo-chemical characteristics of produced water and CBM productivity was elucidated.The key findings are as follows:The ex-tent of water-rock interactions varies under different salinity levels and flow rates.Higher salinity and lower flow rates en-hance water-rock reactions,leading to increased salinity and pH in the effluent.The maximum salinity increment was ob-served at 2 000 mg/L and 0.3 mL/min.Overall,Na++K+and Cl-concentrations increase with rising initial salinity.Specif-ic salinity and flow rate ranges promote the release of Ca2+and Mg2+,whereas elevated salinity and flow rates suppress HCO3-generation.When salinity and flow rate exceed a critical threshold,SO42-becomes significantly enriched.The high productivity of CBM wells is co-controlled by the coupling effect of initial salinity and drainage conditions.In the Fan-zhuang block,high-yield wells correspond to a salinity range of 1 800-2 200 mg/L,mostly exhibiting medium-to-low wa-ter production rates,under which the simulated experimental conditions demonstrated the most intensive water-rock inter-actions.A strong positive correlation exists between Ca2+concentration in produced water and gas production in the No.3 coal seam.However,the productivity response mechanism of Ca2+differs under varying water-gas production regimes.In high-yield water and medium gas production mode,Ca2+release is predominantly governed by water-rock interactions in the underlying sandstone aquifer.Conversely,in low-yield water and high gas production mode,Ca2+mobilization is primarily driven by calcite dissolution within the coal matrix.