Geochemical characterization and interference determination of produced water in coalbed methane wells of Panhe Block

LI Ruiteng; GUO Chen; WANG Fei

doi:10.13347/j.cnki.mkaq.20240426

Abstract：The geochemical characteristics of produced water from coalbed methane(CBM)wells carry abundant geological and production information,which plays a significant role in revealing the hydro-chemical evolution of coal reservoirs,optimizing pro-duction zone combinations,and predicting productivity potential.To investigate the geochemical features of produced water and their implications for productivity in Panhe Block of southern Qinshui Basin,produced water samples were collected from 10 CBM wells in the study area.These samples were analyzed for conventional ion concentrations,hydrogen-oxygen isotopes(δD,δ18O),pH values,and total dissolved solids(TDS).Integrated analytical methods,including Piper trilinear diagrams,Stiff diagrams,and prin-cipal component analysis(PCA),were employed to systematically analyze the geochemical differences in produced water from dif-ferent coal seams and the hydro-chemical formation mechanisms.Furthermore,the correlations between key hydro-chemical para-meters and the productivity of single-zone production wells and commingled production wells were explored.The results indicate that the produced water in the study area is predominantly of the Na-HCO3 type,weakly alkaline and medium salinity,and atmo-spheric precipitation is the main initial source of produced water.The findings demonstrate that cation exchange adsorption and sulfate reduction are the key water-rock interactions controlling the hydro-chemical characteristics of the produced water.The geo-chemical characteristics of produced water in the coalbed methane wells in Panhe Block are controlled by a combination of factors,and its closure is closely related to the production capacity.The single-zone production wells(predominantly horizontal wells)demonstrate superior productivity due to effective reservoir confinement and significant pressure drawdown.However,some wells(e.g.,Well 5)may connect with aquifers,necessitating targeted water shutoff measures.The commingled production wells(vertical wells)are prone to external water influx during drainage,which suppresses pressure depletion and consequently reduces productivity.The closure index(F)exhibits a non-monotonic relationship with the productivity,with an inflection point at approximately 200,in-dicating that moderate closure conditions are favorable for high production.