Geochemical characteristics and geological significance of produced water from coalbed methane wells in Junlian,Sichuan

GU Junyu; YANG Zhaobiao; TIAN Wenguang; HAN Yongsheng; LIU Changqing; LIANG Yuhui; LU Benju; LI Cunlei; PENG Lisha; WANG Haichen

doi:10.13225/j.cnki.jccs.2024.0098

Abstract：The geochemical characterization of coalbed methane(CBM)well drainage water contains rich information on the source of formation water,which can reflect the conditions of CBM preservation,and is of great significance to the study of CBM enrichment mechanisms and exploration and development.Taking 19 typical CBM wells in 9 groups of the Junlian block at the southern margin of the Sichuan Basin as the research objects,experimental tests of conventional ions,trace elements,hydrogen and oxygen isotopes of produced water were carried out,and the source of produced water in CBM wells was discussed in combination with the variation law of produced water conventional ions with drainage time.The results show that the Na++K+ratio in the produced water of coalbed methane wells in Junlian block is more than 90％,and the TDS is 1 203～143 099 mg/L,the Cl-is 709～85 700 mg/L,the SO42-is 12～3 614 mg/L,and the HCO3-is 24～4 516 mg/L,which is unique characteristics of"high TDS,high Cl-,high SO42-,and low H CO3-".During the ten-year dis-charge period,Cl-showed a fluctuating decrease;SO42-was more stable at about 20-25 mEq/L in the early stage and began to fluctuate around 1 500 d;and HCO3-was the lowest.There are mainly three types of produced water in Junlian CBM wells:primary sedimentary water of marine origin,infiltrated water recharged by atmospheric precipitation,and mixed water,and the change of produced water shows a mixing process from infiltration of atmospheric precipitation to primary sedimentary water in both time and space scales.With the infiltration of atmospheric precipitation,the water type of produced water changes from Ca/Mg-Cl type to Na-SO4 type and then to Na-HCO3 type;the content of trace elements decreases gradually,among which Sr and B elements are more sensitive to the source of the produced water;the hydrox-ide isotope value of the produced water decreases gradually,and the hydroxide isotope fitting line is gradually close to the meteoric water line.Various geochemical indicators show that well G1-2,which is located in the axial part of the broad and gentle syncline,are better closed and produce mainly marine sedimentary water,while well G3,which is located in the flank part of the anticline,is poorly closed and the source is mainly atmospheric precipitation,and the rest of the wells are in between,which are a mixture of paleo-seawater and atmospheric precipitation.