Geochemical Characteristics And Response Of Produced Water From Cbm Wells In Western Guizhou

In this paper,about 20 CBM development wells are selected as research objects in Western Guizhou.Based on the fluid background of the original formation in the production area of CBM wells,combined with conventional ions,trace elements,hydrogen and oxygen isotopes and dissolved inorganic carbon isotope tests,the time-varying characteristics,spatial evolution characteristics and influencing factors of geochemical indexes of produced water are analyzed.The relationship is discussed between geochemical indexes of produced water from CBM wells.The characteristics that high production wells drain coal seam water and low production wells produce shallow water or fracturing fluid,and its geochemical indexes are revealed.The geochemical response indexes of produced water are put forward,and the identification templates of produced water source and CBM wells productivity are preliminarily established.The main understandings are as follows:The characteristics of fluid geochemical background of the original formation in the production area of CBM wells are clarified.Coal measures strata in the study area are weak in water-rich and poor in hydrodynamic conditions.The ions of Na⁺+K⁺and HCO₃^-are dominant in formation water,and the water quality is Na-HCO₃ type,which belongs to closed hydrological environment.6 synclines and CBM wells generally have high gas content in the study area.The gas content of coal seams is good,mainly consisting of methane,nitrogen and carbon dioxide.The variation characteristics of geochemical indexes of produced water from CBM wells with time are analyzed.The main cations are Na⁺,K⁺,Ca²⁺and Mg²⁺in produced water,which are mainly affected by water-rock interaction;Cl^-and HCO₃^-are the main anions,which are controlled by fracturing fluid backflow and CBM productivity change,respectively.As the drainage time goes on,the conventional ions,TDS,pH,trace elements,hydrogen and oxygen isotopes and dissolved inorganic carbon isotope fluctuate slightly and the water quality remains unchanged.This is because most wells are discharged for more than 2 to 3 years before sampling and are in stable production stage.The produced water sources are preliminarily identified,mainly including coal seam water,shallow water and fracturing fluid.It is found that the indexes of coal seam water are mostly lower than those of other development wells and its chemical properties are more stable.The spatial evolution characteristics of geochemical inedxes of produced water from CBM wells are analyzed and the geochemical spatial evolution model of formation water in coal measures of CBM well is proposed.The horizontal distribution characteristics of Na⁺,K⁺,Ca²⁺,Mg²⁺,Cl^-,TDS,most trace elements and average daily water production are similar in produced water,and there is a positive correlation and burial depth vertically,this is because with the groundwater runoff,dissolution and fracturing fluid retention promote water and rock reaction.The horizontal distribution characteristics of HCO₃^-,pH and average daily gas production are similar,and there is a negative correlation and burial depth vertically,which is due to the difference of gas and water,resulting in the dissolution of CO₂ in shallow CBM.The horizontal distribution characteristics of hydrogen and oxygen isotopes and dissolved inorganic carbon isotope are not obvious.Vertically,the former is positively correlated with the burial depth,which is caused by the enhancement of water and rock interaction with the increase of burial depth.The latter is weakly related to burial depth because it is controlled by both carbonate dissolution and methanogenesis.The identification templates of produced water source are preliminarily established,and coal seam water,slightly polluted water and heavily polluted water are classified.The d'of a universal D drift index and other geochemical indexes with productivity are proposed.Coal seam water discharged from high-yield wells has lower Na⁺,Cl^-,Mn,Rb,Sr,Ba,?D,?¹⁸O content,higher HCO₃^-and Li element concentration,while shallow water or fracturing fluid polluted water discharged from low-yield wells has opposite characteristics.