Environmental Effects And Control Factors Of Coalbed Methane Production Water In The Songhe GP Well Group In Western Guizhou

The thesis selects 8 CBM development test wells in GP well group of Songhe block in western Guizhou as the research object,and discusses the water quality characteristics,health risk results and geological and engineering control factors of CBM well output water through five years of tracking data of CBM well output water elements,geochemical experiments of coal rock,and geochemical test results of coal rock leaching water.The main understanding is as follows:Firstly,the geochemical characteristics related to the produced water of CBM wells are clarified,and the variation characteristics of elements are discussed on the time and space scales.The conventional anions and cations in the produced water of GP well group in Songhe block have the same characteristics.The produced water and fracturing fluid belong to Cl-Na+K type,and the trace element Ba shows abnormal enrichment in each well.On the time scale,the content of most elements decreased slowly with the drainage,and As and other elements showed a fluctuating rise.On the spatial scale,the content of most elements increases with the increase of buried depth.The produced water is generally polluted by fracturing fluid,and the geochemical characteristics of the produced water in a few wells are close to coal seam water after drainage.The carcinogenic elements Cr and As are common in the well group and the content is high.Secondly,the single factor evaluation method and the fuzzy comprehensive evaluation method were used to evaluate the water quality pollution of the produced water of coalbed methane wells,as well as the characteristics of spatial and temporal changes.A suitable health risk assessment model was established to evaluate the health risk of the produced water.The single factor evaluation method showed that Na~+,Cl~-and Ba in the produced water were heavily polluted,TDS was moderately polluted,and Mn was only slightly polluted in a few wells.It has volatility on the time scale.The results of fuzzy comprehensive evaluation were all V-class water quality and severe pollution.The fuzzy matrix showed that Fe,Mn,Al,Hg and As had different degrees of slight over-standard.On the time scale,it decreases with the drainage and mining,and tends to be gentle in the later period.On the spatial distribution,the evaluation results increase with the increase of buried depth.The health risk model selects the sensitive underground groundwater pollutant exposure scenario by introducing groundwater.The assessment results show that the produced water is almost not dangerous and acceptable;on the time scale,the total health risk R is fluctuating,and some wells rise in the later stage.Except that GP3 well does not pose a danger to the environment,it has the characteristics of increasing with the increase of buried depth on the spatial scale.Thirdly,the geochemical characteristics of coal rock and coal rock leaching water in the study area were clarified,the correlation characteristics between coal rock leaching water and produced water were analyzed,the controlling factors of produced water elements were revealed,and the carcinogenic factors of produced water in the study area were discussed.The content of Fe and Ba in the leaching water is the highest,and the content of Pb is the lowest,which is related to the high content of Fe in coal rock,the low content of Pb and the rapid dissolution of Ba.The element content characteristics of the produced water are the coupling results of the superposition of various geological and engineering factors.It mainly includes geological control factors such as mineral composition and element content,water-rock interaction,burial depth and precipitation in coal and rock,and other engineering control factors such as fracturing fluid,water production and gas production.Among them,geological factors control the types of elements and the content of some trace elements,and engineering factors(mainly fracturing fluid)control the content of conventional anions and cations.In the health risk assessment,Cr and As contributed more to the carcinogenic risk value.The main reason for the deterioration of the health risk assessment results in the middle and late stages of GP3 was the increase of Cr and As content.