Spatiotemporal Evolution Of The Geochemical Characteristics And Productivity Response Of Produced Water From Coalbed Methane Wells In Fanzhuang Block

Hydrogeological condition is one of the important factors affecting the development of coalbed methane.An in-depth study of the geochemical characteristics of produced water from coalbed methane wells is of great significance to control productivity and optimize the drainage and production system.In this thesis,taking the coalbed methane drainage well in Fanzhuang block as the research object,combined with the multi-year and multi-well site water geochemical test results,the temporal and spatial distribution law of water geochemical characteristics of coalbed methane wells is revealed.Based on the water-rock interaction simulation experiment of coalbed methane drainage and production process,the changing trend of coal sample pore structure and experimental solution under the action of water and rock in the drainage and production process is explored,and the geochemical evolution mechanism of produced water is expounded.Based on the further analysis of the relationship between the geochemical characteristics of produced water and the productivity of coalbed gas wells,three modes of gas production and water production of coalbed gas wells are put forward.The main understandings are as follows:It is found that the hydrogeological conditions of Fanzhuang block are generally beneficial to the enrichment of coalbed methane.There is a strong runoff area in the east and a detention area in the west.The northern boundary is the water blocking section,the eastern boundary is the water diversion section,the eastern part of the southern boundary is the water diversion section,and the western section is the water blocking section.The vertical flow ability of each formation water is poor,mainly in the stagnation area and weak runoff area.The salinity of produced water from coal seam gas wells in Guang Coal seam is relatively high,with an average salinity of 2266.42 mg in 2011 a and an average salinity of 2123.47 mg in 2015 a.The average salinity in 2015 a is 1954.37 mg and 2162.8mg/L in 2017 a.On the plane,with the increase of the number of wells and the recharge of meteoric water,the influence of superimposed structural conditions leads to the transfer of high salinity centers.The main hydrochemical cations are Na⁺+K⁺ions,the main anions are HCO₃^-ions,and the water type is mainly Na HCO₃.Most of the chlorine-sodium coefficients are greater than 1,the average annual desulfurization and deterioration coefficients show an upward trend,and the annual equilibrium coefficients of magnesium chloride,calcium magnesium and carbonate show a downward trend.The simulation experiment of water-rock interaction in the process of coalbed methane drainage and production based on hydraulic flushing is carried out.The results show that the water-rock interaction is the strongest under the condition of 1500 mg/L-0.5 m L/min experiment.With the increase of salinity and flow velocity,the peak areas of micropores,micropores and macropores in coal samples show a downward trend,while the peak areas of mesopore show an upward trend.Micropores and small pores are transformed into mesopore through connectivity,while macropores are decomposed and refilled by cementation,and transformed to mesopore.Coal seam water often shows the characteristics of high Na⁺,HCO₃^-,Cl^-concentration and low SO₄^2-,Ca²⁺,Mg²⁺concentration,which is closely related to a series of water-rock interactions,including dissolution,oxidation,cation adsorption alternation,desulfurization,decarbonation,microorganisms and so on.With the change of ions,the hydrogen and oxygen isotopes also change.By analyzing the geochemical parameters and productivity characteristics of coalbed gas wells,it can be found that there is a good correlation between Ca²⁺+Mg²⁺,SO₄^2-,carbonate balance coefficient,calcium and magnesium coefficient and high production wells.The balance coefficients of Mg²⁺+Ca²⁺,SO₄^2-and carbonate decreased with the increase of daily gas production,while the calcium and magnesium coefficients increased with the increase of gas production.Three modes of gas production and water production in coalbed gas wells in the study area are put forward,namely,low water production and high gas production in synclinal wing,medium/low gas production in karst water with high yield in karst water,and low/no gas production in structural water diversion.The corresponding geochemical characteristics of produced water and the spatial expansion characteristics of the model are different.