Prediction Of Water Inflow Of Coalbed Methane Vertical Well In Fractured Reservoir

Reservoir of coal and rock in northern China,especially the Yellow River basin is particularly serious,underground water disasters in the carboniferous and the ordovician karst water development,water,high water head,and each aquifer hydraulic contact closely,hydraulic fracturing technology under the condition of groundwater environment protection faces huge challenges,the construction of a high precision of coalbed methane well water inflow forecast model has very important practical significance.In the past hydraulic fracturing models,coal rock was simplified as dense rock mass,and the influence of primary fractures in coal rock reservoir on hydraulic fracturing behavior was ignored,and the influence of changes in mechanical permeability characteristics of reservoir on water inflow under fracturing and extraction conditions was not fully considered in the existing studies.Based on this,this article in jiaozuo mining area of henan province as an example,in the full collection of hydrogeological conditions and water inflow in the study area,on the basis of the measured data,based on elastic mechanics,fracture mechanics,hydrogeology and groundwater dynamics theory,in-depth analysis of the hydraulic fracturing process,considering the primary fracture geometry characteristic and reservoir fracturing permeability reservoir under the condition of extraction from mechanics properties change rule,A series of researches on the prediction of water inflow in coalbed methane vertical Wells are carried out.The stress field theory was used to analyze the stress state of coal rock around the vertical wellbore.Considering the secondary stress reconstruction of perforating stress and the permeable volume force generated by fracturing fluid permeability,the fracture initiation pressure model of fractured coal rock reservoir was established by introducing the geometric characteristic parameters of primary fractures based on the fracture mechanics crack strength criterion.Combined with the influence of stress damage on the deformation of coal and rock body in the process of hydraulic fracture extension in fractured reservoir,the damage constitutive relation of coal and rock body is established.Based on the fracture initiation pressure model,the calculation formula of hydraulic fracture extension pressure in fractured reservoir is established.Based on the classical PKN hydraulic fracture propagation model,an improved hydraulic fracture propagation model for fractured reservoirs was established based on the conservation law of fracturing fluid volume in fractures,and the relationship between hydraulic fracture length L and fracturing time was obtained.Considering the nonlinear pressure drop law of net pressure in hydraulic fracture in the process of pumping and combining with the influence of the change of pressure gradient of coal and rock reservoir on the geometric size of primary fracture and hydraulic fracture in coal and rock reservoir during the process of pumping and mining,the relationship between the width of hydraulic fracture W and the time of pumping and mining is obtained.According to the n-S equation of fixed viscosity coefficient and density inside the fracture,and combined with the dynamic variation law of hydraulic fracture width,the dynamic permeability coefficient expression of coal and rock reservoir is deduced.Under certain boundary conditions and initial conditions,based on the theory of groundwater dynamics and well flow theory,the equation of fixed solution problem of groundwater movement in coal rock reservoir is established,and then the prediction model of water inflow of coalbed methane vertical well is established by the analysis of fixed solution problem.The model is used to calculate the water inflow of four coalbed methane vertical Wells in the study area during the test period,and compared with the actual production data,the average water inflow difference is about 1.95%～5.96%,and the two are basically consistent,which verifies the correctness of the prediction model of water inflow of coalbed methane Wells.Then the water inflow of four CBM experimental Wells in the production period is predicted and the factors affecting the water inflow of CBM Wells are analyzed.