| The geological conditions of the North China type Permian coal reservoirs in China are very complicated,and the fault block structures in the mining area are extremely developed.The Taiyuan Formation and Ordovician karst water in the floor of No.21 coal seam are mainly mined and the water head is high and the water volume is large.How to accurately control the hydraulic fracturing time and prevent the hydraulic fracture from blindly extending to the water-filled fault in front has always been a technical problem to be solved in the coalbed methane exploitation field.Because there are a large number of primary fractures in coal and rock,hydraulic fracturing is a coupling process of fracture seepage,rock creep and fracturing fluid filtration.However,the traditional hydraulic fracturing research models focus on relatively dense coal body,and the research on fractured coal reservoir is less.In view of this,aiming at the actual hydraulic fracturing project of fractured coal reservoir in Jiaozuo mining area,taking the reservoir mechanical characteristics as the breakthrough point.This paper carries out the corresponding laboratory test and theoretical research from the aspects of primary fracture development,coal rock creep characteristics,reservoir fracture pressure,hydraulic fracture control parameters and hydraulic fracture extension mechanism,and establishes the hydraulic fracturing model of fractured coal reservoir considering creep characteristics In order to achieve the purpose of controlling the accurate extension of hydraulic fractures.Firstly,combined with the triaxial creep test results of coal and rock in Jiaozuo mining area,considering the fracture of actual coal and rock,the fracture element is introduced and connected in series with the generalized Kelvin model and nonlinear viscous element(N I Y body),the creep process of coal and rock is re described,and the seven element nonlinear creep model is established.The results show that the creep model can well represent the whole creep process of coal and rock.Secondly,by analyzing the stress state of coal and rock around the vertical wellbore,considering the secondary stress transformation of the borehole stress and the seepage volume force produced by the fracturing fluid penetration,based on the fracture strength criterion of fracture mechanics,the fracture pressure model of fractured coal reservoir is established;considering the damage phenomenon of stress reduction caused by the primary fractures of the reservoir,the damage constitutive equation of coal and rock is determined.Combined with the fracture pressure model,the hydraulic fracture extension pressure model of fractured coal reservoir is established.Finally,based on the maximum strain failure criterion,creep model and fracture pressure model are combined to establish the calculation model of fracture time t1 of coal reservoir considering creep characteristics;by analyzing the diversion direction of fracturing fluid in the process of hydraulic fracture extension,based on the hydraulic fracture extension pressure model and the principle of fracturing fluid volume conservation,the hydraulic fracture extension law is discussed,and the hydraulic fracture extension model is established The extension time t2 calculation model.Considering the generation and extension process of hydraulic fracture,the total time of hydraulic fracture can be divided into coal reservoir fracture time and hydraulic fracture extension time,namely T=t1+t2.Combined with the reservoir parameters of four test wells in Jiaozuo mining area and the measured data of hydraulic fracturing operation,the coal reservoir fracture time t1 and hydraulic fracture extension time t2 are calculated quantitatively,and compared with the actual construction data,it is found that the two are more consistent,and the error is within the allowable range,so as to verify the correctness of the hydraulic fracturing time model. |
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