| Coalbed methane is an unconventional natural gas resource which is mainly stored either by compression in nature fractures and pores or adsorption on the surfaces of the solid material in coalbeds.As a clean and efficient fossil energy source,CBM has abundant reserves in my country.The economic and efficient development of coalbed methane reservoirs is of great significance for ensuring my country’s energy security,optimize energy structure and promote economic development.Compared with conventional gas reservoirs,coalbed methane reservoirs are quite different in terms of geological characteristics and fluid flow mechanism.The pore structure of coal reservoirs is complex,the pores are multi-scale,and the mechanism of occurrence and migration of coalbed methane is complex.As a result,the seepage theory and development technology of conventional gas reservoirs cannot be directly used to guide the effective development of coalbed methane reservoirs.With the continuous development of hydraulic fracturing technology,the mining methods of fractured vertical wells and fractured horizontal wells have been widely used in the mining of coalbed methane.Due to the complex seepage mechanism of coalbed methane in the reservoir,the research is currently still lagging behind on-site development practices,which leads to certain deviations in the prediction of CBM reservoir development dynamics and fracturing construction design.This paper studies the pore structure characteristics of CBM reservoirs and the mechanism of occurrence and migration of CBM,and determines the mathematics describing coalbed methane adsorption,desorption,diffusion,and seepage laws.On this basis,a dual porosity media coalbed methane reservoir vertical well seepage model is established,combined with modern well test interpretation theories to analyze the characteristics of unstable pressure and production and studied the effect of different diffusion modes,different external boundary conditions,adsorption coefficient,elastic storage ratio,inter-porosity coefficient,dimensionless radius and other influencing factors on the law of the well test curve.Secondly,using the point source solution and the principle of pressure drop superposition,the pressure solution of infinite conductivity vertical fracturing well model of the dual porosity media CBM reservoir is obtained.After the model is solved,the characteristics of unstable pressure and production curve are studied,and studied the effect of adsorption coefficient,elastic storage ratio,inter-porosity coefficient,wellbore storage coefficient on the law of the well test curve.Then,the coalbed methane reservoir model and the finite conductivity fracture model were coupled to establish a dual porosity media coalbed methane reservoir finite conductivity vertical fracturing well model.After the model is solved,the characteristics of unstable pressure and production curve are studied,and studied the effect of adsorption coefficient,elastic storage ratio,inter-porosity coefficient,wellbore storage coefficient,fracture conductivity coefficient,wellbore skin factor on the law of the well test curve.Then,using the point source function method and the mirror reflection principle,the pressure solution of the dual porosity media coalbed methane reservoir horizontal well and a staged fractured horizontal well with the top and bottom closure and infinite boundary conditions were obtained.Then the bottomhole pressure response of staged fractured horizontal wells in dual porosity coalbed methane reservoirs are analyzed,and studied the effect of adsorption coefficient,horizontal well length,horizontal well position,number of fractures,fracture spacing,fracture half-length on the law of the well test curve.Finally,based on the deconvolution well test interpretation method,the unstable pressure and production of different production stages are analyzed by using the actual fracturing coalbed methane wells in Block M,the model has good applicability for mines. |
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