Capacity Prediction Model Of CBM Under Nonlinear Dynamic Flow Pressure

Production capacity prediction of CBM wells has always been a key technical problem in the field of natural gas industry,and it is also an important theoretical basis for the rational development of China's abundant CBM resources.The key to predict long-term coalbed methane production capacity is to establish a seepage output model which can accurately reflect the coalbed methane flow mechanism in coal and rock reservoirs.Due to the multi-pore structure in coal-rock reservoir,matrix diffusion,fracture seepage,failure deformation of hydraulic fracture and dynamic change of bottom-hole flow pressure,the existing capacity prediction model is mainly based on the basic theory of conventional fluid seepage,and coal-rock has not been fully considered.The coupling effect of various complex factors in the reservoir makes it difficult to accurately reflect the permeability mechanism and output law of CBM under nonlinear flow pressure.In view of this,this paper takes the typical"three soft"mining area of Jiaozuo(soft top,soft coal and soft bottom)as the research object,takes the mechanical properties of coal-bearing strata as the entry point,and takes solving the seepage production law of fracturing target layer as the key point.First of all,the influence of source rock stress around wellbore was deeply analyzed,considering the geometric characteristics of primary fractures and fracturing fluid permeability,the fracture stress model of coal rock body with fracture control parameters was introduced.The influence of stress damage on deformation of coal rock body during fracture extension was analyzed,and the constitutive relation of rock mass damage was established.Based on the assumption of classical PKN model and the nonlinear pressure drop law of net pressure in hydraulic fracture,an improved model of hydraulic fracture propagation in coal-rock reservoir was established.Geometric parameters such as length,width and height of hydraulic fractures in coal and rock reservoirs.Then,based on the hydraulic fracture expansion model of coal and rock reservoir,considering the variation law of coal body porosity with fracturing extraction time,and combining the dynamic variation relationship between permeability and coal body porosity,the dynamic permeability model of coal and rock reservoir was established by mathematical and physical equations.Then,based on the dynamic infiltration model,considering the actual stress situation around the vertical well bore,considering the change law of the gas start pressure gradient under the non-linear seepage condition,using Energy Conservation Equation for Fluid Flow to establish the bottom hole dynamic flow pressure model,and the influence of the change of the bottom hole pressure on the seepage output of the coal bed gas was emphatically considered,And then a capacity prediction model was established.On this basis,using the numerical simulation software,the data of the coal bed gas test well and the gas production of the theoretical output test period of 70d were compared and analyzed,and the dynamic change curves of the two were found to be consistent,so that the correctness of the capacity prediction model was verified.In the end,combined with analysis test for gas content determination,isothermal adsorption test,rock mechanical properties test and original permeability test results,the production capacity prediction model was used to predict the long-term production capacity of 2430 d in four CBM fracturing test wells in Jiaozuo mining area.The results show that the production capacity of the four CBM test wells is basically the same.With the decrease of the dynamic flow pressure at the bottom of the well,it shows the trend of first rising and then decreasing.When the No.1 CBM test well is around 30 days,the bottom hole dynamic flow pressure is0.82Mpa,the gas production peaked,the cumulative gas production is 9.92×10~4m~3,and the average daily is production was 40.82m~3;When the No.2 CBM test well drainage period is about 60 days,the bottom hole dynamic flow pressure is 0.27Mpa,the gas production reaches the peak,the cumulative gas production is 1.14×10~5m~3,and the average daily gas production is 46.98m~3;the No.3 CBM test well drainage period When it is about 55 days,the bottom hole dynamic flow pressure is 0.44Mpa,the gas production reaches a peak,the cumulative gas production is 1.96×10~5m~3,and the average daily gas production is 80.63m~3;when the drainage period of No.4 CBM test well is about 50 days The bottom hole dynamic flow pressure is 0.18Mpa,the gas production reaches the peak,the cumulative gas production is 3.97×10~5m~3,and the average daily gas production is 163.31m~3.The research results have certain reference significance for the similar fracturing drainage production prediction.