Reasonable Discharge Rate Of CBM Wells In Primary Production Stage

As a significant unconventional energy,coalbed methane has been widely exploited and applied in our country.Reasonable discharge rate is an important decision basis for economic CBM exploitation and a key factor for increasing CBM stable production period and recovery.However,empirical method is often applied to determine the CBM wells reasonable discharge rate in practical field production.The method is simplex,lacks theoretical basis and is less reliable.The paper considers desorption,diffusion,pressure sensitivity,matrix shrink and coal fine migration in CBM production process;establishes mathematical model of gas-water two phase radial flow;analyzes change law of formation pressure in the primary production stage of CBM;determines reasonable discharge rate for CBM wells in primary production stage from the view of rock mechanics and seepage mechanics.The method provide scientific basis for CBM rational development.According to the paper' study,main achievements are as follows:(1)Establish the coalbed permeability model in consideration of pressure sensitivity and matrix shrink.Through determining the relation between fracture width and coalbed permeability and the effect of pressure sensitivity and matrix shrink on fracture width,determine the effect of pressure sensitivity and matrix shrink on coalbed permeability.(2)Determine the relation between coal fine yield and coalbed permeability.Through studying coal fine production mechanism and damage,determine the coalbed ground stress according to thick wall cylinder theory;determine coal fine migration condition using Von-Mises rock failure criterion;establish mass conservation equation for coal fine migration;determine the effect of coal fine migration on coalbed permeability.(3)Deduce the pressure drop formula in CBM primary production stage.Establish mathematical model of pressure drop in CBM primary production stage;combined with initial and boundary condition,deduce gas-water two phase radial flow formula;calculate coalbed water saturation for every little time step using material balance equation;calculate pressure distribution in CBM primary production stage.(4)Determine the reasonable discharge rate of CBM wells in primary production stage.Combined with the hazard caused by unreasonable discharge rate,determine CBM wells minimum discharge rate through critical flow rate when wellbore produce with coal fine;determine CBM wells maximum discharge rate through critical condition of hole collapse;in the range of the minimum and maximum discharge rate,determine the reasonable discharge rate of CBM wells in primary production stage by keeping the coalbed permeability maximum to provide a good seepage channel for next production stage.