Method for determining productivity of coalbed methane well without shutting down the well

Abstract

A method for determining a productivity of a coalbed methane well without shutting down the well includes steps of: obtaining coalbed methane basic data; based on PVT experimental data, determining a relationship table between a pressure and a coalbed methane deviation factor, and a relationship table between the pressure and a pseudo-pressure; recording daily gas production rates, bottomhole flow pressures, and cumulative gas productions at each stabilized flow pressure test moment in at least three different production stages; determining formation pressures corresponding to each stabilized flow pressure test moment based on a material balance equation; according to the formation pressures, the bottomhole flow pressures and the production rates, determining coefficients in a deliverability equation of the coalbed methane well for determining the deliverability equation; substituting the formation pressures and the bottomhole flow pressures into the deliverability equation for obtaining corresponding productivity.

What is claimed is: 
     
         1 . A method for determining a productivity of a coalbed methane well without shutting down the well, comprising steps of:
 step 1: obtaining basic data of a target coalbed methane well, comprising coalbed methane PVT experimental data, an original formation pressure P Ri , a bottomhole flow pressure P wf  of the coalbed methane well during production, and a daily production rate q sc ;   step 2: based on the coalbed methane PVT experimental data obtained in the step 1, determining a relationship table between a pressure P and a coalbed methane deviation factor Z, and further determining a relationship table between the pressure P and a pseudo-pressure P s , wherein P s =P/Z;   step 3: performing stabilized bottomhole flow pressure test under constant production conditions in no less than three production stages of the coalbed methane well, and recording daily gas production rates q sc(1) , q sc(2) , . . . , q sc(n) , bottomhole flow pressures P wf(1) , P wf(2) , . . . , P wf(n) , and cumulative gas productions GP (1) , GP (2) , . . . , GP (n)  of the coalbed methane well at each stabilized flow pressure test moment;   step 4: setting an initial iterative assumption value G0 of a gas well dynamic reserve, combining the cumulative gas productions GP (1) , GP (2) , . . . , GP (n)  at each stabilized flow pressure test moment in different production stages, and determining formation pressures P Rm(1) , P Rm(2) , . . . , P Rm(n)  corresponding to each stabilized flow pressure test moment based on a material balance equation;   step 5: according to the formation pressures P Rm(1) , P Rm(2) , . . . , P Rm(n)  corresponding to each stabilized flow pressure test moment determined based on the material balance equation, the bottomhole flow pressures P wf(1) , P wf(2) , . . . , P wf(n)  and the production rates q sc(1) , q sc(2) , . . . , q sc(n) , determining coefficients A and B in a binomial deliverability equation for the coalbed methane well: P R   2 −Pwf 2 =Aq sc +Bq sc   2 , wherein P R  is the formation pressure, P wf  is the bottomhole flow pressure of the coalbed methane well, and q sc  is the daily production rate of the coalbed methane well;   step 6: based on the binomial deliverability equation for the coalbed methane well P R   2 −Pwf 2 =Aq sc +Bq sc   2  determined in the step 5, combining the bottomhole flow pressures P wf(1) , P wf(2) , . . . , P wf(n)  and the production rates q sc(1) , q sc(2) , . . . , q sc(n)  corresponding to each stabilized flow pressure test moment obtained in the step 3, and adopting P R =√{square root over (Pwf 2 =Aq sc +Bq sc   2 )} to determine formation pressures P Rp(1) , P Rp(2) , . . . , P Rp(n)  corresponding to each stabilized flow pressure test moment, which means obtaining the formation pressures P Rp(1) , P Rp(2) , . . . , P Rp(n)  based on the binomial deliverability equation; and   step 7: performing an error test between the formation pressures P Rm(1) , P Rm(2) , . . . , P Rm(n)  based on the material balance equation and the formation pressures P Rp(1) , P Rp(2) , . . . , P Rp(n)  based on the binomial deliverability equation corresponding to each stabilized flow pressure test moment; if an error between the formation pressures obtained by different methods fails to meet a preset accuracy requirement, then repeating the steps 4-6 to iterate until the preset accuracy requirement is satisfied, wherein the binomial deliverability equation obtained when the preset accuracy requirement is met is a deliverability equation for the coalbed methane well; substituting the formation pressures and the bottomhole flow pressures into the deliverability equation of the coalbed methane well for solving, thereby obtaining the productivity of the coalbed methane well under corresponding formation pressure and bottomhole flow pressure conditions.   
     
     
         2 . The method, as recited in  claim 1 , wherein in the step 4, the formation pressures corresponding to each stabilized flow pressure test moment are determined as follows:
 determining formation pseudo-pressures P Rs(1) , P Rs(2) , . . . , P Rs(n)  corresponding to each stabilized flow pressure test moment based on a pseudo-pressure material balance equation, and then adopting interpolation or function fitting to calculate the corresponding formation pressures P Rm(1) , P Rm(2) , . . . , P Rm(n)  based on the relationship table between the pressure P and the pseudo-pressure P s .   
     
     
         3 . The method, as recited in  claim 1 , wherein in the step 5, the coefficients A and B in the binomial deliverability equation for the coalbed methane well are determined by:
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          x (i) =q sc(i) , i=1, 2, . . . , n; according to the gas production rates q sc(1) , q sc(2) , . . . , q sc(n)  of the coalbed methane well at each stabilized flow pressure test moment in different production stages and corresponding bottomhole flow pressures P wf(1) , P wf(2) , . . . , P wf(n) , obtaining a series of observation points (y (i) , x (i) ); 
         processing observation point data with linear fit, so that A is equal to an intercept of a linear equation obtained from the linear fit, and B is equal to a slope of the linear equation obtained from the linear fit.