Study Of Quantitative Design On Drainage In CBM Wellbore

With the increasing demand for energy and further exploration of unconventional oil and gas resource,the CBM has become the most valuable recovered gas resources outside the tight sandstone gas and shale gas.The CBM exploration will alleviate the tension of clean energy demand,reduce mine gas explosion accidents and greenhouse gas emissions.The coal rocks are brittle and fragile,so the reservoir may be destroyed and pulverized coal or sand production may be serious with the increasing drainage rate in the CBM production.If the pulverized coal deposited and plugged the channel,the permeability of the coal reservoir may be reduced.If the pulverized coal got into the wellbore,it was seriously possible to cause the downhole failures such as stuck pump or pump leakage,that would add significant cost to the CBM production and cause poor production continuity.This paper aimed to reduce the reservoir damage,researched on damage type and damage situation of coal reservoir based on the theoretica l analysis and simulation,extracted and optimized the key drainage and mining parameters that had great influence to the reservoir damage,and designed an optimized producing system,which could ensure the CBM production smoothly.This paper detailed researched on the CBM reservoir damage mechanism including clogging by coal fines,stress closure,and downhole failures.For the problem of clogging by coal fines,this paper established a mathematical model of coal failure,and got the critical bottom hole pressure that could not cause serious damage to the coal bed.And studied the transport conditions of coal fines in gas channel by means of theoretical analysis and simulation,got the accumulated liquids flow rate that can reduce the clogging by coal fines.For the problem of stress closure this paper established the permeability model and pressure drop model for the coal reservoir,and got the proper drainage rate which could avoid the gas channel stress closuring and ensure the pressure drop could spread far and wide.For the problem of downhole failures,this paper studied the transport condition of coal fines in wellbore by means of theoretical analysis and simulation,and got the drainage rate that could avoid the downhole failures such as stuck pump caused by the coal fines subsided.Based on the theoretical analysis,this paper developed an analysis software to CBM reservoir which could analyze the damage,permeability and pressure drop,and show the results,which could provide the technicians and wo rkers with a basis to control drainage and mining parameters.At last,this paper chosen a sample well in Hancheng to analyze and calculate the drainage and mining parameters,and established an optimized producing system to ensure the CBM production stably and high yield.