Optimization Of CBM Drainage Schedule And Mechanism Of CO₂-ECBM Recovery For Superposed Gas-bearing System

Coalbed methane（CBM）,as an accessory product of the coalification process,is an unconventional natural gas that is stored in coalbed mainly in adsorbed states,of which the main component is methane.The production of CBM is of great significant for energy structure improvement,environmental pollution prevention and coal mine gas disaster control.There are very rich CBM resources in Yunnan,Guizhou,Sichuan,Chongqing and other southwestern areas.However,the distinct accumulation characteristics of superposed gas-bearing system（SGS）in these areas make the conventional CBM extraction technology difficult to be widely used.Moreover,the low permeability of coal in the areas seriously restricts the efficient production of CBM.The CO₂-enhanced coalbed methane recovery（CO₂-ECBM）is an effective technology with better development prospects that not only improve CBM recovery,but also geogicially storage CO₂.Therefore,this paper took the coal in Jinjia coal mine as the the research object based on the accumulation characteristics of SGS in the southwestern areas.A series of CBM drainage and CO₂-ECBM tests were conducted based on the self-developed apparatus.The dynamic evolution of coal reservoir,the mechanism of interlayer interference,and the effect of CO₂-ECBM were explored systematically.The mechanism of CO₂-ECBM was also discussed based on the basic theory such as CBM geology,seepage mechanics,rock mechanics and porous media theory,etc.The main conclusions of the research are as follows:（1）The novel physical simulation methods of CBM drainage and CO₂-ECBM was proposed in the laboratory based on the accumulation characteristics of SGS.The apparatus of CBM drainage and CO₂-ECBM with multiple boreholes was also developed by ourselves.The apparatus can realize the true triaxial stress loading of large-scale coal specimen,the synchronous real-time acquisition of multiple parameters.It provides the new research approaches of CBM drainage and CO₂-ECBM under multi-physics coupling and multi-factor levels.（2）The CBM drainage tests with multiple boreholes inside the single layer coalbed were carried out.The dynamic evolution of coal reservoir parameters and their mutual coupling were studied,and the effect of borehole superposition was quantitatively characterized and analyzed based on gas pressure.The multi-dimensional visualization of reservoir parameters during coalbed methane production is realized by Matlab software programming.The space-time distribution characteristics of gas flow field and reservoir permeability were analyzed,and the calculation model of effective drainage radius was proposed.It is concluded that the realation between the effective drainage radius and the drainage time follows the power function.（3）The CBM commingled drainage tests of SGS under different initial gas pressure difference conditions were carried out.It was found that the gas pressure of low pressure coalbed increased obviously accompanied by gas backfilling phenomenon in the early period,and the gas pressure enlargment range and gas backfilling capacity increased with the increase of initial pressure difference.The mechanism of interlayer interference during commingled drainage process was revealed and the successive drainage and progressive drainage processes were proposed.At last,the optimization effect based on the difference coefficient was evaluated.（4）The CO₂-ECBM tests with single well and multiple wells under different injection pressures were carried out.There was a complex nonlinear relationship between the CBM recovery and the injection pressure as well as the number of production wells.They can improve CBM recovery obviously in the eraly stage with increasing injection pressure or production wells.However,it had a limited effect or even a negative impact in the late stage.The CO₂-ECBM tests with gas injection at the same time,after gas injection,and combing drainage and gas injection were conducted.The corresponding CBM recovery increment compared to commingled drainge was increased by 11.4%,15.2%and 23.3%respectively.（5）Based on the CO₂-ECBM experimental results,the desorption-diffusion-seepage of CO₂-CH₄ binary system was clarified considering both favorable and unfavorable factors in the process of CO₂ injecting.The mechanism of CO₂-ECBM under the combined actions of competitive adsorption with CO₂-CH₄ binary system,coal matrix swelling reducing gas diffusion,and gas energy enhancement and permeability reduction was revealed.