Experimental Study On Evolution Of Reservoir Parameters And Influencing Factors During Commingled Drainage Process Of Coalbed Methane In Superposed Gas-bearing System

The production of coalbed methane（CBM）can not only prevent coal mine gas explosions and gas outbursts,but also can be used as clean energy.Recent studies have shown that more CBM reservoirs in China are superposed gas-bearing system（SGS）,and conventional CBM drainage technology has defects such as low gas production efficiency.Therefore,a series of the CBM commingled drainage tests of SGS such as physical simulation tests of SGS with different adsorption gases,different reservoir pressure gradients and different ground stress were conducted based on the self-developed apparatus.Through analyzing the evolution law of reservoir parameters such as reservoir pressure,coal temperature,deformation and production flow during the mining process,and explore the interlayer interference mechanism of SGS.The main conclusions of the research are as follows:（1）The physical simulation test of CBM drainage in the SGS under different adsorption gases（CH₄,CO₂ and N₂）was carried out.The results show that with the increase of gas adsorption,the cumulative flow rate of each coal seam during mining is larger,and the longer the gas backfilling phenomenon time is,the recovery rate of the SGS is gradually reduced.In the low-pressure coal reservoirs where the backfilling phenomenon occurs,the rate and magnitude of reservoir pressure rise are negatively correlated with the gas adsorption strength.（2）The physical simulation test of CBM drainage in the SGS under different reservoir pressure gradients was carried out.The results show that with the increase of reservoir pressure gradients,in the early stage of CBM drainage,low-pressure coal reservoirs have obvious gas backfilling phenomenon,and the gas backfilling volume and backfilling time increased gradually.The contribution rate of the coal reservoirs exploitation gradually decreased,and the pressure increase of the coal reservoir in the reversed coal seam gradually increased.Otherwise,the phenomenon of interlayer interference is more serious.（3）The physical simulation test of CBM drainage in the SGS under different ground stress gradients was carried out.The results show that in the early stage of CBM drainage,gas backfilling occurred in low potential coal reservoirs,and the contribution rate of mining decreased with the increase of ground stress gradient,indicating that the gas production of the coal reservoir was inhibited;The analysis of the reservoir pressure evolution in the SGS shows that the pressure drop rate of the internal measuring points of each coal reservoir in the CBM drainage process is not much different,and the coal temperature drop gradually decreases with the increase of the ground stress gradient,and the coal deformation gradually Increase.（4）The calculation method of interlayer interference coefficient is established by the gas well binomial productivity equation,and the quantitative characterization of interlayer interference phenomenon is realized.The analysis results show that after the start of the combined mining,the interference coefficient rises rapidly to the peak value in the initial stage of SGS,then gradually slows down and tends to be stable,and the interference coefficient is positively correlated with the gas adsorption strength,the reservoir pressure gradient and the ground stress gradient of the SGS.