Experimental study on CBM coproduction in superposed gas-bearing systems under constant gas production rate and constant wellbore pressure

In order to study the production characteristics of multi-pressure system under coalbed methane (CBM) coproduction based on the special gas reservoir model of superposed gas-bearing systems,three groups of CBM coproduction tests of conventional,constant wellbore pressure and constant gas production rate were conducted using the self-developed physical simulation test system of multi-field coupling testing system for CBM exploitation.The gas production characteristics and fluid flow characteristics of superposed gas-bearing system under different CBM coproduction modes were expounded,and the mechanism of fluid disturbance effect in the process of CBM coproduction were revealed.Research results obtained are as follows:① It is easy for the coal reservoir with low initial fluid energy to suffer from pressure disturbance by limiting the wellbore outlet,which leads to pressure rise near the wellbore.However,the pressure disturbance mainly exists in the initial stage of CBM coproduction and weakens with time.② The pressure disturbance will change the spatial distribution of potential energy in the coal reservoir,especially in the coal reservoir with low initial fluid energy,that is,a high potential zone will be formed near the well which changes the original flow characteristics of the fluid.③ The CBM coproduction with constant wellbore pressure will result in a “dynamic pressure balance” relationship between each coal reservoir with different pressures,and a part of gas from the coal reservoir with high initial fluid energy to the gas-bearing system with low initial fluid energy.The whole process of CBM coproduction with constant-rate can be divided into stable production period and decline production period.The gas production capacity of each coal reservoir is redistributed under the effect of constant-rate during the stable production period,and a “dynamic distribution balance” gas production mode is formed.That is,when the gas production capacity of the coal reservoir with high fluid energy exceeds the constant-rate,a part of the fluid from the coal reservoir with high initial fluid energy is injected into the coal reservoir with low initial fluid energy in the reverse direction,which aggravates the fluid disturbance effect.