Multiple layers superposed CBM system commingled drainage schedule and its optimization

The apparatus for the commingled drainage of multiple layers superposed coalbed methane( CBM) system has been developed specifically for the special CBM reservoir-forming characteristics of the “ multiple layers super- posed CBM system”. The following results were obtained:(1) Conventional commingled drainage test was carried out with four layers superposed coal seams and the reservoir pressures were 1. 0 MPa,1. 4 MPa,1. 8 MPa and 2. 2 MPa respectively. During the process of drainage,gas in high reservoir pressure coal seam flowed into the coal seams with low reservoir pressure,which suppressed the output of low reservoir pressure coal seam and was harmful to commingled drainage;(2) Progressive drainage test was carried out by optimizing gas production time in different coal seams and gas inversion between different coal seams was avoided effectively. The recovery ratio of lowest reservoir pressure coal seam and superposed coal seams were increased by 6. 5% and 1. 3% ,respectively;(3) Progressive drainage can re- duce the gas production contribution rate of high reservoir pressure coal seams and increase the gas production contri- bution rate of low reservoir pressure coal seams,which make gas production distribution more reasonable;(4) Super- posed difference coefficient showed rising firstly and then decreasing,which indicated that the difference between conventional commingled drainage and progressive drainage was changing dynamically during drainage. It reached the maximum in the mid-term of drainage and decreased slightly in the late stage of drainage.