Characteristics and revelation of pressure drop of reservoir during combined CBM production of multi-coal seams in Songzao mining area

Coalbed methane exploration and development of Late Permian Longtan Formation in Southwest China has achieved good results, and weak aquifer in this region restricts the drainage of coalbed methane. In order to reveal the pressure drop characteristics of different reservoirs in the coal seam group, based on the plane radial seepage theory, using the geological engineering data of well Q1 in Songzao mining area, based on the comprehensive analysis of the bottom hole flow pressure, the set pressure, the daily water production and the daily gas output data, the mathematical model of the bottom hole flow pressure of different reservoirs was established vertically, and the pressure drop of different reservoirs features was analyzed. The results show that the initial gas production time of coal seams M₆, M₇ and M₈ was 45 d, 162 d and 217 d. The reservoir pressure and permeability controlled the reservoir feeding capacity, leading to the decrease of pressure drop effect as the layer lowered, and the critical desorption pressure of the reservoir(4.07 MPa) and the low water production(0.23 m³) affected the combined CBM extraction effect of multiple coal seams. It is an important work direction to strengthen the comprehensive study of the selected layers, the analysis of the characteristics of water production and the study of the timing of water injection, to carry out research and development of progressive production-related process equipment and engineering exploration.