Recoverability evaluation on coalbed methane of multiple coal seams in Zhongling-Pingshan Block of Guizhou Province

The recoverability evaluation plays an important role in exploration and development of coalbed methane (CBM), which depends on a number of geological parameters. The Qianxi area is abundant in coalbed methane in multiple coal seams, the recoverability of which deserves attention greatly. In this paper, Zhongling-Pingshan in Guizhou is selected as the research object to evaluate the development potential of coalbed methane resources in Qianxi area. Based on analysis of the tectonic characteristics, multi-seams development characteristics, gas content, and adsorptivity, the equivalent reservoir pressure, critical desorption pressure, gas saturation, and recoverability were evaluated using equivalent coal reservoir pressure isothermal adsorption method. The results show that the thin to medium thick seams were developed in the upper Permian Longtan formation which contains as many as over 50 layers. Among all the layers, 10 to 12 layers are recoverable, with gas content 8.25 to 15.44 m3/t and an average of 11.71 m3/t. The equivalent reservoir pressure gradient was 0.59～0.95 MPa/hm with an average of 0.80 MPa/hm. The critical desorption pressure is 1.07～1.87 MPa with an average of 1.56 MPa and the gas saturation is 62%～82%with an average of 72%. The calculated recoverability is favorable which is 27.54%～48.66%, with an average of 40.4%, higher than the national average. The isoline maps of gas contents, pressure and calculated results show that the distributions of gas content, reservoir pressure, critical desorption pressure and gas saturation is consistent. In high gas content area, all the equivalent reservoir pressure, critical desorption pressure, and the gas saturation have high values. Since mining multiple coal seams is an effective way to greatly improve coalbed methane production and recoverability, it will be the first choice for coalbed methane development method for Guizhou.