Characteristics of coalbed methane isothermal adsorption and its controlling on productivity

Logarithm equation(Q=alnP+b) is, for the first time, used to express characteristics of coal isothermal adsorption based on large experiment data, which contains two parameters, a and b. It is obvious that coalbed methane desorption efficient (a/P) is increasing with reservoir fluid pressure decreases, and starting pressure (P=1.5a), sensitive pressure (P=√2a/2) and abandoned pressure (P=exp((a-b)/a)) are defined according to the curvature of logarithm equation subsequently. Furthermore, according to starting pressure and sensitive pressure, it is classified into 3 stages during dewatering, which are slow desorption, transitional desorption and sensitive desorption. There is a significant difference of desorption efficient in every stage and it shows that it is mainly effective stress effect in the stage of slow desorption and matrix shrinkage effect in the stage of sensitive desorption. Therefore, coal permeability changes in different stages and so does the CBM productivity. If initial reservoir pressure is close to sensitive pressure, CBM wells are apt to high productivity in a short time, while if it is bigger than starting stress and drainage is difficult, CBM wells are most low productivity. Liulin district is now a hot CBM development target in Hedong coal field, in eastern Ordos basin, China. Although abundant CBM resources have been verified, and Hedong coal field is treated as "San Juan" in China, the CBM well productions are not satisfied considering the large water production, especially in Taiyuan Formation. Based on the theory above and initial reservoir pressure and hydrology characters in Liulin district, it is well explained the fact that high CBM accumulation area and high CBM production area is not consistent, and high CBM production wells are confined at a specific depth window about 350m-550m now. The deeper areas are going to face its high production with the development of dewatering, and a large scale of production should be from shallow areas to deep areas step by step.