Effect of water occurrence in coal reservoirs on the production capacity of coalbed methane by using NMR simulation technology and production capacity simulation

The occurrence of gas and water is a critical factor affecting the production capacity of coalbed methane. Therefore, occurrence and dynamic interaction between gas and water in coal reservoirs need to be studied. T2 spectrum of all the samples were studied by NMR technique at 0, 5, 9, and 28 h of natural evaporation. Then, multifractal model was used to characterize water distribution heterogeneity, and a novel evaluation coefficient was proposed to characterize distribution heterogeneity of water content, the correlation among evaluation coefficient, water distribution heterogeneity and pore structure parameters was analyzed. The results are as follows. In the saturated state, the moisture distribution of three types is heterogeneous. Type A of adsorption pore-developed exhibits strong moisture variation heterogeneity of small pores, in contrast to the uniform moisture variation in large pores. Type B of fracture-developed has the highest coefficient of variation in moisture distribution. The coefficient of variation A(1) for moisture content can determine the speed of moisture transport and evaporation in the pore structure. A lower A(1) indicates a greater moisture transport volume and faster moisture transport rate over the same time. The coefficient of variation A(2 )for moisture distribution represents the heterogeneity of moisture content variation in the pore structure. That is, a smaller A(2) indicates better heterogeneity in the distribution of moisture content in the pore structure. Numerical simulation indicates the average gas production and the peak gas production time increase and decrease with the initial moisture content (Sgrw) increases, respectively. These findings provide practical guidance for coalbed methane development, especially in optimizing coalbed methane production capacity prediction and improving recovery efficiency.