Multi-stage gas diffusion and its implications for the productivity of coalbed methane in the southern Qinshui Basin, north China

The behavior of coalbed methane(CBM)diffusion considerably influences gas productivity.Based on the multi-porous diffusion model and on-site CBM desorption data of coal cores,the behavior of CBM diffusion and its implications on the gas productivity of No.3 coal seam in the southern Qinshui Basin(SQB)were elaborately analyzed.Results indicate that CBM diffusion of No.3 coal seam demonstrates noticeable three-stage characteristics,including the fast diffusion,transitional diffusion,and slow diffusion stages.During the gas diffusion process,the gas content and/or the degree of developed pores and fractures/cleats in coal seams can affect the desorption of CBM and the amount of diffused CBM by influencing the changes in gas pressure in pores,thus controlling the behavior of gas diffusion in different stages.Because gas content and the developed degree of pores and fractures/cleats are closely associated with the deformation degree of the coal seams,variably deformed coal seams exhibit unique characteristics of gas diffusion.The low-deformation degree of the coal seams have a relatively uniform distribution of gas production over the history of a well.By contrast,the moderate-deformation degree of the coal seams have a relatively high rate and amount of gas diffusion in the fast and transitional diffusion stages,producing most of the gas in the early-to-intermediate stages of the wells.Finally,the high-deformation degree of the coal seams has a high rate and amount in the fast diffusion stage,indicating that most of the production stage occurs during the early stage of the gas production history of a well.In summary,the behavior of gas diffusion can be used for predicting gas production potential.

P-wave and S-wave response of coal rock containing gas-water with different saturation: an experimental perspective

The acoustic response of gas and/or water saturated coal rock is fundamental for establishing the correspondence between the physical properties of the coal reservoir and the characteristics of the well-logging response,which is the technology essential for the geophysical exploration of coalbed methane(CBM).This acoustic response depends on water(Sw)and gas(Sg)saturation among other factors.In this study,we performed acoustic tests on dry and different gas-water saturated coal samples with different degrees of metamorphism and deformation,collected from several coal mining areas in China.These tests enabled us to analyze the influence of coal type and gas-water saturation on the acoustic response of CBM formations.Our results show that the acoustic velocity of P-wave and S-wave(Vp and Vs,respectively),and the relative anisotropy of and Vs,increased with increasing vitrinite reflectance,density,Vp and Sw.WithSw increasing from 0 to 100%,the growth rate of the acoustic velocity decreased with increasing vitrinite reflectance.The Vp/Vs ratio of tectonic coal was generally higher than that of primary coal.The growth rate of the relative anisotropy in tectonic coal was markedly higher than that in primary coal.