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The characteristics of hydrogen (H) and oxygen (O) isotopes in produced water of coalbed methane (CBM) wells have abundant geochemical indication information. On the basis of the tests of conventional ions and H and O isotopes of 20 water samples produced from CBM wells in western Guizhou, the characteristics of H and O isotopes and productivity response of the wells in the study area have been analyzed in combination with the geological background. The research shows that, if the H and O isotopic compositions of produced water from a CBM well have obvious D drift (D drift means that, when δ18O takes the same value, the measured value of δD that is higher than the theoretical value of δD is obtained from the regional atmospheric precipitation line equation), the well is likely to obtain high production, and if the H and O isotopic compositions have O drift (O drift means that, when δD takes the same value, the measured value of δ18O that is higher than the theoretical value of δ18O is obtained from the regional atmospheric precipitation line equation), the well is not likely to obtain high production. As drainage continues, the water–rock interaction gradually strengthens and the H and O isotope compositions tend to become heavier, but seasonal rainfall can cause their values to decrease. From shallow to medium-deep formations, δD and δ18O values increase continuously, and the wells with the lowest δD and δ18O values in the shallow formation have the highest average daily gas production. d′, D drift comprehensive index, is proposed, which is of universal significance. With the increase of d′, the average daily gas production decreases, and in combination with the productivity performance, the high production wells and low production wells can be distinguished. d′ of less than 0 indicates high production, which can be used as a geochemical index to evaluate potential productivity. It is proposed that δD = −50‰, d′ = 0, and Cl– = 2000 mg/L can be used as a criterion for judging whether produced water of the CBM well is polluted and δD = −25‰ and Cl– = 4000 mg/L can be used to distinguish the degree of formation of water pollution. Moreover, the identification template of the produced water source based on δD, d′, and Cl– has been established preliminarily, and three ranges, coal seam water, light polluted water, and severe polluted water, can be identified according to the template. Moreover, it is considered that the CBM production will drop as the pollution degree of produced water increases.

Coalbed is the carrier for coalbed methane (CBM) enrichment and migration. The pore structure characteristics of coal and their main geological controlling factors are critical to the exploration and development of CBM. In this paper, 20 coal samples were collected from eastern Yunnan and western Guizhou, China. Based on vitrinite reflectance, proximate analysis, maceral analysis, and low-temperature N 2 adsorption/desorption (LT-N 2 GA) experiments, the hysteresis coefficient of low-temperature N 2 desorption was proposed, the types of pore structure were identified, and the effects of coal facies and rank on the pore structure were revealed. The results show that the R o,max values of the 20 coal samples are between 0.74 and 3.38%, which belong to medium- and high-rank coal. In the coal macerals, the vitrinite is mainly collodetrinite. The inertinite is dominated by semifusinite, and some coal samples contain exinite. The coal samples investigated can be divided into two types. Type A samples mainly contain open pores, while type B samples are rich in bottle-shaped pores. Compared with type A coal samples, type B samples have the characteristics of smaller total pore volume (TPV), smaller average pore diameter (APD), larger specific surface area (SSA), and larger hysteresis coefficient. The coal samples are located in three regions of different coal facies, including low-level swamp (reed) facies, wetland herbaceous swamp facies, and wet forest swamp facies. The tissue preservation index (TPI) values of most coal samples are less than unity, which indicates that herbaceous plants have absolute dominance in the coal-forming plants in eastern Yunnan and western Guizhou. The maximum vitrinite reflectance ( R o,max ), gelification index (GI), TPI, vitrinite content ( V ), inertinite content ( I ), Barrett-Joyner-Halenda pore volume ( V BJH ), Brunauer–Emmett–Teller SSA ( S BET ), and low-temperature N 2 desorption total hysteresis coefficient ( H t ) were clustered using the R-type cluster analysis method. It is found that TPI is the main controlling factor of the pore structure of type A coal samples, while the pore structure of type B coal samples are jointly controlled by TPI and coal rank. Type B coal samples are mainly located in Zhuzang and Laochang high-rank coal research areas, while the distribution of type A coal samples is mainly in other medium–high-rank coal research areas. These results will contribute to the exploration and development of CBM and also guide the study of pore structures of other unconventional gas reservoirs.

The differences of coalbed methane (CBM) desorption-diffusion from coal drilling-core under various drilling fluid medium are not considered in the present calculating methods of lost CBM quantity, which leads possibly to the inaccuracy of CBM quantity in coal seam. Here we took the desorption of CBM from coal core under drilling fluid medium as a pressure-swing process, and based on the Langmuir equation and Fick-first law, established the desorption-diffusion model and numerical modeling method of lost gas (including free CBM) calculation in coal core under various drilling fluid mediums through physical simulation test and by considering comprehensively primary factors. The results showed that the physical simulated t-Qt curves can be rightly fitted by the numerical modeling data, which indicated the ultimate desorption quantity from the numerical modeling was adjacent to that from the physical simulation as a whole. It was found that the lost CBM quantity from the modeling method was generally higher than that from the direct method when lost time was relatively long. Thus, we suggest that it is necessary to emend the active China national standard through further investigation, since the lost CBM quantity from coal drilling-core was generally underestimated using the method in the current standard. drilling fluid, CBM, lost gas quantity, desorption-diffusion model, numerical modeling method Citation: Yang Z B Qin Y, Wang Z F, et al. Desorption-diffusion model and lost gas quantity estimation of coalbed methane from coal core under drilling fluid medium.