Methane adsorption constrained by pore structure in high‐rank coals using FESEM, CO₂ adsorption, and NMRC techniques

Abstract

To evaluate the impacts of nanopores of high‐rank coals on coalbed methane adsorption and storage, 12 anthracite and semianthracite coal samples from Yangquan and Shouyang blocks in the Qinshui Basin were investigated. Field emission scanning electron microscopy (FESEM) and CO2 adsorption combined with nuclear magnetic resonance cryoporometry (NMRC) experiments were used to evaluate the pore structure with diameters ranging from 0 to 500 nm and their impact on adsorption capacity based on qualitative and quantitative analysis. The results show that a coalification jump from semianthracite to anthracite occurred in the study area due to the magmatic intrusion. In the process, the volume of supermicropores and micropores largely increased while the volume of transition pores and mesopores decreased slightly. Additionally, vitrinite gets purified and enriched during the rapid maturation of coal reservoir, which is beneficial to the microporous structure development. The pore size distribution (PSD) of anthracite is mainly divided into two types, which are in serrated and decreasing forms, respectively. Higher vitrinite content can promote the formation of decreasing type (type II), which corresponds to a lower degree of complexity. The fractal dimensions indicate that the heterogeneity of coal samples is increasing with the decrease in pore size. Accordingly, the increase in pore heterogeneity corresponds to the lower adsorption capacity. The main pore sizes that contribute to CBM adsorption include two parts: 25‐30 nm and 50‐60 nm. For the supermicropores with large specific surface areas, the pore system detected by CO2 molecules is not conducive to CBM adsorption, while the increase in pore volume can improve the adsorption rate and capacity of CO2. These findings are vital for a precisely understanding of nanoscale pores as well as future CBM exploitation.

A comparative study of the nanopore structure characteristics of coals and Longmaxi shales in China

“…(a) Both are “self‐generation and self‐storage” reservoirs, meaning they can generate and store natural gas by themselves. (b) Both contain a certain amount of adsorbed gas . The adsorbed gas content accounts for more than 80% of the total CBM gas content, and its saturated methane adsorption capacity can exceed 10 m 3 /t .…”

“…(b) Both contain a certain amount of adsorbed gas . The adsorbed gas content accounts for more than 80% of the total CBM gas content, and its saturated methane adsorption capacity can exceed 10 m 3 /t . However, the adsorbed gas content in shale only accounts for 30%‐60% of the total gas content, with a saturated adsorption capacity that is usually in the 2‐4 m 3 /t range .…”

“…Similar experimental methods are used in the pore structure characterization of coal and shale gas reservoirs, which primarily consist of fluid injection and noninvasive imaging methods. The fluid injection method includes low‐pressure nitrogen gas adsorption (LP‐N 2 GA), low‐pressure carbon dioxide gas adsorption (LP‐CO 2 GA), high‐pressure mercury injection (HPMI), and low‐field nuclear magnetic resonance (LF‐NMR) . The noninvasive imaging method includes micro‐ and nano‐CT, scanning electron microscope (SEM), focused ion beam scanning electron microscope (FIB‐SEM), and small‐angle neutron scattering (SANS) .…”

Characteristics of in situ stress and its influence on coalbed methane development: A case study in the eastern part of the southern Junggar Basin, NW China

“…24,56,57 Among them, the G value tends to increase with the increase in the R o and P o , whereas the T em , moisture content, and ash yield are not beneficial to gas absorption of coal reservoirs. 58,59 When the R o , T em , and coal macerals are relatively stable, as discussed above, the current in situ stress regime controls the P o and affects the G and saturation of coal reservoirs. To discuss the influence of the vertical change of in situ stress regime on the G of coal reservoirs, two CBM wells (A and B) from the Miquan and Fukang regions have been selected to analyze the relationship between the G and the P o with the H. Seen from Figure 15, the G obviously increases with increasing H (<1000-1150 m), though the outliner of the G might occur in local positions, due to the difference of hydrodynamic or roof lithology.…”

Gas seepage laws based on dual porosity and dual permeability: Numerical simulation and coalbed methane extraction practice

“…The formation of coal is accompanied by abundant biogenic and thermal coalbed methane (CBM) 3‐5 . China is under a huge amount of CBM resource, and the geological resource of CBM with a buried depth of more than 2000m is about 36.8 × 10 12 m 3 4‐6 . At the same time, CBM is an important clean energy resource, which is of profound significance for reducing greenhouse gas emissions 4,7,8 …”