Microbial degradation and its influence on components of coalbed gases in Enhong syncline,China

Coalbed gases (CBG) in Enhong syncline are characterized by high concentration of C₂₊ (C_2–5), with the highest content of ethane over 30%. However, the concentrations of C₂₊ are not evenly distributed in the syncline. Based on the analysis of δ¹³C₁, δ¹³C₂, δ¹³C₃, δ¹³CO₂, δD_CH4 of CBG and their origin diagrams in the normal and abnormal areas, this research shows that gases in both areas are thermogenic gases and the reason for the uneven distribution of C₂₊ is that the microbial degradation action on gases is stronger in the normal area than in the abnormal area. The secondary biologic gases in the normal area are mainly characterized by that the carbon isotopes become obviously lighter in methane and become heavier in ethane, whereas the molecular and isotopic compositions of CO₂ change little. These features indicate that the secondary biologic gases are mainly generated by the microbial degradation of C₂₊, not generated by the reduction of CO₂. The degradation process is selective to make the residual ethane being enriched in ¹³C and the generated methane rich in ¹²C.     

Selective transport of CO2, CH4, and N2 in coals: insights from modeling of experimental gas adsorption data

Selective adsorption and transport of gases in coal are important for natural gas recovery and carbon sequestration in depleted coal seams for environmental remediation. Gases are stored in coal mainly in the adsorbed state. In this study, the interaction energies of adsorbates (CO₂, CH₄, and N₂) and micropores with various widths are investigated using a slit-shape pore model. The experimental adsorption rate data of the three gases conducted on the same coal sample are numerically simulated using a bidisperse model and apparent diffusivities of each adsorbate in the macropore and micropore are determined. The results indicate that the relative adsorbate molecule size and pore structure play an important role in selective gas adsorption and diffusion in micropores. Generally, the microporous coals diffusion is activated and the apparent micropore diffusivities of gases in coal decrease strongly with increase in gas kinetic diameters. Apparent micropore diffusivity of CO₂ is generally one or two order of magnitude higher than those of CH₄ and N₂ because their kinetic diameters have the relation: CO₂ (0.33 nm)<N₂ (0.36 nm)<CH₄ (0.38 nm). In contrast to theoretical values, apparent macropore diffusivity of CO₂ is also larger than those of CH₄ and N₂, suggesting that coal has an interconnected pore network but highly constricted by ultra micropores with width <∼0.6 nm. It is also found that the apparent diffusivity strongly decreases with an increase in gas pressure, which may be attributed to coal matrix swelling caused by gas adsorption. Therefore, rigorous modeling of gas recovery and production requires consideration of specific interaction of gas and coal matrix.     

Reservoir reconstruction technologies for coalbed methane recovery in deep and multiple seams

Multiple coal seams widely develop in the deep Chinese coal-bearing strata. Ground in situ stress and coal seam gas pressure increase continuously with the increase of the mining depth, and coal and gas outburst disasters become increasingly severe. When the coal is very deep, the gas content and pressure will elevate and thus coal seams tends to outburst-prone seams. The safety and economics of exploited firstmined coal seams are tremendously restricted. Meanwhile, the multiple seams occurrence conditions resulted in different methane pressure systems in the coal-bearing strata, which made the reservoir reconstruction of coal difficult. Given the characteristics of low saturation, low permeability, strong anisotropy and soft coal of Chinese coal seams, a single hydraulic fracturing surface well for reservoir reconstruction to pre-drain the coalbed methane(CBM) of multiple seams concurrently under the different gas pressure systems has not yet gained any breakthroughs. Based on analyses of the main features of deep CBM reservoirs in China, current gas control methods and the existing challenges in deep and multiple seams, we proposed a new technology for deep CBM reservoir reconstruction to realize simultaneous high-efficiency coal mining and gas extraction. In particular, we determined the first-mined seam according to the principles of effectiveness and economics, and used hydraulic fracturing surface well to reconstruct the first-mined seam which enlarges the selection range of the first-mined seam. During the process of mining first-mined seam, adjacent coal seams could be reconstructed under the mining effect which promoted high-efficiency pressure relief gas extraction by using spatial and comprehensive gas drainage methods(combination of underground and ground CBM extraction methods). A typical integrated reservoir reconstruction technology, ‘‘One well for triple use", was detailed introduced and successfully applied in the Luling coal mine. The application showed that the proposed technology could effectively promote coal mining safety and simultaneously high-efficiency gas extraction.