Adsorption Factors in Enhanced Coal Bed Methane Recovery: A Review

Enhanced coal bed methane recovery using gas injection can provide increased methane extraction depending on the characteristics of the coal and the gas that is used. Accurate prediction of the extent of gas adsorption by coal are therefore important. Both experimental methods and modeling have been used to assess gas adsorption and its effects, including volumetric and gravimetric techniques, as well as the Ono–Kondo model and other numerical simulations. Thermodynamic parameters may be used to model adsorption on coal surfaces while adsorption isotherms can be used to predict adsorption on coal pores. In addition, density functional theory and grand canonical Monte Carlo methods may be employed. Complementary analytical techniques include Fourier transform infrared, Raman spectroscopy, XR diffraction, and 13C nuclear magnetic resonance spectroscopy. This review summarizes the cutting-edge research concerning the adsorption of CO2, N2, or mixture gas onto coal surfaces and into coal pores based on both experimental studies and simulations.

The failure behavior of buckling pin valve: Theoretical, numerical, and experimental study

At present, buckling pin in the bypass of piping as pressure relief valve has been gradually utilized in the low-concentration coal-bed methane (CBM), which bends to release pressure when the main valve fails leading to pipeline blockage. However, current researches mainly focused on the buckling behavior of hydraulic cylinder rod or rod string, and less consideration was given to the operational reliability of buckling pin valves. This paper deduced the calculation formula of the critical failure load based on Euler formula in the buckling pin under buckling load. Besides, three finite element models (FEM) based on Johnson−Cook constitutive model were compared to predict failure strength of buckling pin which were verified by experiment. In addition, the defect sensitivity analysis of the buckling pin under different initial geometric defects rate was carried out. The results showed that a) the experimental value of the critical failure load in the buckling pin was 206.04 N and the bending position was in the middle of the buckling pin; b) the analysis result adopting explicit dynamic method was in best agreement with the experimental results within deviation of 0.24%; and c) the initial geometric defect of buckling pin should be controlled within 1%. This study provides an important reference to predict the critical failure load of the buckling pin valve and achieve safe transportation of low-concentration CBM.

Experimental Investigation on the Propagation of Hydraulic Fractures through Coal-Rock Interfaces

Although hydraulic fracturing has been one of the primary stimulation methods for coal-bed methane (CBM) exploration, it is difficult to be applied in soft and low-permeability coal seams due to the instability of wells in such geological structures. In order to solve the problem, an idea of indirect fracturing is proposed, that is, fractures are initiated in stable and hard rocks and then propagated to coal seams in which crack networks can be formed. To verify the feasibility of such an approach, the true triaxial hydraulic fracturing experiments were conducted using two-dimensional and three-dimensional coal-rock combination samples, respectively. This study investigates the fracture patterns, pressure variation, and fracture morphology. The results show that in the process of fracture propagation from sandy mudstones to coals, the strain energy release rate in the sandy mudstones is 10.69∼25.53 times greater than that in the coal. When the fracture has a tendency to deflect toward the lower strength coal strata, under the condition of large K2/K1, the deflection criterion will be met first and the fracture will deflect and grow into the coal strata. In addition, the complex crack network can be generated when the hydrofracture intersects the coal-rock interface and the fracture pattern is analyzed.

Analysis of the Influence of Coal Petrography on the Proper Application of the Unipore and Bidisperse Models of Methane Diffusion

The analysis of phenomena related to gas transport in hard coal is important with regard to the energetic use of coal bed methane (CBM), the reduction of greenhouse gas emissions to the atmosphere (CO2) and the prevention of natural hazards such as methane hazards and gas and rock outbursts. This article presents issues concerning the feasibility and scope of applying the unipore and bidisperse diffusion models to obtain knowledge concerning the kinetics of methane sorption and its diffusion in the carbon structure, depending on its petrography. Laboratory tests were carried out on coal samples which varied in terms of petrography. Quantitative point analyses were carried out, based on which content of groups of macerals was determined. The degree of coalification of coal samples was also determined based on measurements of vitrinite reflectivity R0 and the volatile matter content Vdaf. Sorption kinetics were also investigated, and attempts were made to adjust the unipore and bidisperse models to the real sorption kinetic courses. This allowed the identification of appropriate coefficients controlling the course of sorption in mathematical models. An attempt was also made to assess the possibility of applying a given model to properly describe the phenomenon of methane sorption on hard coal.

A Leap into Automation for Advanced Fracture Characterization

Fractures are the prime conduits of flow for hydrocarbons in reservoir rocks. Identification and characterization of the fracture network yields valuable information for accurate reservoir evaluation. This study aims to portray the benefits and limitations for various existing fracture characterization methods and define strategic workflows for automated fracture characterization targeting both conventional and unconventional reservoirs separately. While traditional seismic provides qualitative information of fractures and faults on a macro scale, acoustics and other petrophysical logs provide a more comprehensive picture on a meso and micro level. High resolution image logs, with shallow depth of investigation are considered the industry standard for analysis of fractures. However, it is imperative to understand the framework of fracture in both near and far field. Various reservoir-specific collaborative workflows have been elucidated for a consistent evaluation of fracture network, results of which are further segregated using class-based machine learning techniques. This study embarks on understanding the critical requirements for fracture characterization in different lithological settings. Conventional reservoirs have good intrinsic porosity and permeability, yet presence of fractures further enhances the flow capacity. In clastic reservoirs, fractures provide an additional permeability assist to an already producible reservoir. In carbonate reservoirs, overall reservoir and production quality exclusively depends on presence of extensive fracture network as it quantitatively controls the fluid flow interactions among otherwise isolated vugs. Devoid of intrinsic porosity and permeability, the presence of open-extensive fractures is even more critical in unconventional reservoirs such as basement, shale-gas/oil and coal-bed methane, since it demarcates the reservoir zone and defines the economic viability for hydrocarbon exploration in reservoirs. Different forward modeling approaches using the best of conventional logs, borehole images, acoustic data (anisotropy analysis, borehole reflection survey and stoneley waveforms) and magnetic resonance logs have been presented to provide reservoir-specific fracture characterization. Linking the resolution and depth of investigation of different available techniques is vital for the determination of openness and extent of the fractures into the formation. The key innovative aspect of this project is the emphasis on an end-to-end suitable quantitative analysis of flow contributing fractures in different conventional and unconventional reservoirs. Successful establishment of this approach capturing critical information will be the stepping-stone for developing machine learning techniques for field level assessment.

Comparative Study of Pore Structure Characteristics between Mudstone and Coal under Different Particle Size Conditions

In order to investigate the difference of pore structure characteristics between mudstone and coal under different particle size conditions, samples acquired from Henan province were smashed and screened into three different particle sizes (20–40, 80–100, and >200 mesh) to conduct the experiments, using the high-pressure mercury intrusion porosimetry (MIP) and low-temperature N2 adsorption (LT-N2A) techniques. The results demonstrated that the proportion of open pores or semi-enclosed pores increased, and the pores became preferable contacted each other for both mudstone and coal during the crushing process. These variations of pore structure characteristics in the coal were beneficial to methane storage and migration. The total specific surface areas and pore volumes all showed a tendency of increasing continually for both mudstone and coal, as the particle sizes decreased from the LT-N2A test. The mudstone and coal were non-rigid aggregates with micropores, plate-shaped pores, and slit-shaped pores developed inside. The effect of the crushing process on the pore shape for the mudstone and coal was inappreciable. Moreover, the influence of the particle sizes on the mesopore was the most significant, followed by the macropore; and on the micropore, the influence was negligible for both mudstone and coal. The crushing process only had a significant impact on the pore structure of mudstone with a particle size of less than 100 mesh, while it could still alter the pore structure of coal with a particle size of larger than 100 mesh. It is believed that this work has a significant meaning to explore the diffusion and migration rules of coal-bed methane in coal.

Primary studies on the effect of coal bio-gasification in situ in the Qinshui basin

Coal bio-gasification is one in situ coal gasification technology that utilizes the digestion of organic components in coal by methanogenic bacteria. It is not only an effective technology to enhance the recoverable reserves of coalbed methane, but also an important technical method to promote clean coal utilization. Relevant laboratory researches have confirmed the technical feasibility of anthracite bio-gasification. However, in the complex environment of coal bed, whether in situ gas can be yield with methanogenic bacteria needs to be verified by in situ experiments. In this study, a vertical well and a horizontal well were used in Qinshui basin to perform field experiments to confirm the technical industrial feasibility. The concentration of Cl− ion and number changes of Methanogen spp. were used to trace nutrition diffusion. Gas production changes and coalbed biome evolution were used to analyze technical implementation results. The trace data and biome evolution identified that: (1) The development of Methanoculleus spp. has a significant positive correlation with culture medium diffusion; (2) the structure of coalbed microbial community was significantly changed with the injection of nutrition, and the newly constructed methanogenic community was more suitable for fermentation of coal; and (3) the evolution of dominant microflora has further enhanced bio-gasification of coal. Gas production data showed that the gasification of coal lasted 635 and 799 days and yielded 74,817 m3 and 251,754 m3 coalbed methane in Z-159 and Z-7H wells, respectively. One nutrition injection in coalbed achieved an average of 717 days of continuous gas production in experimental wells. Results confirmed that coalbed methane enhancement with bio-gasification of coal is a potential technology to achieve the productivity improvement of coalbed methane wells. And the findings of this study can help to further understand the mechanism of in situ coal bio-gasification and provide theoretical support for the development of biomining of coal.