A Review on the Application of Molecular Dynamics to the Study of Coalbed Methane Geology

Over the last three decades, molecular dynamics (MD) has been extensively utilized in the field of coalbed methane geology. These uses include but are not limited to 1) adsorption of gaseous molecules onto coal, 2) diffusion of gaseous molecules into coal, 3) gas adsorption-induced coal matrix swelling and shrinkage, and 4) coal pyrolysis and combustion. With the development of computation power, we are entering a period where MD can be widely used for the above higher level applications. Here, the application of MD for coalbed methane study was reviewed. Combining GCMC (grand canonical Monte Carlo) and MD simulation can provide microscopic understanding of the adsorption of gaseous molecules onto coal. The experimental observations face significant challenges when encountering the nanoscale diffusion process due to coal structure heterogeneity. Today, all types of diffusion coefficients, such as self-, corrected-, and transport-diffusion coefficients can be calculated based on MD and the Peng-Robinson equation. To date, the MD simulation for both pure and multi-components has reached a situation of unprecedented success. Meanwhile, the swelling deformation of coal has been attracting an increasing amount of attention both via experimental and mimetic angles, which can be successfully clarified using MD and a poromechanical model incorporating the geothermal gradient law. With the development of computational power and physical examination level, simulation sophistication and improvements in MD, GCMC, and other numerical models will provide more opportunities to go beyond the current informed approach, gaining researcher confidence in the engagement in the estimation of coal-swelling deformation behaviors. These reactive MD works have clarified the feasibility and capability of the reactive force field ReaxFF to describe initial reactive events for coal pyrolysis and combustion. In future, advancing MD simulation (primarily characterized by the ReaxFF force field) will allow the exploration of the more complex reaction process. The reaction mechanism of pyrolysis and spontaneous combustion should also be a positive trend, as well as the potential of MD for both visualization and microscopic mechanisms for more clean utilization processes of coal. Thus, it is expected that the availability of MD will continue to increase and be added to the extensive list of advanced analytical approaches to explore the multi-scaled behaviors in coalbed methane geology.

Integrated technology for multistage drainage of coal seams before high-rate cutting

An increase in the depth of underground coal mining leads to an increase in gas content, to a decrease in gas permeability, and, as a result, to a decrease in the mine drainage efficiency. The article presents the actual data on implementation of the integrated multistage pre-mine drainage technology using surface and underground boreholes in Kirov Mine of SUEK-Kuzbass. The actual estimate of the improved underground hydraulic fracturing efficiency is made by a number of objective factors identified during underground coal production in the hydraulic fracturing zones and in the check zones. These objective factors are the increase in output per face, decrease in methane emission in longwalls and the reduction in downtime of mining equipment by gas criterion. The study results on the effective parameters of the underground fracturing technology are presented, namely, the fluid injection rate, effective (useful) length of fracturing wells and the fluid injection volume. The article gives the actual data on hydraulic impact exerted on a coal seam unloaded from rock pressure using surface boreholes. It is shown that the main impact mode is hydraulic splitting, sometimes with microhydrofractures created at high rates of fluid injection into the coal seam. The prospects of further improvement in hydraulic coal splitting from ground surface are based on implementation of selfsustaining coal destruction and on using the effect of coal swelling in the process zone. The authors appreciate participation of A. V. Malafeev, I. A. Kurmanov, E. A. Kvitko, V. I. Gavrilov, I. A. Komissarov, S. L. Demin, A. B. Letashkov, I. V. Chaldin and other experts from SUEK-Kuzbass in the preparation and implementation of the tests.

A theoretical model for coal swelling induced by gas adsorption in the full pressure range

The phenomenon of coal swelling caused by gas adsorption is well known. For Enhanced Coal Bed Methane Recovery and carbon storage, coal swelling induced by gases adsorption may cause significant reservoir permeability change. In this paper, based on the assumption that the surface energy change caused by adsorption is equal to the change in elastic energy of the coal matrix, a theoretical model is derived to describe coal swelling induced by gas adsorption in the full pressure range. The Langmuir constant, coal density, solid elastic modulus, and Poisson’s ratio are required in this model. These model parameters are easily obtained through laboratory testing. The developed model is verified by available experimental data. The results show that the presented model shows good agreement with the experimental observations of swelling. The model errors are within 14% for pure gas, and within 20% for mixed gas. It is shown that this model is able to describe coal swelling phenomena for full pressure range and different gas type including pure gas and mixed. In addition, it is also shown that the errors of the presented model and the Pan’s model are almost the same, but the presented model is solved more easily.

Analysis of Coal Swelling Deformation Caused by Carbon Dioxide Adsorption Based on X-Ray Computed Tomography

The geologic sequestration of carbon dioxide by coal beds leads to the swelling deformation of coal. In order to investigate the swelling deformation characteristics at the microscopic scale, X-ray computed tomography (CT) scanning technology was used. X-ray CT scanning technology detects the internal structure, deformation, and porosity of coal at different gas pressures. Results show that swelling deformation is nonuniform, which is caused by the heterogeneity of the coal structure. Through quantitative measurement of the distance between fractures and pseudocolor processing of CT images, we observed that fractures gradually close with the increase of adsorption pressure. As adsorption pressure increases, the porosity of coal decreases, and the density of coal increases.