Gas Transportation and Enhanced Coalbed Methane Recovery Processes in Deep Coal Seams: A Review

Abstract

Coalbed methane (CBM) is a potential green energy supply for addressing the worldwide energy crisis. However, the recovery of economically viable amounts of methane requires the application of production-enhancement techniques. The greater effectiveness of enhanced coalbed methane (ECBM) recovery compared to traditional pressure depletion and hydraulic stimulation techniques has been identified in terms of higher CBM recovery with minimal pollution risk and the ability to contribute to CO2 sequestration. Gas transport behavior in a coal seam is the governing factor for ECBM recovery, which includes sorption/desorption and diffusion in the matrix and advective flux in cleats. The interactions among sorption, diffusion, and flow indicate the complexity and abstruseness of gas transport in coal. Therefore, the purpose of this paper is to provide comprehensive knowledge of the gas transport process in deep coal seams, particularly in relation to the ECBM process. According to the review, the dual-porosity system in coal provides sorption sites, and CO2 has much higher adsorption affinity to coal compared to that of CH4. Gas adsorption capacities for CH4 and CO2 are greatly reduced with temperature and the presence of moisture and increased with pressure. However, the adsorption capacity for supercritical CO2 decreases with increasing pressure due to changes in the associated CO2 properties. Regarding the diffusion process, CO2 has the highest diffusivity for its smallest kinetic diameter and the diffusion capability may be reduced with the existence of moisture for moisture adsorption-induced coal swelling. Seam temperature has a positive influence on gas diffusion due to the enhanced kinetic energy and, the effect of pressure on diffusion is still open to debate. Upon sorption/diffusion, gas moves toward the cleat system through gas flow, which is controlled by permeability and is in turn greatly altered by gas adsorption/desorption-induced swelling/shrinkage effects during ECBM recovery. With high chemical reactive potential, CO2 creates the greatest coal matrix swelling for its higher adsorption capacity. Seam permeability increases with increasing injection pressure due to the associated pore expansion and reduces with enhanced swelling. Coal mass swelling reduces with increasing temperature due to the exothermic nature of gas adsorption. Dewatering coal seams increases coal permeability through the reduced moisture content that provides more sorption places for CO2 adsorption. However, this in turn may cause reduced permeability through the enhanced swelling effect.

Numerical study on enhancing coalbed methane recovery by injecting N2/CO2 mixtures and its geological significance

“…The fluid‐solid model was used to rationalize and explain the trends of experimental data. Due to the complex coupling responses of ternary gases, numerical simulations are assumed to be an effective method to represent the process of ECBM recovery by gas injection . Sayyafzadeh et al optimized the injected mixture with less CO 2 by a semisynthetic simulation model, which improved methane recovery ratio by 21.1% comparing to natural depletion.…”

“…Due to the complex coupling responses of ternary gases, numerical simulations are assumed to be an effective method to represent the process of ECBM recovery by gas injection. 34,35 Sayyafzadeh et al 31 optimized the injected mixture with less CO 2 by a semisynthetic simulation model, which improved methane recovery ratio by 21.1% comparing to natural depletion. Although some important factors, such as heat transfer and coal permeability, were ignored, the numerical simulation of mixed gas injections has provided a useful theoretical basis for coalbed methane recovery enhancement.…”

Effects of Water and Brine Saturation on Mechanical Property Alterations of Brown Coal

“…The relatively clean-burning nature of CBM has drawn much attention for its potential to provide energy source to a world in need of clean energy supplies. Enhanced coalbed methane (ECBM) technology, which implemented by injecting CO 2 (CO 2 -ECBM) or N 2 (N 2 -ECBM) or flue gas into the coal seam to replace the adsorbed CH 4 , has become a promising technique to recover CBM, with less environmental pollution and higher recovery rates and CO 2 -ECBM has the additional benefit of immobilizing large amounts of CO 2 , which is the main greenhouse gas responsible for global warming [9,10]. However, during this process, the adsorption/desorption of gas/fluids create significant changes in the mechanical properties of coal mass.…”

“…However, during this process, the adsorption/desorption of gas/fluids create significant changes in the mechanical properties of coal mass. Changes in coal strength properties may occur due to gas and water sorption during the ECBM process and this leads to the reduction of coal seam permeability and recovery rates [9]. Therefore, an investigation of the changes of coal mechanical property associated with adsorbate type and content is of great significance, not only for a better understanding of the outburst phenomenon during coal mining but also for a full appreciation of coal seam permeability changes, the key factor controlling CBM recovery during the ECBM process.…”

Mathematical Modeling and Simulation on the Stimulation Interactions in Coalbed Methane Thermal Recovery

“…The adsorption capacity of methane decreases by approximately 19%, 32% and 45% for coals at micro-pore with sizes of 0.7-0.9 nm, 1.0-1.3 nm and size that larger than 1.4 nm respectively. Although many researchers have pointed out the effects of temperature on gas sorption, a theoretical presentation based on experiments that can concisely and usefully describe the effects is still lack.Change of reservoir temperature causes a succession of interactions to CBM seepage behavior and the evolution of coal permeability [25][26][27]. Wang et al [28] found that the evolution of coalbed permeability with increasing temperature may be divided into two stages: coal permeability decreases due to the internal swelling of coal matrix at the first stage and increases at the domination of rising gas pressure.…”