Thermo-hydro-mechanical-chemical couplings controlling CH4 production and CO2 sequestration in enhanced coalbed methane recovery

The coupling of in situ stress, seepage, and fracture makes the strata exhibit complex strain behavior and permeability evolution under the stress path of cyclic loading-unloading. In this study, the cyclic loading-unloading experiments of constant amplitude axial stress of sandstone under the combination of different initial confining pressures and loading-unloading rates are conducted. The experimental resultsshow that the heterogeneity of sandstone, Poisson's ratio produced by adjacent loading-unloading, and confining pressure determine the deformation characteristics of sandstone in lateral and axial directions. Sandstone can produce significant shear dilatancy at a low rate; this is because stress perturbation can activate more particle slippage and fracture structure changes. However, the fracture preferentially propagates along the end or edge of the crack with strong stress sensitivity to form a shear failure plane at high rate. The normalized permeability of sandstone decreases with an increase in the loading-unloading rate before failure. The evolution of normalized permeability is closely related to the shear slip of fracture structure and sandstone particles. For the prevention of rock engineering disasters, the high loading-unloading rate results in lower normalized permeability, which favors the prevention of gastype disasters in rocks with higher gas potential energy; however, it is not conducive to the prevention of rockburst. K E Y W O R D Sconstant amplitude cyclic loading-unloading, loading-unloading rate, normalized permeability, shear effect, volumetric strain | 453 LIU et aL.

Section: Permeabilit Y Evolutionmentioning

confidence: 99%

Experimental study on strain behavior and permeability evolution of sandstone under constant amplitude cyclic loading‐unloading

Liu

Zhang

et al. 2019

“…In addition, dame sheets should be arranged outside the block wall and at the corner of the transportation roadway and the ventilation roadway of working face #10414. The anchor cable parts should be removed to fully cave the top coal at the corner of the roadway, reduce air leakage, and prevent heat accumulation (Figure ) …”

Section: Discussionmentioning

confidence: 99%

Study of the influence of the characteristics of loose residual coal on the spontaneous combustion of coal gob

Shu

et al. 2019

Mine fires are becoming a serious issue as the intensity of mining increases, especially in deep mines. Loose coal gob has a hidden ignition location and a high possibility of spontaneous combustion, which makes fire prevention difficult. Therefore, based on the theory of gas seepage and the characteristics of loose coal, a model of air leakage and spontaneous combustion in gob is established in this paper. Using working face #10414 in the Yangliu coal mine as an example, the relationship between the three spontaneous coal combustion (CSC) zones and the three stress zones is analyzed and verified by combining a FLAC3D simulation with field monitoring. In addition, the influence of advancing speed on the CSC is discussed, and suggestions for fire prevention are presented. The results show that the variation in the calorific value of the CSC with increasing degree of looseness of the residual coal in the gob forms an arch‐shape. There is a one‐to‐one relationship between the distribution of the three stress zones and the three CSC zones. In addition, as the advancing speed increases, the contact time between the loose coal body and the air decreases and the possibility of CSC decreases. This study provides a scientific basis for fire prevention and control in mines.

“…Assuming the ternary gas content in the REV is M n , the mass change of the REV over d t follows the continuity equation of ternary gases flowing in the coal seam. This equation can be expressed as:

\frac{\partial M_{normaln}}{\partial t} + \nabla \cdot false(ρ_{gn} q_{gn} false) + \nabla \cdot (- D_{normaln} \cdot \nabla φ C_{normaln}) = Q_{sn}

where ρ gn = C n · M gn ,

q_{gn} = - k R_{0} T_{0} \cdot \nabla C_{normaln} / μ_{normaln}

. The subscript n represents different gas components; M n is the mass of gas component n contained per unit volume of coal (kg/m 3 ); M gn is methane molar mass of gas component n (kg/mol); ρ gn is the gas density of gas component n (kg/m 3 ); q gn is the seepage velocity vector of gas component n (m/s); k is the coal permeability (m 2 ); R 0 is the universal gas constant (J/(mol·K)); T 0 is the initial coalbed temperature (K); C n is the concentration of gas component n (mol/m 3 ); μ n is the dynamic viscosity coefficient of gas component n (Pa·s); D n is the dispersion coefficient of gas component n (m 2 /s); and φ is the coal porosity (%).…”

Section: Fully Coupled Equations Of the Thm Fields For Gas Mixture Ecmentioning

confidence: 99%

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

Fan

Wang

Deng

et al. 2019

As an effective carbon utilization technology, the injection of N2/CO2 mixtures into coal seams has significant potential for improving coalbed methane recovery. Considering the technical barrier that injection of pure CO2 decreases the well injectivity index and pure N2 injection leads to the rapid methane recovery, a method of injecting N2‐enriched gas mixtures with a constant component is proposed. In this study, a thermo‐hydro‐mechanical (THM) coupling numerical model for enhanced coalbed methane (CBM) recovery by injecting N2/CO2 mixtures is established. This model includes complex interactions of coal deformation, competitive adsorption, ternary gas seepage, and heat transfer. The THM coupling model is first validated, and then applied to investigate the evolution of mixed gas concentrations, reservoir permeability, reservoir temperature, CH4 production, and N2/CO2 storage during N2/CO2 enhanced CBM recovery. The results show that the displacement radius and concentration of the mixed gas in the coal seam increased with gas injection pressure increase. The concentration of CH4 gradually decreased with time, and the early decline is faster than the later stage. The sweep of the N2 flow accelerates CH4 desorption and migration, promoting a reduction in reservoir temperature near the production well. Reservoir permeability evolution results from the combined effects of ternary gases (CH4, CO2, N2) competitive adsorption, gas pressure, and geostress on the coal seam within the THM fields. At the methane natural depletion stage (within 250 days), the permeability of coal reservoir first decreases and then increases. With the arrival of the N2/CO2 mixture, the permeability decreases dramatically. From the perspective of cumulative CH4 production, the optimal composition is dominated by the synergistic effect of maximizing breakthrough time and minimizing coal matrix swelling. For 30% CO2‐70% N2, the CH4 recovery ratio reached 71.76%, representing an increase of 16.67% compared to natural depletion.