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

Abstract

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.

Reserves Estimation for Coalbed Methane Reservoirs: A Review

“…Recently, several methods have been developed in order to enhance the recovery of the coalbed methane, such as injecting CO 2 , N 2 , or a mixture of both into the coal seams. The objective of using the gas injection to produce CBM is to reduce the partial pressure of methane, which decreases the required time for the desorption process [76].…”

“…Besides these attempts, other studies were conducted by considering different coal reservoir parameters, several compositions of gas mixtures, and concentrations of mixed gases in order to reveal the coupling mechanism of the ternary gases (CH 4 , CO 2 , and N 2 ). Establishing the coupling mechanism can lead to predicting the most effective approach for developing a certain coalbed methane reservoir [76].…”

Effective Methane Extraction Radius after High-Pressure Water Jet Slotting

“…Equation (7) provides a time-independent solution of the effective radius and influence radius, but the extraction time remains unknown and the effect of temperature in methane extraction is ignored. establish the THM coupling model for methane extraction after high-pressure water jet slotting, we suggest the following basic assumptions [30][31][32]: (1) the coal seams are homogeneous and isotropic porous media; (2) the deformation of coal skeleton and porosity is minimal and can be described by the linear elasticity criterion;…”

Analysis of Gas Content and Permeability Change Pattern of a Coal Reservoir in the Tectonic Positions Based on a THM Coupled Model

“…According to the physical parameters of coalbed and CBM measured in the study area, a coefficient partial differential equation (PDE) is established. Its simultaneous solution can describe the heat conduction equation and linear elastic deformation equation of CBM flowing in the coalbed [35][36][37].…”