Reasonable start time of carbon dioxide injection in enhanced coalbed methane recovery involving thermal-hydraulic-mechanical couplings

doi:10.1007/s11707-022-1029-7

Front. Earth Sci.

2023, Vol. 17

Issue (3) : 832-843 https://doi.org/10.1007/s11707-022-1029-7

RESEARCH ARTICLE

Reasonable start time of carbon dioxide injection in enhanced coalbed methane recovery involving thermal-hydraulic-mechanical couplings

Chaojun FAN¹, Lei YANG¹(

), Bin XIAO¹, Lijun ZHOU², Haiou WEN¹, Hao SUN¹

¹. College of Mining, Liaoning Technical University, Fuxin 123000, China
². College of Safety Science and Engineering, Liaoning Technical University, Huludao 125105, China

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Abstract

Injection of gas (CO₂) into coal seams is an effective method to benefit from both CO₂ geological storage and coalbed methane recovery. Based on the dual pore structure of coal mass, and the Weibull distribution of fracture permeability, a thermal-hydraulic-mechanical (THM) coupling mathematical model is proposed involving the non-isothermal adsorption of binary gases, dynamic gas diffusion between matrix and fractures, multiphase seepage, coal deformation, heat conduction and heat convection. This mathematical model is applied to study the process of CO₂-enhanced coalbed methane recovery (CO₂-ECBM). Results show that the CH₄ content of CO₂-ECBM in coal seam decreases significantly when compared with that of regular drainage, and decreases rapidly in the early stage but slowly in the later stage. Coal seam permeability evolution is triggered by changes in gas adsorption/desorption, temperature and effective stress. For regular drainage, the early permeability shows a decreasing trend dominated by the increase of effective stress, while the later permeability shows an increasing trend dominated by the CH₄ desorption caused shrinkage of coal matrix. For CO₂-ECBM, the permeability in coal seam generally shows a downward trend due to both matrix swelling induced by gas adsorption and thermal expansion, particularly near injection well. There appears an increased and delayed peak production rate of CH₄. The CH₄ production rate of CO₂-ECBM is always higher than that of regular drainage. The CH₄ cumulative production and CO₂ cumulative storage linearly increase with time, and the CH₄ cumulative production of CO₂-ECBM increased by 39.2% in the duration of 5000 d compared with regular drainage. Reasonable CO₂ injection starting time can overcome the issue of early CO₂ breakthrough and ineffective increase of CH₄ production. In the studied case, the optimal injection starting time is 2500 d. Compared with the simultaneous CH₄ extraction and CO₂ injection, the CH₄ cumulative production of optimal time has increased by 30.1%. The research provides a reference for determining the reasonable CO₂ injection time under similar conditions.

Keywords CO₂ sequestration coalbed methane reasonable injection start time thermo-hydro-mechanical coupling model numerical simulation

Corresponding Author(s): Lei YANG

Online First Date: 16 June 2023 Issue Date: 12 December 2023

Cite this article:

Chaojun FAN,Lei YANG,Bin XIAO, et al. Reasonable start time of carbon dioxide injection in enhanced coalbed methane recovery involving thermal-hydraulic-mechanical couplings[J]. Front. Earth Sci., 2023, 17(3): 832-843.

URL:

https://academic.hep.com.cn/fesci/EN/10.1007/s11707-022-1029-7
https://academic.hep.com.cn/fesci/EN/Y2023/V17/I3/832

Fig.1 Physical model of double-porous single-permeability medium. (a) Simplified model of coal; (b) equivalent fracture distribution of coal; (c) representative element volume.

Fig.2 Geometric model for CO₂-ECBM simulation.

Parameter	Value	Parameter	Value
Initial CH₄ pressure in fracture (p_f10, MPa)	5.24	Specific heat capacity of coal (C_s, J/(kg·K))	1350
Initial CH₄ pressure in matrix (p_m10, MPa)	5.24	Specific heat capacity of water (C_w, J/(kg·K))	4187
Initial CO₂ pressure in fracture (p_f20, MPa)	0.2	Specific heat capacity of CH₄ (C_g1, J/(kg·K))	2220
Initial CO₂ pressure in matrix (p_m20, MPa)	0.2	Specific heat capacity of CO₂ (C_g2, J/(kg·K))	844
Young’s modulus of coal seam (E, MPa)	2815	Density of coal (ρ_c, kg·m⁻³)	1470
Young’s modulus of coal skeleton (E_S, MPa)	8469	Klinkenberg factor (b_k, MPa)	0.76
Poisson’s ratio of coal (v)	0.32	Relative permeability of water (k_rw0)	1.0
Fracture stiffness (K_n, GPa/m)	2.8	Relative permeability of gas (k_rg0)	0.875
Langmuir volume constant of CH₄ (V_L1, m³·kg⁻¹)	0.0256	Capillary pressure (p_cgw, MPa)	0.035
Langmuir pressure constant of CH₄ (P_L1, MPa)	2.07	Adsorption time of CH₄ (τ₁, d)	0.221
Langmuir volume constant of CO₂ (V_L2, m³·kg⁻¹)	0.0447	Adsorption time of CO₂ (τ₂, d)	0.334
Langmuir pressure constant of CO₂ (P_L2, MPa)	1.38	Initial temperature in coal seam (T₀, K)	305.5
Dynamic viscosity of CH₄ (μ_g1, 10⁻⁵pa·s)	1.34	Gas molar constant (R, J·mol⁻¹·K⁻¹)	8.314
Dynamic viscosity of CO₂ (μ_g2, 10⁻⁵pa·s)	1.84	Initial porosity of matrix (ϕ_m0)	0.045
Dynamic viscosity of water (μ_w, 10⁻³pa·s)	1.01	Initial porosity of fracture (ϕ_f0)	0.011
Adsorption strain constant of CH₄ (ε_L1)	0.0128	Initial permeability of fracture (k₀, m²)	5.14 × 10⁻¹⁶
Adsorption strain constant of CO₂ (ε_L2)	0.0237	Thermal conductivity of coal (λ_s, W/(m·K))	0.1913
Temperature coefficient (c₁, 1/T)	0.021	Thermal conductivity of CH₄ (λ_g1, W/(m·K))	0.0301
Pressure coefficient (c₂, 1/MPa)	0.071	Thermal conductivity of CO₂ (λ_g2, W/(m·K))	0.0137
Isosteric heat of CH₄ adsorption (q_st1, kJ/mol)	16.4	Initial water saturation (s_w0)	0.8
Isosteric heat of CO₂ adsorption (q_st2, kJ/mol)	19.2	Irreducible water saturation (s_wr)	0.42

Tab.1 Parameters related to numerical simulation

Fig.3 Probability distribution of coal permeability with various homogeneity index m.

Fig.4 CH₄ content in coal seam during regular drainage.

Fig.5 CH₄ content on the reference line AB during regular drainage.

Fig.6 CH₄ content in coal seam during CO₂-ECBM.

Fig.7 CH₄ content on the reference line AB during CO₂-ECBM.

Fig.8 Contour of CO₂ content during CO₂-ECBM.

Fig.9 Evolution of CO₂ content on the reference line AB during CO₂-ECBM.

Fig.10 Evolution of permeability on the reference line AB during regular drainage.

Fig.11 Evolution of permeability on the reference line AB during CO₂-ECBM.

Fig.12 Gas production/sequestration rate.

Fig.13 Gas cumulative production/sequestration.

Tab.2 Scheme for gas injection time optimization

Fig.14 Gas production rates at different injection start times.

Fig.15 Cumulative CH₄ production at different injection times.

Fig.16 CO₂ cumulative sequestration at different injection times.

Fig.17 Variation law of CH₄ production/CO₂ sequestration and breakthrough time with the CO₂ injection start time.

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