Numerical analysis of dual porosity coupled thermo-hydro-mechanical behaviour during CO2 sequestration in coal

Abstract

This study presents a coupled dual porosity thermal-hydraulic-mechanical (THM) model of nonisothermal gas flow during CO2 sequestration in coal seams. Thermal behaviour is part of the disturbed physical and chemical condition of a coal seam caused by CO2 injection, and must be understood for accurate prediction of CO2 flow and storage. A new porosity-permeability model is included for consideration of the fracture-matrix compartment interaction. The new model is verified against an analytical solution and validated against experimental measurements, before being used to analyse coupled THM effects during CO2 sequestration in coal. A simulation of CO2 injection at a fixed rate shows the development of a cooling region within the coal seam due to the Joule-Thomson effect, with the temperature in the vicinity of the well declining sharply before recovering slowly. The temperature disturbance further from the well is more gradual by comparison. Under the simulation conditions studied, CO2 injection increases coal matrix porosity and decreases the porosity and permeability of the natural fracture network, especially in the vicinity of the injection well, due to adsorption-induced coal swelling. Compared with the effects of gas pressure and temperature, the matrix-fracture compartment interaction plays an important role in changes of porosity and permeability. Considering the temperature disturbance caused by CO2 injection under the set of representative conditions studied, the coupled model can provide an insight into the associated effects on CO2 flow and storage during its sequestration in coal seams.

Modeling Non-isothermal Transport Behavior of Real Gas in Deformable Coal Matrix

“…In order to understand the effects of nanoscale pore size, different pore sizes of 1, 5, 10, 25, and 50 nm are considered (permeability ranging from 1.5 × 10 −23 to 3.9 × 10 −20 m 2 ). The parameters used for the numerical simulation are listed in Table 4, which are chosen from validation tests above and work by Chen et al 26 Figure 9 shows the varitions of gas pressure and adsorbed gas concentration both at the P1 and P2 observation points of simulation domain. It can be seen from Figure 9 that gas transport behaviors in micropores (below 2 nm) and mesopores (between 2 and 50 nm) are different.…”

“…In this section, a set of governing equations for describing gas transport in deformable porous media under non-isothermal conditions is presented. The details of the coupled THM model including multicomponent gas and chemical transport are available in Thomas and He, 24 Masum and Thomas, 25 Chen et al, 3 and Hosking et al 26 Here, the theoretical framework is extended to include the nanopore gas transport processes. The coupled model consists of four main components: the thermoporoelastic mechanical model, free and adsorbed gas phase transport models, and heat transfer model under local thermal equilibrium.…”

“…where J h is the heat flux through the rock matrix and λ e is the effective thermal conductivity of media, which is approximated as 26 n where λ g and λ a are the thermal conductivity of free gas and adsorbed gas, respectively, and λ s is the thermal conductivity of solid skeleton.…”

“…The stress–strain constitutive relationship is where G = E e /2­(1 + v ) is the shear modulus, λ = E e v /(1 + v )­(1 – 2 v ) is the Lamé constant, E e is Young’s modulus, ε ij e represents the component of the elastic strains, and ε v e = ε 11 e + ε 22 e + ε 33 e is the elastic volumetric strain.…”

“…The total strain, ε ij , can be expressed as where ε T is the thermal expansion-induced volumetric strain and ε ad is the adsorption-induced swelling strain.…”