*2.4. ECLIPSETM\_VISAGETM THM Modelling*

The THM modelling is performed using two commercial software packages, referred to here as the ECLIPSE™-VISAGE™ coupling. ECLIPSE™ is a conventional reservoir simulator used as the flow simulator for the multiphase flow processes. In the following study, the ECLIPSE™ 100 finite difference black oil simulator is used for the flow calculations. VISAGE™ is one of the most advanced and comprehensive stress analysis simulators commercially available. It is designed for applications in which nonlinear mechanics play a greater role, for example, in disciplines such as rock mechanics and geomechanics. It is flexible and can also be used in many other scientific disciplines, e.g., fluid mechanics, heat transfer, materials science, etc. The system provides high computational power and sophisticated modelling for many analytical situations, which currently include mining, civil engineering, reservoir engineering, and geothermal energy.

The advanced and comprehensive finite element code (VISAGE™) has been coupled with the ECLIPSE™ reservoir simulator to include geomechanical processes in this modelling study. Fluid flow is first calculated in ECLIPSE™, and the results are then transferred from ECLIPSE™ to VISAGE™ via the ECL2VIS interface for specified time steps. Updates of porosity and permeability can be calculated via constitutive relations, e.g., Kozeny–Carman.

The coupled THM models are based on three fundamental laws, namely Hooke's law of elasticity, Fourier's law of heat conduction, and Darcy's law of fluid flow in porous media. The governing equations are discussed above.

One-way coupling is usually sufficient for THM modelling of gas reservoirs, as gas compressibility dominates the bulk rock compressibility, and the mass balance is mainly controlled by gas pressure rather than by the stresses of solid rock [31]. The THM-coupled model presented in this paper is a one-way coupled model. With this approach, pressure data are transferred from the reservoir simulator (fluid flow simulator) ECLIPSE™ to the mechanical finite element simulator (geomechanical simulator) VISAGE™ at regular and/or critical times. The fluid pressure drives the geomechanics, but mechanically induced changes in porosity and permeability are not fed back into the dynamic reservoir simulation. It is therefore assumed that permeability and porosity are the same for each time step in the modelling and are not influenced by changes in stresses or ageing effects of the reservoir. This type of approach is feasible in this case study due to the high permeability of up to 80 mD, which is characteristic of the reservoir.
