Improved Convergence for Air-Water and CO2-Water TOUGH2 Simulations

Abstract

Numerical modelling has become an important tool in managing geothermal systems and planning their exploitation. The TOUGH2 simulator is the industry standard tool for developing numerical models. It includes several different equation-of-state modules and thus can be used for modelling many different kinds of geothermal fields as well as other complex sub-surface flow problems. Under certain conditions TOUGH2 has difficulty in running a model up to the very large times required for a natural state simulation as it stalls at a relatively small time step size. This problem leads to slow model development and also poses a significant obstacle to inverse modelling using iTOUGH2 or PEST as forward simulations are more computationally expensive and may not finish. In this paper two conditions leading to stalled simulations are identified and analysed. A correction to the air-water and CO2-water equation-of-state modules eliminates the first problem. The second is eliminated by an adjustment to the saturation temperature calculation. Results are presented showing a dramatic improvement in the simulation time required to achieve a steady state for large models of real geothermal systems.