Adapting TOUGH2 for general equations of state with application to geological storage of CO2

The currently distributed version of the heat and mass flow model, TOUGH2, requires that pressure be a primary variable. This may limit the functionality and efficiency of the program, both in terms of convergence properties and ease of programming. TOUGH2 was adapted to allow for a general choice of independent (primary) variables. The changes to the code include increasing the size of the secondary parameter array for storage of pressure and changing the references to pressure in the flow equation module. Testing showed that the new code accurately replicates results of the original code. The program was applied to problems of relevance to geological storage of CO2 using the modified code (D-EOS), and the original code (P-EOS), as primary variables. Two scenarios, addition of heat and addition of mass, were simulated using both versions of TOUGH2. The simulations produced very similar results; however, in most instances, the density-based routine solved the problems faster and satisfied analytical constraints placed on the problems more accurately than the pressure-based runs. It was generally the case that the greater the modelled pressure gradients the better D-EOS performed. The new formulation adds flexibility to TOUGH2, both in terms of improved convergence behaviour and ease of programming. The new flexibility to choose any primary variables which uniquely specify the equilibrium state of a system allows for increased efficiencies in coding EOS routines, and may have important consequences in optimization of the solution of the flow equations for a given problem.