Proposal of a suction approximation to simulate the behaviour of real gases

Modelling the gas behaviour is important in reservoir engineering and geomechanical problems related to the analysis of geological energy production and storage, as well as the storage of spent nuclear fuel and high-level radioactive waste. In such systems, the temperature and pressure of the gas can be so high that more advanced equations of state than the ideal gas hypothesis are required. In this case, formulations in which the variable of state of each gaseous species is its concentration will be of interest, and the pressure of the gas phase will be obtained with all concentrations. Assuming equilibrium between liquid water and its vapour, the density of the water vapour is determined by suction. When the matrix suction is taken to be equal to the capillary suction, the vapour concentration becomes a function of the gas pressure. As previously mentioned, this pressure is, in turn, a function of the concentrations of all the gaseous species, particularly of the water vapour density. This results in an implicit relationship that significantly complicates the numerical simulation. This work proposes a strategy to estimate suction without solving the above implicit equation. A study is then carried out to determine the error introduced when this strategy is adopted.