Compositional Reservoir Simulation Using (T,V) Variables-Based Flash Calculation
D. Paterson, M.L. Michelsen, E.H. Stenby and W. Yan
Event name: ECMOR XVI - 16th European Conference on the Mathematics of Oil Recovery
Session: Physical Modelling II
Publication date: 03 September 2018
Info: Extended abstract, PDF ( 435.74Kb )
Price: € 20
Phase equilibrium calculation is at the heart of compositional reservoir simulation. The conventional example is the isothermal isobaric flash (T,P) which must be solved in every grid-block at each iteration during simulation. This is conventionally solved through the repeated use of stability analysis and phase-split calculation. To accurately represent the fluid properties it is useful to use an equation of state. Most compositional reservoir simulations use the cubic equations of state (SRK or PR). More advanced equations of state which, for example, take account of association are attractive alternatives (e.g. CPA or PC-SAFT) for some simulations. Each of these models are functions for the Helmholtz energy with the natural variables (T,V,n), using the conventional flash framework it is necessary to solve the equation for volume (at a given pressure) at each iteration. Though simple for the cubic equations of state this is a more significant issue when using advanced equations of state where the association equations must also be solved iteratively at each volume iteration. This increase in computational cost is one reason that the more advanced equations of state are not yet in common use. An alternative framework to solve the isothermal isobaric flash problem is possible. Instead of solving the equation of state for volume at each iteration the pressure of the phases is matched only at the final equilibrium point. This allows for the natural variables, (T,V,n), of the equation of state to be used to co-solve the equation of state with the equilibrium equations. Using this framework means that the equation of state does not need to be solved for volume at each iteration. This means that the more complex equations of state are only marginally more computationally expensive than the simple cubics. In this work we will present a method to solve the (T,P) flash problem using the natural variables of the equation of state, (T,V,n). The resulting framework will be used in a multiphase, compositional, 3D reservoir simulator and demonstrated using a number of examples. The computational cost of the proposed method will be compared with the conventional method for solving the (T,P) flash problem when solving the same simulation problem.