Unstructured Grids for Numerical Reservoir Simulation - Using TOUGH2 for Gas Storage

For numerical reservoir simulation structured grids are commonly used. These grids are following the geological structure and are bound to the layering of the reservoir In principal these grids should be regular for the task to solve the differential equations describing the flow of fluids in a reservoir in space and time. For the means of a better adaptation of the grid to the geological structure so called corner point grids have been introduced, which strictly follow the reservoir layering. At faults these corner point grids encounter the problem that grid blocks belonging to different layers should communicate with each other Formally these non neighbor connections do not comply with the mathematical scheme of the finite difference solution approach. Another approach for the simulation of fluid and heat flow in reservoirs is the unstructured gridding as it is e. g. used in the TOUGH2 simulator Grid cells of any geometry can be defined. The location in space of a grid cell is not used in the simulation. Only the volume of a grid cell and the relation to its neighboring cells has to be defined. For each cell the net flow of mass and heat is calculated during the simulation. The flow equations are then represented as Taylor series of the primary variables like temperature or pressure. The Taylor series is truncated after the first term and the resulting system of linear equations for the residuals can be solved using standard lgorithms. Typically the system is solved implicitly in a few iteration steps. The TOUGH2 simulator has been developed by K Pruess at the University of California in Berkeley. It is written in Fortran and the source code can be obtained under a license. The way the software is designed makes it very flexible but this concept also makes it difficult to use. The typical reservoir modeling tools applicable in the oil and gas industry cannot be directly used to create a simulation model with TOUGH2. This paper describes how input data from a simulation input deck such as for Eclipse can be used and how corner point grids can be converted into TOUGH2 input data. The TOUGH2 code can be extended to output data in a format used in oil and gas field simulations. The work flow is discussed by means of examples for gas injection and production.