Numerical tuning in reservoir simulation: it is worth the effort in practical petroleum applications

Abstract

Giant carbonate oil fields in Brazilian pre-salt area present a high level of heterogeneity. Despite the fact that geoscientists aim at modelling these complex models in a high-block cell resolution, it is important to reduce the computational effort and speed up some processes such as forecasting the production during a risk analysis in a probabilistic approach. This practice must be evaluated to keep numerical and geological consistency of reservoir models. As a result, creating a procedure to assist petroleum scientists and engineers so as to reduce computational time, without losing accuracy, is the main goal of this proposed work. A methodology based on numerical parameter evaluation, tuning technique, diagnosis and application is defined to provide a robust and effective starting point of the numerical control settings during reservoir simulation model runs. All of the analyses are based on a good understanding of the reservoir heterogeneities (geologists) and characteristics (reservoir engineering) that may result in reduced central processing unit time, number of time-step cuts, solver failures and material balance compared to the default approach considered for these problems. In addition, to check the consistency of the proposed procedure, one building risk curve application for two case studies was selected, assuming that we are executing the tuning and the application in sequence of a common reservoir application. The results of the two case applications under our proposed assumptions showed that it is possible to (1) speed the simulation runs up to three times in comparison with the base case which uses the default numerical parameters of a commercial simulator, (2) reduce the number of cuts and failures and (3) control the material balance equation error within a previously defined tolerance. Despite decreasing the simulation run time in practical reservoir study applications, we are also avoiding bad combination of numerical inputs which can result in unfeasible reservoir numerical models. To conclude, depending on the amount of applications, number of runs and complexity of reservoir numerical model, the results showed that we can save time in the reservoir engineer’s routine activities while maintaining the consistency of reservoir models by dedicating an initial time to better understand and optimise the numerical parameters. Even though the vast majority of reservoir numerical models are created in high-resolution grid cells to maintain the level of heterogeneity and, consequently, increase the computational effort, more suitable numerical model quantifications should be conducted in order to reduce the running time. Nonetheless, a consistency in the reservoir model must be maintained during the entire procedure. Thus, the most important step before starting any reservoir simulation workflow is to set the numerical parameters to avoid convergence problems, to keep the consistency of the numerical simulation results (approaching the solution of nonlinear equations to the true solution) and to assist geoengineering in starting the reservoir numerical parameters consciously. In addition, we can make sure that appropriate controls of the numerical model are then being used efficiently as a practice in reservoir simulation studies, mainly before starting to run a large amount of simulations.