Ali H. Dogru

A Massively Parallel Reservoir Simulator for Large Scale Reservoir Simulation

A Massively Parallel reservoir simulator (POWERS) has been developed and fully integrated with Suadi Aramcos pre and post processing graphical environment. The simulator has been validated against published results and other industrial simulators. Three different giant oil field studies using million grid blocks were carried out by using the new simulator. Benchmark runs indicated near linear performance improvement with linearly increasing number of processors. With the current capabilities mega cell construction, preprocessing and post processing is quite fast. History match runs involving over million grid block and 30-50 years of history is obtained in less than one hour on a low cost parallel computer.

Studies of Robust Two Stage Preconditioners for the Solution of Fully Implicit Multiphase Flow Problems

The solution of the linear system of equations for a large scale reservoir simulation has several challenges. Preconditioners are used to speed up the convergence rate of the solution of such systems. In theory, a preconditioner defines a matrix M that can be inexpensively inverted and represents a good approximation of a given matrix A. In this work, two-stage preconditioners consisting of the approximated inverses M1 and M2 are investigated for multiphase flow in porous media. The first-stage preconditioner, M1, is approximated from A using four different solution methods: (1) constrained pressure residuals (CPR), (2) lower block Gauss-Seidel, (3) upper block Gauss-Seidel, and (4) one iteration of block Gauss-Seidel. The pressure block solution in each of these different schemes is calculated using the Algebraic Multi Grid (AMG) method. The inverse of the saturation (or more generally, the nonpressure) blocks are approximated using Line Successive Over Relaxation (LSOR). The second stage preconditioner, M2, is a global preconditioner based on LSOR iterations for the matrix A that captures part of the original interaction of different coefficient blocks. Several techniques are also employed to weaken the coupling between the pressure block and the nonpressure blocks. Effective decoupling is achieved by: (1) an IMPES-like approach designed to preserve the integrity of pressure coefficients, (2) Householder transformations, (3) the alternate block factorization (ABF), and (4) the balanced decoupling strategy (BDS) based on least squares. The fourth method is a new technique developed in this work. The aforementioned preconditioning techniques were implemented in a parallel reservoir simulation environment, and tested for large-scale two-phase and three-phase black oil simulation models. This study demonstrates that a two-stage preconditioner based on balanced decoupling strategy (BDS) or ABF combined with Gauss-Seidel sweeps, that also incorporate nonpressure solutions for M, delivers both the fastest convergence rate and the most robust option overall without compromising parallel scalability.

Using Unstructured Grids for Modeling Densely-Spaced Complex Wells in Field-Scale Reservoir Simulation

Reservoir simulation predominately uses structured grids, but they have difficulties for field-scale simulation with maximum reservoir contact (MRC) wells. In this work, the grid resolution for numerical convergence using structured grids is studied. Results showed that sufficiently fine grid is needed to obtain converged solution. In common practice, the geocellular models are simply upscaled and used in history matching. This study highlights the need for grid resolution to resolve flow dynamics in reservoir simulation and improve well inflow performance calculation. Otherwise, the near-well flow may not have converged and the fidelity of the models for performance prediction is in question. Improving the fidelity of simulation results by adequately modeling the complex multiphasic flow dynamics near complex wells in full-field simulation is of paramount importance nowadays. This work introduces a full-field unstructured gridding method which can optimally place unstructured grid cells where the resolution is needed. The method produces a consistent discretization that is efficient to compute by using a parallel unstructured reservoir simulator. For a giant Middle-East carbonate reservoir that was developed primarily using complex MRC wells, unstructured grid models were used to improve the accuracy of near-well flow and to better represent well inflow performances. The unstructured grid models are compared against the original structured grid models that show computational cost saving and require few grid cells. Simulation results demonstrate an unstructured workflow that is practical and can be used to validate near-well modeling accuracy for the existing structured grid simulation results. The method is particular attractive for situation with denselyspaced complex wells where a structured local grid refinement (LGR) method will be ineffective. An unstructured grid is well suited to honor the near-well flow geometry and to focus grid resolution where it is needed. Perpendicular bisection (PEBI) grids are orthogonal by construction. This reduces computational complexity because two-point flux approximation (TPFA) can be applied. In field-scale simulation, this results in a significant improvement to accuracy and computational cost savings.

Interior boundary-aligned unstructured grid generation and cell-centered versus vertex-centered CVD-MPFA performance

Grid generation for reservoir simulation must honor classical key constraints and be boundary aligned such that control-volume boundaries are aligned with geological features such as layers, shale barriers, fractures, faults, pinch-outs, and multilateral wells. An unstructured grid generation procedure is proposed that automates control-volume and/or control point boundary alignment and yields a PEBI-mesh both with respect to primal and dual (essentially PEBI) cells. In order to honor geological features in the primal configuration, we introduce the idea of protection circles, and to generate a dual-cell feature based grid, we construct halos around key geological features. The grids generated are employed to study comparative performance of cell-centred versus cell-vertex control-volume distributed multi-point flux approximation (CVD-MPFA) finite-volume formulations using equivalent degrees of freedom. The formulation of CVD-MPFA schemes in cell-centred and cell-vertex modes is analogous and requires switching control volume from primal to dual or vice versa together with appropriate data structures and boundary conditions. The relative benefits of both types of approximation, i.e., cell-centred versus vertex-centred, are made clear in terms of flow resolution and degrees of freedom required.

Correction to: Interior boundary-aligned unstructured grid generation and cell-centered versus vertex-centered CVD-MPFA performance

Due to an oversight, some author’s corrections were not carried out during Performing proof corrections stage. The Publisher apologizes for these mistakes. The original article was corrected.

Seismic-to-simulation Integration for Autonomous Fields and Real-time Monitoring: The Advantage of Gigacell Simulation

Reservoir simulation technology capable of handling billions of cells makes it possible to model reservoirs with unprecedented geological and fluid characterization detail, either minimizing or eliminating upscaling and its adverse effects. The speed of these simulations, aided by new computer technologies, will enable us to make inroads into new paradigms, such as autonomous fields coupled with seismic-to-simulation integration.