Giga Cell Compositional Simulation
Authors: Obi E. , Eberle N. , Fil A. , Cao H. 
Published: May 2016

Abstract
Simulation models are often compromised (coarse cells, property up-scaling, incomplete physics), due to limitations in simulator technology and access to high computing power. Both computing power and technology have vastly evolved over the past 30 years. Unfortunately, previous choices made in the legacy reservoir simulators have limited their ability to adopt, and hence prevented them from harnessing these advances in an optimal fashion resulting in inefficiencies in parallel runs.
We present the results of a next generation highly scalable commercial simulator on a Giga cell multi-component, compositional model of a gigantic field with several hundred wells and several years of production.
The previous model was a 5.7 million active cell, multi-component compositional model that did not properly capture the main lateral and vertical heterogeneities. These heterogeneities consist of very thin high permeability streaks that play a major role in the pressure depletion. With the acquisition of a next generation simulator technology, a higher resolution model, with 47 million active cells was built. The performance of our next generation simulator, complemented with in-house developments, was a substantial 4-fold faster in CPU time than the same case using a legacy commercial simulator.
However, the average cell size was still in the order of hundreds of meters laterally for this new model. A refinement of the cell size resulted in a billion cell (Giga) model. This model was simulated using our high-performance cluster computer. We also discuss the challenges of the simulation workflow in terms of pre-post Processing and IT environment.

Introduction
Reservoir rocks have a highly complex disorderly structure over a wide range of length scales. The heterogeneities in these rocks play a very dominant role in the transport of fluid within them. Therefore, better understanding of the processes that determine the amount of hydrocarbon that can be recovered is an important objective for the oil industry.
To understand and better predict how fluids will flow in the reservoir, it is necessary to determine the nature, extent and distribution of non-reservoir intervals. The latter highlights the importance of understanding the nature of reservoir heterogeneity, which occurs at various scales from large-scale faults, external and internal sand body characteristics, to microscopic features such as porosity and permeability. To deal with these uncertainties, the conventional approach is to build a detailed geological model, which is then upscaled and entered into a legacy reservoir simulator. Important geological features are most times lost during this upscaling process.
Legacy reservoir simulators were developed at a time when high performance computing technology was still at its infancy. Unfortunately, choices made in the development of these simulators limited their ability to adopt, and hence prevented them from harnessing the advancement in computing power and technology in an optimal fashion. This led to some compromises in the building of reservoir simulator models. However, with the advancement of computer technology especially in the domain of parallel and high performance computing, modifying the legacy simulators to take full advantage of these technologies was not only cumbersome but compromises had to be made.