Practical Use of Scale Up and Parallel Reservoir Simulation Technologies in Field Studies
Authors: Tchelepi H.A. , Durlofsky L.J. , Chen W.H. , Bernath A. , Chien M.C.H. 
Published: December 2007

Abstract
Scale up and parallel reservoir simulation represent two distinct approaches for the simulation of highly detailed geological or geostatistical reservoir models. In this paper, we discuss the complementary use of these two approaches for practical. large scale reservoir simulation problems. We first review our recently developed approaches for upscaling and parallel reservoir simulation. Then, several practical large scale modeling problems, which include simulations of multiple realizations of a waterflood pattern element, a four well sector model and a large, 130 well segment model, are addressed. It is shown that, for the pattern waterflood model, significantly coarsened models provide reliable results for many aspects of the reservoir flow. However, the simulation of at least some of the fine scale geostatistical realizations, accomplished using our parallel reservoir simulation technology, is useful in determining the appropriate level of scale up. For models with large numbers of wells, the upscaled models can lose accuracy as the grid becomes coarse, due in part to the lack of areal resolution between wells. In these cases, though field-wide performance can still be predicted with reasonable accuracy, parallel reservoir simulation is required to maintain sufficiently refined models capable of accurate flow results on a well by well basis. Finally, some issues concerning the use of highly detailed models in practical simulation studies are discussed.

Introduction
Reservoir description and flow modeling capabilities continue to benefit from advances in computing hardware and software technologies. However, the level of detail typically included in reservoir characterizations continues to exceed the capabilities of traditional reservoir flow simulators by a significant margin. This resolution gap, due to the much larger computational requirements of flow simulation, has driven the development of two specific technologies: scale up and parallel reservoir simulation. These two technologies represent very distinct approaches - scale up methods attempt to coarsen the simulation model to fit the hardware while parallel reservoir simulation technology attempts to extend computing capabilities to accommodate the detailed model.
The purpose of this paper is to present and discuss ways in which to utilize these two technologies in a complementary fashion for the solution of practical large scale reservoir simulation problems. Toward this end, we first discuss our previously developed capabilities for scale up and parallel reservoir simulation. Next, the two technologies are applied to several reservoirs represented via highly detailed (i.e., on the order of one million cells) geostatistical models. Various production scenarios are considered. It will be shown how the direct simulation of the highly detailed models (using parallel reservoir simulation technology on an IBM SP/2) can be used to assess and guide the scale up procedure and to establish the appropriate level of coarsening allowable. We will show that, once this level is established, upscaled models can be used to evaluate multiple geostatistical realizations.
We additionally apply the detailed simulation results to develop general guidelines for the degree of scale up allowable for various types of simulation models; e.g., pattern, sector and large segment models. Our general conclusion is that our scale up technology, as currently used, is quite reliable when sufficient refinement is maintained in the coarsened model. Thus, single processor workstations can be used to obtain accurate flow performance predictions for many pattern and sector models (e.g., 40,000-100,000 cell models). We show that when many wells are to be simulated, the upscaled models can begin to lose accuracy, particularly when well by well production is considered. This is due in part to the fact that, in the coarse models, wells are separated by very few grid blocks, and degradation in accuracy results. In this case. parallel reservoir simulation technology is essential to accurately predict local flow behavior.
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