Reservoir Simulation: State of the Art (includes associated papers 11927 and 12290 )
Authors: Coats Keith 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

Introduction
The purpose of this paper is to describe the current level of development inreservoir simulation. This requires some discussion of what a simulation modelis and why it is needed or used. Following a brief history of simulation and ageneral description of a simulation model, two sections describe the reservoirsimulator through discussions of recovery mechanisms and model methodology. Thesecond of these sections discusses past and recent developments and summarizesthe technology currently used in simulation models. The two descriptivesections are followed by a discussion of why simulation is used (i.e., typicalreservoir performance questions addressed by computer simulation), a sectionwith examples pertinent to simulation today, and a summary.

A Brief History
In it broad sense. reservoir simulation has been practiced since thebeginning of petroleum engineering in the 1930's. Simulation is simply the useof calculations to predict reservoir performance (to forecast recovery orcompare economics of alternative recovery methods). Before 1960. thesecalculations consisted largely of analytical methods, zerodimensional materialbalances, and one-dimensional (ID) Buckley-Leverett calculations. The term"simulation" became common in the early 1960's. as predictive methods evolvedinto relatively sophisticated computer programs. These programs represented amajor advancement because they allowed solution of large sets offinite-difference equations describing two- and three-dimensional(2- and 3D), transient, multiphase flow in heterogeneous porous media. This advancement wasmade possible by the rapid evolution of large-scale, high-speed digitalcomputers and development of numerical mathematical methods for solving largesystems of finite-difference equations. During the 1960's. reservoir simulationefforts were devoted largely to two-phase gas/water and three-phase black-oilreservoir problems, Recovery methods simulated essentially were limited todepletion or pressure maintenance. It was possible to develop a singlesimulation model capable of addressing most reservoir problems encountered.This concept of a single, general model always has appealed to operatingcompanies because it significantly reduces the cost of training and usage, and, potentially, the cost of model development and maintenance. During the 1970's, the picture changed markedly. The sharp rise in oil prices and governmentaltrends toward deregulation and partial funding of field pilot projects led to aproliferation of enhanced-recovery processes. This led to simulation of newprocesses that extended beyond conventional depletion and pressure maintenanceto miscible flooding, chemical flooding, C02 injection, steam or hot waterstimulation/ flooding. and in-situ combustion. A relatively comfortableunderstanding of two-component(gas and oil) hydrocarbon behavior in simpleimmiscible flow was replaced by a struggle to unravel and characterize thephysics of oil displacement under the influence of temperature, chemicalagents, and complex multicomponent phase behavior. In addition to simplemultiphase flow in porous media, simulators had to reflect chemical adsorptionand degradation, emulsifying and interfacial tension (IFT) reduction effects, reaction kinetics, and other thermal effects and complex equilibrium phasebehavior. The proliferation of recovery methods in the 1970's caused adeparture from the single-model concept as individual models were developed torepresent each of these new recovery schemes. Thus, the emphasis today is onexamining and fine tuning the equations and related assumptions pertinent tothese techniques. Research during the 1970's resulted in many significantadvances in simulation model formulations and numerical solution methods. Theseadvances allowed simulation of more complex recovery processes and/or reducedcomputing costs through increased stability of the formulations and efficiencyof the numerical solution methods.