Reservoir Simulation: State of the Art

Abstract

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing 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. "Today, desktop computers and a wide range of reservoir. simulation systemsprovide the engineer with an economic means to solve complex reservoir.engineering problems with efficiency." Introduction This paper describes the state of the art in reservoir simulation for thenonexpert, focusing primarily on advances that have occurred since a similarpaper was published by Coats1 in 1982. For completeness, a historyof simulation is given first, followed by a brief and simple review of thesimulation process. Advances in simulation technology are then described, highlighting those in reservoir description and simulation of naturallyfractured reservoirs, hydraulic fracturing, and horizontal wells. The future ofreservoir simulation is discussed last. Many papers have been presented since Coats' review in 1982. Rather thanrepeat the exhaustive list provided in the SPE indices, a reduced set ofpertinent references is given. I offer my apologies in advance to the authorsof the many excellent papers contributed during the past decade that are notmentioned here. History of Simulation In the broadest sense, reservoir simulation has been practiced since thebeginning of petroleum engineering. By the late 1940's, the potential ofreservoir simulation was recognized and several companies embarked on thedevelopment of both analog and numerical models to enhance the existinganalytical solutions, material balances, and ID Buckley-Leverett displacementcalculations. By the late 1950's, major commitments to fundamental research on thenumerical solution of flow equations began to bear fruit. The results werecrude, but certainly useful, computer programs for simulating reservoirs. Theseprograms represented a major advance and used the solution of sets offinite-difference equations to describe 2D and 3D, transient, multiphase flowin heterogeneous-porous media. For the first time, reservoir engineers couldsolve complex problems. The driving force for development of numerical-simulation systems was theability to predict the future performance of a reservoir under various "whatif" conditions so that better reservoir-management decisions could be made. This force still drives the use of reservoir simulation today. Current businessincentives for virtually all reservoir-simulation applications fall into one ormore of the following categories: economics; credibility and reliability;decision-making; arbitration and unitization; performance monitoring;nonelective studies resulting from safety, environmental, or regulatoryconcerns; and communications and personnel utilization.2 Throughout the 1960's, reservoir-simulation development was devoted largelyto three-phase, black-oil reservoir problems. Recovery methods that weresimulated included pressure depletion and various forms of pressuremaintenance. Many computer programs were developed to address most of thegeneral reservoir problems encountered. These programs operated on mainframecomputers and used decks of cards for input. During the 1970's, the picture changed remarkably because of theproliferation of research in EOR processes, advances in numerical-solutiontechniques, and the decreased size and increased speed of computers. Mathematical simulators were developed that extended beyond conventionalprimary and secondary recovery into such areas as chemical flooding, steamflooding, and in-situ combustion. Research during this period resulted insignificant advances in characterizing the physics of hydrocarbon displacementsunder the influences of temperature, chemical agents, and complexmulticomponent phase behavior. Simulators had to reflect chemical adsorptionand degradation, reaction kinetics, interfacial-tension-reduction effects, andmore complex equilibrium phase behavior. The range of reservoir-simulation application continued to expand during the1980's. Reservoir description, finally becoming a major topic on its own, advanced to the use of geostatistics to describe heterogeneities and to providereservoir definition not expressed in reservoir models. The technology to modelnaturally fractured reservoirs, including compositional effects, was alsodeveloped, with extensions into the simulation of hydraulic fracturing andhorizontal wells. and application to such complex processes as reservoirmonitoring.