Simulation of High Relief Reservoirs, Rainbow Field, Alberta, Canada
Authors: McCulloch R.C. , Langton J.R. , Spivak A. 
Published: December 2007

A comparison of the three-dimensional and one-dimensional models for both natural depletion and crestal gas injection shows that for high relief reservoirs, a one-dimensional model is adequate for many reservoir engineering applications.

Introduction
The Rainbow field of northwestern Alberta was discovered in 1965 and currently contains 44 separate reef reservoirs with areal extents up to 4,600 acres, and productive oil pay zones up to 600 ft. Production is from the Rainbow member of the middle Devonian Keg River formation, which occurs at a depth of approximately 6,000 ft. Average porosity values range from 4 to 15 percent and typical average values of vertical and horizontal permeability are 40 and 200 md, respectively. Excellent reservoir communication has allowed wide well spacing ranging from 160 to 640 acres. Oil in place varies from less than 10 million bbl in the smaller pinnacle reefs to 260 million bbl in the larger atolls. Reef buildups in the Rainbow area are shown schematically in Fig. 1. In the Rainbow field, many different types of secondary recovery projects have been initiated, including bottom-water drive, projects have been initiated, including bottom-water drive, crestal gas injection and vertical downward miscible slug displacement. Gravity drainage controls all recovery mechanisms, and typical predicted recovery factors are 55 percent for natural depletion, 65 percent for gas or water injection and 95 percent for percent for gas or water injection and 95 percent for miscible displacement.
Under the Alberta oil proration system, allowables are distributed among pools on the basis of recoverable reserves. Therefore, to define allowables in the Rainbow reservoirs it is necessary to evaluate accurately the recoverable reserves for both primary depletion and secondary recovery projects. To perform these studies, several mathematical reservoir simulators were used. These included one-, two- and three-dimensional, three-phase, compressible and incompressible models. The use of these different models for studying various depletion mechanisms for many different reservoirs offered a unique opportunity to evaluate their speed, cost and areas of application. This paper discusses the use of geology in defining the spatial distribution of reservoir properties; some particular problems that arose in the simulation of particular problems that arose in the simulation of Rainbow-type reefs and the methods used to solve some of these problems. The functional steps in the multidimensional simulation of performance are shown graphically in Fig. 2.

Use of Geological Studies to Define the Spatial Distribution of Reservoir Properties
The wide well spacing in the Rainbow area led to extensive use of geological studies to define the spatial distribution of reservoir rock properties (porosity, permeability, capillary pressure, relative permeability) for use in mathematical models. Detailed permeability) for use in mathematical models. Detailed studies have been performed on the reefs in the Rainbow area, including the examination of data from 45 wells in 13 separate reservoirs and 15,000 ft of core. These studies revealed the presence of 14 geological rock types or lithofacies. The 14 rock types represent six depositional environments: basin, forereef, organic reef, backreef, lagoon and shoal.
JPT
P. 1399