The Role of Geology in Reservoir Simulation Studies

Abstract

Reservoir simulation models are becoming increasingly sophisticated, resulting in part from the development of both geological and engineering reservoir-description methods. The geologist's research usually follows a four-step procedure that is applicable to most exploration and production operations. production operations. Introduction This paper describes the geological activities required for constructing realistic mathematical reservoir models. The methods used by a geologist are pointed out and the typical data provided by these methods are presented. Also, the geological controls on reservoir properties and the significance of these controls in reservoir performance are summarized. The information presented should be helpful to both the engineer and the geologist involved with reserves estimation and performance prediction. Reservoir simulation models are used increasingly to evaluate both new and mature fields, and to determine the most efficient management scheme. Part of the information contained in the model is provided by the geologist, based on his studies of the physical framework of the reservoir. To be useful, such studies must develop quantitative data. The geologist must also understand the type of engineering data needed and how the engineer can aid the quantification of the framework data. In the following discussion, reservoir simulation models are described briefly. This provides a base for discussion of the geologist's activities in simulation work and the assistance that he requires from the engineer. This discussion is restricted to the reservoir-description phase of simulation studies. The main topic is demonstration of the geological activities used in a study of a pilot-test site. The illustrations indicate the information that is often required for simulation modeling and demonstrate how geologic data can be synthesized to provide quantified information. The general application of the methodology is also described. The technology required for future studies is discussed. Reservoir Simulation Models Over the past few years, improved technology has made it possible to develop detailed and complex computer programs for simulating fluid flow in new and in programs for simulating fluid flow in new and in developed reservoirs. These dynamic models are based on the Darcy equation for fluid flow and on material-balance calculations. The simulation may be in one, two, or three dimensions and may involve one, two, or three fluid phases. Engineering literature contains further phases. Engineering literature contains further information on the variety and historical development of simulation models. A common simulation model is the two-dimensional, multicell cross-section shown in Fig. 1. Such a model is capable of handling vertical variations in porosity, permeability, and capillary properties for a limited (or permeability, and capillary properties for a limited (or typical) area. The variation is represented by assigning values to the essential parameters (Table 1) characterizing each cell or element. These and other data are entered into a computer program, and changes in fluid volumes, fluid state, etc., are calculated for each cell on a predetermined production (or injection) schedule. production (or injection) schedule.The distributions of the reservoir- and nonreservoir-rock types and of the reservoir fluids determine the geometry of the model and influence the type of model to be used. For example, the number and scale of the shale (or dense carbonate) breaks in the physical framework determine the continuity of the physical framework determine the continuity of the reservoir facies and influence the vertical and horizontal dimensions of each cell. Real variations in reservoir parameters may require several cross-sections or a parameters may require several cross-sections or a three-dimensional model. P. 625