Detailed reservoir description studies usually precede the selection of an enhanced recovery process and the establishment of operational procedures for a particular field. Considerable attention sometimes must be paid to the rock framework because some recovery processes are very sensitive to changes in physical properties.
Reservoir geology studies have several objectives:
determining the number and the vertical and lateral limits of reservoir units,
establishing the thickness distribution of reservoir units and the occurrence of "shales" in these units.
developing the three-dimensional distribution of porosity permeability, and hydrocarbon saturation properties for each unit.
locating areas where various reservoir units are in vertical pressure communication, and
providing information on the storage capacity and permeability capacity of the aquifer.
Using a pilot test site as an example. a four-step procedure for geologic studies is presented. The four steps are essentially functional in nature:
determination of reservoir continuity and geometry.
evaluation of reservoir quality, and
integration of framework data.
Sources for data include cuttings and cores, well logs, core plugs, seismic surveys, well tests, and pressure-production records. Experience gained from outcrop and subsurface studies often guides the data analysis.
This approach to detailed reservoir description is designed to determine the relative influence of depositional and post-depositional processes an reservoir characteristics. In the case of sandstones, for example. we first need to know whether the reservoir units are fluvial, deltaic, near shore or turbidite deposits. Subsequently, we must as certain how compaction, faulting (and fracturing), cementation, and leaching have modified the depositional character.
The accuracy of the description commonly is evaluated by computer modeling, pilot studies or both. As in the earlier steps, the geologist, geophysicist, and engineer must work closely together to provide the best possible description.
Many candidates for enhanced recovery operations require careful geologic study to ascertain the physical framework that contains residual hydrocarbons. Accurate knowledge of this physical framework not only permits firm calculations of "remaining reserves" but provides critical information that will aid the selection of the most appropriate recovery scheme.
Because stratigraphic properties are usually the most common framework factor needing to be determined this paper will deal mostly with methods for developing reservoir geometry, continuity, and quality. Although discussion is limited to sandstone reservoirs, the methods presented are applicable to carbonates, with some modification required where secondary porosity is important. Fractured reservoirs mayor may not be suitable for analysis according to the procedures described herein.
The main topic of this paper1 is the presentation of a four-step procedure for reservoir geology analysis. The procedure will be described in general and the various steps will be demonstrated by means of (I case study. Before proceeding to the main topic, brief mention will be made of the principal factors that influence reservoir geometry, continuity, and quality. This will provide a context for the description procedure.