Synergism in Reservoir Management The Geologic Perspective

Abstract

Improved hydrocarbon recovery can be obtained through the coordinated use of engineering, geology, and geophysics. Particularly in synergistic studies, geologists must be able to identify the rock properties that will be significant to oil and gas recovery. This paper presents an overview of geologic technology and the concepts and principles that guide its use. Introduction Rather than being homogeneous tanks or uniformly layered entities, most reservoirs exhibit complex variations of reservoir continuity and thickness patterns and of pore-space attributes (porosity, permeability, and capillary-pressure properties). The reservoir interval is commonly subdivided vertically and areally into "pay zones" that are separated by impermeable rock units; the pay zones themselves often contain thin shale or tight pay zones themselves often contain thin shale or tight carbonate streaks. Thickness distributions of pay zones may be sheet-like or linear and, within the rock framework, pore-space attributes may vary in a predictable or random manner. It is this complexity of rock framework and pore-space variation that challenges petroleum scientists to apply their technology and experience in reservoir description, with the aim of improving recovery. The best way to identify and quantify rock framework and pore-space variations is through the deliberate and integrated use of engineering and earth-science technology. Reservoir studies are more effective when geologists and engineers determine jointly, at the outset, the course of investigation, the work-area responsibility for each professional on the description team, and the target dates for combining results. Such an approach to reservoir description requires an understanding of the technology used by other professionals and an awareness of the principles and concepts upon which the technology is based. Furthermore, this understanding and awareness will promote the free exchange of ideas - a fundamental facet of synergistic activities. This paper presents an overview of geologic technology and the concepts and principles that guide its use. Four topics are covered. First, the geologic procedures used in reservoir description are presented together with a brief discussion of the emphasis required for different recovery projects. The next two topics deal with the details of the technology and guiding principles and concepts used to describe sandstone and carbonate reservoirs. Finally, synergy needs for the future are discussed. Geologic Activities in Reservoir Studies General Activities The steps commonly followed by the geologists are indicated in Fig. 1, together with some work areas where the geologist needs to combine his efforts with those of the engineer. These steps are listed in their general order of accomplishment, although sometimes it is worthwhile to "look ahead" before completing a particular step. Rock studies involve using cuttings, cores, well logs, and routine core-analysis data to identify the rock types (both potential reservoir and nonreservoir types that make up the reservoir interval) and to interpret the depositional origin of the interval. These data provide fundamental information for predicting reservoir continuity and thickness patterns and variations in pore-space properties. Typical of the information developed in this step properties. Typical of the information developed in this step are core-description graphs and porosity-permeability cross-plots. Framework studies determine the geometric configuration of the trap and the vertical and lateral distribution of the rock types identified previously. JPT P. 761