Variables Affecting Restored-State Determination of In-Place Oil

One of the important functions of the petroleum engineer is the volumetric analysis of the reservoir for an estimate of in-place oil. The reliability of the estimate is a joint function of the accuracy of basic data used and of the correctness of application. Frequently, basic data are obtained from laboratory determinations of porosity and interstitial water saturation. It follows that errors in laboratory measurement may have a serious effect on reservoir evaluation. A better understanding of the methods used, of the variables that affect it, and of consistent variations that tend to yield high or low results will allow the laboratory worker to produce more accurate values. Similarly, a better understanding on the part of the engineer will permit him to use the data more intelligently so that unavoidable errors may be corrected rather than compounded, and so that an increased reliability of reservoir evaluation may result. This paper presents the results of investigations of variables which may lead to errors in some standard porosity methods, and in the restored-state [disc] determination of interstitial water. Variables of preferential wettability, clay hydration and gas adsorption are shown to be interrelated in their effects. Additional variables related to equipment, technique, capillary pressures, fluid phases and calculation procedures are considered. It is shown that a report would more properly present porosity available for hydrocarbon storage than simply porosity and interstitial water content. Data obtained statistically from a quantity of restored-state and routine core analysis work are offered as supporting evidence. The variables are each treated individually, so that each may be understood separately, as well as in its interrelationship with others. Introduction The restored-state concept has been widely utilized as an indirect approach to petroleum reservoir evaluation. The term denotes a laboratory simulation of reservoir processes of oil accumulation and production in a sample of reservoir rock. Determination of interstitial water has been a primary concern of restored-state work during its relatively short life. Publications have dealt with variously devised interstitial water methods and modifications of methods. In one particular aspect much of the reported work has been somewhat idealized in that implicit assumption has been made that porosity measurement is not a problem as associated with determination of interstitial water. In this paper, consideration is given variations between some usual porosity methods which are utilized in routine core analysis and restored-state work.