A Critical Review of Core Analysis Techniques

Abstract

Abstract Considerations are presented for a coordinated program to secure and analyze cores that will yield desired objective; i.e. (1) porosity, permeability, lithology, residual fluid saturations – and probable production, (2) areal changes in porosity, permeability and lithology to characterize the reservoir, (3) interstitial water saturation and (4) samples unaltered in wetness and suitable for special core analysis tests. A review is made of currently used techniques for conventional core analysis with discussions of the strength and weakness of each. Included are comments on full-diameter, plug-size, sidewall, rubber sleeve core and combined core analysis approaches required in sands containing hyrdratable clays. Special core analysis tests available to assist in defining solutions to various reservoir problems are listed in tabular form for easy reference. Selected tests are discussed and illustrations of the effect of core wettability on certain results arc presented. INTRODUCTION CORE ANALYSIS remains the cornerstone upon which formation evaluation rests. Rock samples recovered by coring furnish information concerning lithology, storage capacity (porosity), and fluid content and deliverability (permeability). (See Table 1). Many of the basic principles used in core analysis today are the same as those originally established by the pioneers in the field. Techniques have been expanded and improved and instrumentation has undergone change; however, attention to detail and selection of the proper analytical technique for the rock type recovered are still essential to secure valid data. Changes occur in the in-situ saturations during (1) the coring operation and (2) the subsequent pressure reduction and gas expansion that occurs as the core is brought from reservoir temperature and pressure to atmospheric conditions. Core residual saturations determined in the laboratory reflect (1) coring fluid used and, hence, the filtrate lost, (2) the degree of flushing, (3) reservoir fluid properties, (4) rock relative permeability characteristics, (5) core packaging and preservation after removal from the core barrel to the time of analysis and (6) in some cases, the analytical technique. Control of the coring fluid, the core packaging and preservation, the analytical technique and to some degree the flushing, is within the grasp of the person establishing the coring program. The procedures followed should correspond to the objectives to be obtained. The remaining factors are inherent reservoir properties and are beyond control. Controllable factors are normally recorded and become an important part of the information needed for data analysis. The remaining factors influence residual gas, oil and water values and establish the ranges of saturation that indicate to the analyst whether a given formation will produce gas, oil or water. Wettability of cores is of little consequence in conventional core analysis. It is important when special tests are to follow core analysis, because certain tests made on cores non-representative of reservoir wettability yield invalid data. Objectives of the core analysis program Ideally, objectives of a core analysis program should be carefully defined before coring commences. In some cases the objectives conflict and it is impossible to satisfy all requirements on a given well.