Core Analysis for Aid in Reservoir Description

Abstract

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering. Introduction Core analysis provides a varied menu of laboratory data for reservoirdescription. Included are measured values, visual observations, andphotographs. Heterogeneity, degree of consolidation, and other formationcharacteristics impose unique core-analysis procedures to aid in understandingreservoir anatomy. procedures to aid in understanding reservoir anatomy. Theseprocedures. as well as laboratory data available from cores that assist inreservoir description, are discussed here. The complexity of formations thoughtto be homogeneous and continuous has resulted in large quantities ofunrecovered hydrocarbons. This complexity exists in both macroscopic andmicroscopic scale. Physical examination and testing of core samples yieldsinsight into this complexity and furnishes data useful in reservoirdescription. "Routine core analysis" that furnishes measured porosity, permeability. residual fluids, lithology, and porosity, permeability. residualfluids, lithology, and texture remains the most common and most importantsource of rock characterization. However, these measurements comprise only asmall portion of information available from cores. Complementary tests listedin Table 1 furnish additional analytical results as well as visual records ofthe core. Specialized core data used for reservoir description (Table 2) aretargeted for specific application and are made on a more limited basis.Quantitative studies of non-reservoir rocks now are used routinely to identifysource rocks from which hydrocarbons may have been generated. Source-rockstudies indicate probability of the presence of hydrocarbons in a new basin andwhether they are likely to be gas or oil. In established basins, this knowledgeis used to link oils in reservoirs with their source rock, which in turnidentifies oil migration paths and future areas for exploration. paths andfuture areas for exploration. Characteristics and Studies That Impose UniqueAnalytical Procedures Heterogeneous Carbonates. Lithology and pore space in carbonates may behighly variable, and the porosity can exist as microporosity, intergranular, vuggy, fracture, or a combination of all four. Intergranular porosity incarbonates of uniform texture can be porosity in carbonates of uniform texturecan be determined by means of procedures and small plug size samples similar tothose employed for homogeneous sands. Analysis of heterogeneous, fractured, and/or vuggy rocks illustrated in Fig. 1 requires samples as large as can beobtained, such that pore spaces are small in relationship to bulk volume ofpore spaces are small in relationship to bulk volume of the samples.Consequently, these types of formations are analyzed with full-diameterprocedures. Samples in the form of a right cylinder up to 10 in. (25 cm) longand approximately 5 in. (13 cm) in diameter may be used. Data generated includeBoyle's Law porosities utilizing helium as the saturating medium. Thismeasurement combines matrix, vug, and fracture porosity in a single value, Twohorizontal permeabilities are determined. When fractures or vugs permeabilitiesare determined. When fractures or vugs are present, one of the permeabilitymeasurements is oriented visually through the more permeable section, and thesecond permeability is at right angles to this measurement. In this manner, theeffect of vugs or fractures on horizontal permeability is indicated. Verticalpermeabilities also are determined frequently. JPT p. 2483