Summary
Whole-core analysis is critical for characterizing directional permeability in heterogeneous, fractured, and/or anisotropic rocks. Whole-core measurements are essential for heterogeneous reservoirs because small-scale heterogeneity may not be appropriately represented in plug measurements. For characterization of multiphase-flow properties (special core analysis) in heterogeneous rocks, whole-core analysis is also required.
Few commercial laboratories are equipped to conduct routine measurements on whole cores up to 4 in. in diameter and up to 8 in. long and, importantly, under simulated reservoir net confining stress (NCS). Special whole-core analyses are rarely conducted because of the difficulties associated with establishing a representative water saturation in drainage capillary pressure experiments and measuring directional effective permeabilities. Electrical properties also can be measured on whole cores to determine porosity and saturation exponents for situations in which resistivity tools are used in horizontal or highly deviated wells.
In this paper, we provide an overview of routine and special core-analysis measurements on whole cores. Results from selected heterogeneous sandstone and carbonate rocks will be discussed. We also will show how the results relate to data obtained from plug analysis, with particular emphasis on directional absolute permeability, trapped-gas and fluid saturations, and the effect of NCS. Finally, we will describe a novel apparatus for special core analysis on whole cores and provide examples of the capabilities of the system.
In this paper, we will present:• Recommended techniques for the determination of directional absolute and effective permeability and for establishing initial water saturation in whole cores.• Improved understanding of the effect of scale (sample size) on the measured properties.• Description of a novel whole-core apparatus with measurement of fluid-saturation distribution using in-situ saturation monitoring.
Introduction
Reservoir rocks are heterogeneous, especially carbonate rocks, in which more than 50% of the world's hydrocarbon reserves are deposited. Fig. 1 shows an example of variability in rock characteristics as observed in a carbonate-rockout crop in Oman. The heterogeneous nature of these rocks tends to become more apparent as attempts are made to measure their petrophyscal properties at various scales. An example of permeability variation in a plug from a carbonate formation is shown in Fig. 2. Single-phase air permeability varies by three orders of magnitude over the distance of a few centimeters in this core plug. This dual-porosity behavior impacts the spontaneous-imbibition performance significantly (Fig. 3).
Technology at Commercial Laboratories
Selected commercial laboratories have capabilities to appropriately clean and prepare whole cores, perform core X-ray imaging, and measure basic properties such as directional permeability and porosity under a maximum confining stress of 5,000 psi. Available technologies for imaging, sample preparation, and routine core analysis are summarized in the following sections.
Special-core-analysis capabilities at commercial laboratories are rare. Only one or two laboratories are capable of measuring primary-drainage gas/water capillary pressure and gas/water or oil/water electrical properties on whole cores at confining stress.
Whole-Core Imaging and Screening
Whole-core photography and X-ray imaging provide information about surface features and internal structure. The computed tomography (CT) scan provides evidence of fractures, vugs, and heterogeneities as indicated by the extent in the variation of CT density.
X-ray fluoroscopy and CT are two of the most practical X-ray scanning techniques used to characterize core-level heterogenieties and to explain their effect on horizontal and vertical permeabilities. CT-scanning algorithms should often be modified to obtain images free of artifacts and with better than0.5-mm horizontal and 1-mm vertical resolutions.
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