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-rock
outcrop 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 than
0.5-mm horizontal and 1-mm vertical resolutions.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
18 November 2003
- Revised manuscript received:
8 August 2005
- Manuscript approved:
23 August 2005
- Version of record:
15 December 2005
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