Whole Core Analysis - Experience and Challenges
- Mehdi M. Honarpour (ExxonMobil Upstream Research Co.) | Nizar F. Djabbarah (ExxonMobil Upstream Research Co.) | Krishnaswamy Sampath (ExxonMobil Upstream Research Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- December 2005
- Document Type
- Journal Paper
- 460 - 469
- 2005. Society of Petroleum Engineers
- 5.8.6 Naturally Fractured Reservoir, 5.6.2 Core Analysis, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 2.4.3 Sand/Solids Control, 5.5.2 Core Analysis, 5.2.1 Phase Behavior and PVT Measurements, 1.2.3 Rock properties, 4.1.2 Separation and Treating, 5.3.2 Multiphase Flow, 5.5.11 Formation Testing (e.g., Wireline, LWD), 1.6.9 Coring, Fishing, 5.5 Reservoir Simulation, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.3.4 Scale, 5.2 Reservoir Fluid Dynamics, 5.4.2 Gas Injection Methods, 5.1 Reservoir Characterisation, 4.1.5 Processing Equipment
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Whole-core analysis is critical for characterizing directional permeabilityin heterogeneous, fractured, and/or anisotropic rocks. Whole-core measurementsare essential for heterogeneous reservoirs because small-scale heterogeneitymay not be appropriately represented in plug measurements. For characterizationof multiphase-flow properties (special core analysis) in heterogeneous rocks,whole-core analysis is also required.
Few commercial laboratories are equipped to conduct routine measurements onwhole cores up to 4 in. in diameter and up to 8 in. long and, importantly,under simulated reservoir net confining stress (NCS). Special whole-coreanalyses are rarely conducted because of the difficulties associated withestablishing a representative water saturation in drainage capillary pressureexperiments and measuring directional effective permeabilities. Electricalproperties also can be measured on whole cores to determine porosity andsaturation exponents for situations in which resistivity tools are used inhorizontal or highly deviated wells.
In this paper, we provide an overview of routine and special core-analysismeasurements on whole cores. Results from selected heterogeneous sandstone andcarbonate rocks will be discussed. We also will show how the results relate todata obtained from plug analysis, with particular emphasis on directionalabsolute permeability, trapped-gas and fluid saturations, and the effect ofNCS. Finally, we will describe a novel apparatus for special core analysis onwhole cores and provide examples of the capabilities of the system.
In this paper, we will present:
• Recommended techniques for the determination of directional absolute andeffective permeability and for establishing initial water saturation in wholecores.
• Improved understanding of the effect of scale (sample size) on themeasured properties.
• Description of a novel whole-core apparatus with measurement offluid-saturation distribution using in-situ saturation monitoring.
Reservoir rocks are heterogeneous, especially carbonate rocks, in which morethan 50% of the world's hydrocarbon reserves are deposited. Fig. 1 shows anexample of variability in rock characteristics as observed in a carbonate-rockoutcrop in Oman. The heterogeneous nature of these rocks tends to become moreapparent as attempts are made to measure their petrophyscal properties atvarious scales. An example of permeability variation in a plug from a carbonateformation is shown in Fig. 2. Single-phase air permeability varies by threeorders of magnitude over the distance of a few centimeters in this core plug.This dual-porosity behavior impacts the spontaneous-imbibition performancesignificantly (Fig. 3).
Technology at Commercial Laboratories
Selected commercial laboratories have capabilities to appropriately cleanand prepare whole cores, perform core X-ray imaging, and measure basicproperties such as directional permeability and porosity under a maximumconfining stress of 5,000 psi. Available technologies for imaging, samplepreparation, and routine core analysis are summarized in the followingsections.
Special-core-analysis capabilities at commercial laboratories are rare. Onlyone or two laboratories are capable of measuring primary-drainage gas/watercapillary pressure and gas/water or oil/water electrical properties on wholecores at confining stress.
Whole-Core Imaging and Screening
Whole-core photography and X-ray imaging provide information about surfacefeatures and internal structure. The computed tomography (CT) scan providesevidence of fractures, vugs, and heterogeneities as indicated by the extent inthe variation of CT density.
X-ray fluoroscopy and CT are two of the most practical X-ray scanningtechniques used to characterize core-level heterogenieties and to explain theireffect on horizontal and vertical permeabilities. CT-scanning algorithms shouldoften be modified to obtain images free of artifacts and with better than0.5-mm horizontal and 1-mm vertical resolutions.
|File Size||4 MB||Number of Pages||10|
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