Proper Coring and Wellsite Core Handling Procedures: The First Step Toward Reliable Core Analysis
Authors: Skopec R.A. 
Published: December 2007

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Coring and wellsite core handling should follow the best possible practices because the value of all core analysis is limited by this initial operation. The objectives of any coring and core preservation program should be to obtain rock that is representative of the formation while minimizing physical alteration of the rock during coring and handling. Core analysis is an important component of formation evaluation and is especially advantageous when complex reservoirs are to be developed (see Table 1).

The major problems encountered during coring, handling, and preserving reservoir rocks are (1) designing a bottomhole coring assembly and drilling-fluid program to minimize mud invasion and maximize drilling parameters, (2) selecting a nonreactive core preservation material and method to prevent fluid loss or the absorption of contaminants (e.g., wettability altering drilling fluid components), and (3) applying appropriate core handling and preservation methods based on rock type, degree of consolidation, and fluid type.

Different rock types may require additional precautions in obtaining representative core data. Double-tube core barrels have effectively replaced rubber-sleeve coring methods for unconsolidated and fractured rock. Disposable inner barrels made of fiberglass or aluminum work best because of their low friction coefficients and ability to prevent jamming. Full-closure core catcher systems have contributed to obtaining better core recoveries of intact unconsolidated rock. Coring technology has advanced significantly in the last 3 years to maximize recoveries in most rock types while minimizing core damage. Low-invasion coring equipment is widely available and has proved critical when advanced reservoir studies are to be conducted for petrophysical and reservoir engineering purposes (e.g., electrical properties, relative permeability testing, and capillary pressure measurements).

There is no one best method for handling and preserving cores. Core preservation is an attempt to maintain core, before analysis, in the same condition that existed when it was removed from the core barrel. In the process of cutting, recovering, and bringing the core to the surface, the fluid content of the rock is altered by unavoidable changes in pressure and temperature; pressure-retained cores attempt to minimize this effect. Experience can help determine the most satisfactory preservation method for the rock type in question. The core preservation method should depend on the composition, degree of consolidation, and distinguishing features of the rock. The techniques required to preserve cores for testing may depend on the length of time for transportation, storage, and the nature of the test(s) to be performed.

Rock types that require special procedures for coring and wellsite preservation are unconsolidated rocks to include those containing both heavy and light oil, vuggy carbonates, evaporites, fractured rock, rocks rich in clay minerals, shale, low-permeability rock (tight gas sand), coal, and diatomite.

Preferred methods to preserve cores for laboratory analysis include mechanical stabilization; environmentally controlled preservation through chilling, freezing, and regulated humidity control (freezing is probably the most common and most controversial method for preserving unconsolidated rocks); storage in heat-sealable plastic laminates or plastic bags; use of dips and coatings; sealing in disposable inner core barrels; and containment in specialized sealed devices, such as anaerobic jars.

Totally unconsolidated rock is best handled frozen because mechanical alteration is likely during handling and preparation before analysis. The full effect of freezing on a core's petrophysical properties is unknown; however, plug preparation of unfrozen unconsolidated rock is wrought with many difficulties (e.g., plunge cutting). Unconsolidated frozen core should be packed with a surface ice cake to reduce fluid evaporative losses by sublimation. Consolidated rocks should never be frozen because freezing may cause irreversible structural damage to the core.

The fundamental objective of core analysis is to obtain data representative of in-situ reservoir rock properties. Coring, handling, and preservation should be conducted in a manner that prevents both loss of interstitial fluids and contamination with foreign fluids. To obtain reliable core analysis, speed is essential in removing, laying out, labeling, and preserving the core. Sampling of core material at the well site should be limited to plugging for invasion or wettability studies. Core should never be washed and must be protected from temperature extremes, moisture, and dehydration. A multidisciplinary team must be assembled to ensure representative core analysis results. This team should include drilling engineers, reservoir engineers, geoscientists, and other individuals involved with laboratory testing. Suitable acquisition and testing protocol must be adopted to obtain meaningful data. A properly designed program will benefit not only the near-term user but also future users of the core material.

References

Skopec, R.A.: "Recent Advances in Rock Characterization," The Log Analyst (1992) 33, No. 3, 270-85. Rathmell, J.J., Gremley, R.B., and Tibbitts, G.A.: "Field Applications of Low Invasion Coring," paper SPE 27045 presented at the 1993 SPE Annual Technical Conference and Exhibition, Houston, Oct. 3-6. " Wellsite Core Handling Procedures," API, Dallas (in preparation).

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