Core alteration and preservation
| Development Geology Reference Manual | |
| |
| Series | Methods in Exploration |
|---|---|
| Part | Wellsite methods |
| Chapter | Core alteration and preservation |
| Author | Caroline J. Bajsarowicz |
| Link | Web page |
| PDF file (requires access) | |
Considerable resources are invested in core analysis programs designed to furnish information on geological and petrophysical rock properties and on engineering and completion data (Keelan, 1985)[1]. The economic implications of the accuracy and credibility of the data obtained from these analyses can be significant, especially in equity determinations. It is important to obtain data that relate as closely as possible to virgin reservoir conditions. Thus, alteration of the core during recovery, wellsite handling, shipment, and storage must be minimized.
Core alteration during recovery
Changes in the core and fluid content during coring are unavoidable. However, changes can be minimized by understanding the processes that affect the core during recovery. Cores can be damaged during recovery by
- Filtrate invasion
- Fluid expansion and expulsion
- Physical damage to the rock
Filtrate invasion
During core acquisition and retrieval, the mud filtrate often invades the core. Invasion can displace over half of the native fluid, which can change the in situ fluid saturations in the core. Invasion can also alter rock properties through interaction with the core minerals and fluids. For example, the filtrate may cause clays either to swell or to shrink.
The amount of native fluid displaced by mud filtrate depends on the rate of bit penetration, permeability of the formation, viscosity and compressibility of the native fluid and the filtrate, mud cake permeability, pressure differential and relative permeability of the formation to the mud filtrate, and core diameter (Basan et al., 1988Cite error: Closing </ref> missing for <ref> tag):
- Select a bit that directs the drilling fluid away from the core rather than toward it.
- Increase the coring speed. The faster the core enters the core barrel, the less time there is for invasion to occur.
- Establish a low pressure differential between the drilling fluid and the reservoir.
- Optimize the fluid loss properties of the drilling fluid.
- Increase the diameter of the core cut to increase the area of uninvaded central core.
Evaluation of fluid invasion can be tested by doping the coring fluid with a suitable tracer and then checking the tracer concentration in the fluids extracted from the recovered core. The effect of invasion on fluid saturations is measured using "plug and donut" analysis.
Fluid expansion and expulsion
As the core barrel is brought to the surface, the core and fluids are subjected to a reduction in pressure and temperature from reservoir to atmospheric conditions. Only minor changes occur to the rock matrix. However, the fluids undergo substantial changes in volume. Oil releases gas from solution, resulting in shrinkage of the oil. The gas dissolved in the oil and water expands and escapes from the core, leading to expulsion of the fluids. These phenomena result in surface saturations that are different from those downhole (American Petroleum Institute, 1960; Keelan and Donohue, 1985Cite error: Closing </ref> missing for <ref> tag. A variety of dry and wet preservation methods used by the industry are summarized in Table 1. Note that none of these methods provide an ideal solution to core preservation.
Dry core preservation methods
Air-tight metal cans
Metal cans are excellent vapor barriers, but they can react with water. Consequently, canned cores should be prewrapped to prevent moisture loss. Prewrapping also minimizes headspace in the container, preventing movement
of the core in the container and reducing evaporation and condensation losses. The prewrap should be inert so that it does not react with the formation fluids, and it should be nonporous so as not to affect core saturations (American Petroleum Institute, 1960).
Core sleeves, liners and barrels
Rubber, plastic and aluminum sleeves, fiberglass liners, and pressure (steel) core barrels can be cut into suitable lengths and then capped for storage of core. This preservation technique provides some protection to the core during surface handling, particularly for fractured and unconsolidated cores.
Except for aluminum and steel tubes, none of these materials are effective vapor barriers. Therefore, this preservation method should be used for temporary storage only. Excess mud should be drained out of the tubes to minimize exposure of the core to the drilling fluid. However, leaving the space filled with air can result in evaporation and dehydration of the core.
Plastic bags
The simplest preservation method is to wrap the core in plastic or heat-sealed plastic bags. Plastic is not an absolute oxygen or water vapor barrier; it only reduces the rate of evaporation. Studies by Auman (1989)[2] show that cores in heat-sealed plastic bags lose 6% of their water in 10 days. One pinhole more than triples water loss. Cores wrapped in plastic lose about 30% of their water in 3 days.
Hot wax or strippable plastic
Coating cores with hot wax or strippable plastic is a widely used preservation method that involves wrapping the core in plastic wrap and aluminum foil and then dipping the core in paraffin or a plastic sealant. The steps to preserve a core using this method are as follows:
- Wrap the core in several layers of plastic wrap or film to prevent fluids in the core from contacting the outer wrapping of aluminium foil. Of the commercially available food wraps, Saran Wrap® has been found to be the least reactive with formation fluids. However, the wrap has been found to degrade with some hydrocarbon compositions (Table 2) (Hunt and Cobb, 1988)[3]. Barex® film, which is relatively inert against organic solvents and corrosive fluids, can be used, but it is inflexible and difficult to wrap around core (Hunt and Cobb, 1988)Cite error: Closing
</ref>missing for<ref>tag. - For protection during shipment, the individual pieces should be wrapped in pads or bubble wrap.
This preservation method provides a good vapor barrier, superior to the hot wax or strippable plastic method (Auman, 1989Cite error: Closing </ref> missing for <ref> tag. The most common method of freezing core is with dry ice. However, light hydrocarbon fractions are not maintained at dry ice temperatures (temperature::-78.5°C), thus liquid nitrogen temperatures (temperature::-195.8°C) are required.
The exact effects of freezing on the rock and its petrophysical properties are still unknown. A variety of studies examining the effects of freezing on porosity and permeability show contradictory results (Wisenbaker, 1947[4]; Lebeaux, 1952[5]; Kelton, 1953[6]; Torsaeter, 1985<ref name=Torsaeter_1985 \>). The freezing process may affect the rock structure due to ice formation and may affect the wettability due to precipitation of hydrocarbons onto pore surfaces. To minimize damage by ice, the cores should be frozen quickly to reduce ice crystal growth.
Sublimation from the core surface must be prevented during storage. One method is to quick freeze a layer of brine on the surface of the core. The brine does not enter the frozen core; subsequent sublimation comes from this layer, not the core.
Wet core preservation methods
Cores can be preserved by submerging them in jars of deoxygenated formation brine or diesel. A bactericide, generally formaldehyde, is added to prevent bacterial growth during storage. The jars are closed and the system purged with nitrogen. This system inhibits most oxidation (Basan et al, 1988)<ref name=Basan_etal_1988 \>.
Polycarbonate or anaerobic jars are the most commonly used containers. Other containers that can be used are made of steel, PVC, or glass. Caution must be exercised when using steel because it can rust in the presence of water. PVC containers are not optimal because they permit diffusion of water and oxygen (Basan et al., 1988)<ref name=Basan_etal_1988 \>. Glass containers are excellent preservation containers, but they are difficult to use in the field without breaking.
The wet method of core storage is often used when the core analysis program requires maintaining wettability. There still some debate about which fluid should be used in the containers. Wet preservation cannot be used when cores are cut to evaluate interstitial water, to measure fluid levels, or to interpret gas, oil, or water production. This is because exposure of the core to a fluid results in imbibition of that fluid and alteration of saturations.
Preserving and storing core with a wet system has a high cost and requires regular maintenance. Each jar must be purged with nitrogen every two weeks.
References
- ↑ Keelan, D. K., 1985, Coring Part 1--Why it's done: World Oil, v. 200, n. 4, p. 83-90.
- ↑ Auman, J. B., 1989, A laboratory evaluation of core preservation materials: SPE Formation Evaluation, v. 3, n. 4, p. 691-695.
- ↑ Hunt, P. K., and S. L. Cobb, 1988, Core preservation with a laminated, heat-sealed package: SPE Formation Evaluation, v. 3, n. 4, p. 691-695.
- ↑ Wisenbaker, J. D., 1947, Quick freezing of cores preserves fluid contents: Oil Weekly, v. 124, n. 9, p. 42-46.
- ↑ Lebeaux, J. M., 1952, Some effects of quick-freezing upon the permeability and porosity of oil well cores: Journal of Petroleum Technology, v. 4, n. 11, p. 19-20.
- ↑ Kelton, F. C., 1953, Effect of quick-freezing versus saturation of oil well cores: Petroleum Transactions, AIME, v. 198, p. 312-314.
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