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	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. 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.

Filtrate invasion can be minimized several ways:

• 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

Figure 1 Typical fluid contents from reservoir to surface. (a)Oil-productive formation. (b) Gas-productive formation. (Courtesy of Core Laboratories, a Division of Western Atlas International.)

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. The magnitude of saturation changes that can occur during coring and recovery with water-based and oil-based coring fluids are illustrated in Figure 1.

A pressure coring tool is designed to maintain reservoir pressure in the core by enclosing the core in a pressurized chamber before it is brought to the surface. This helps prevent the fluid changes that occur with expansion and expulsion. Saturation measurements from pressure cores are much more accurate than those from conventional core. However, they are still not 100% accurate, as pressure cores can still be subject to flushing during the coring process. A sponge core liner system can also help minimize errors in saturation measurements due to fluid expansion and expulsion by the retention of the expulsed formation fluids in a sponge or foam lining.

Physical damage

Petrophysical properties can be altered when the rock is damaged during the coring process. Physical damage to the core can occur in many ways:

• Fractures induced due to stress relief or jarring of the core barrel during retrieval
• Disaggregation and fracturing of unconsolidated sediments
• Crushed grains due to the high impact from percussion sidewall coring

Core alteration during wellsite handling

Although changes in the core and its fluid content during coring are unavoidable, it is important to minimize any further damage to the core during wellsite handling which would make the core even less representative of the reservoir. The time a core is exposed to the atmosphere and the drilling fluid during wellsite handling will affect subsequent core analysis measurements.

Depending on atmospheric conditions, exposure of cores for even a short period of time can cause significant loss of water and light hydrocarbon fractions. Tests show exposure for even 30 min can result in 10 to 25% loss in water (American Petroleum Institute, 1960). To prevent saturation changes, the time the core is exposed to the atmosphere should be minimized (see Core Handling for additional information on core handling techniques).

Core preservation during shipment and storage

Core preservation is an attempt to maintain a core during shipment and storage in the same condition it was in when the core was originally removed from the core barrel.

Core preservation techniques should keep the core in correct sequence, prevent breakage during shipment and storage (which is very important for soft or poorly consolidated cores), minimize core alteration, and preserve the volume and distribution of the core fluids.

Problems that core preservation methods must address include the following:

• Dehydration and salt precipitation
• Oxidation
• Redistribution of fluids
• Evaporation and condensation
• Hydrocarbon deposition
• Clay collapse
• Bacterial growth

Preservation should be quick in order to minimize exposure time. Head space in preservation materials should be small to reduce the amount of air in the package and decrease evaporation and condensation losses. Porous materials that can affect saturations should not be used in the preservation package. Temperature fluctuations that can cause problems with evaporation and condensation of core fluids should also be minimized.

Methods of core preservation

Ideally, all core should be preserved. The method of preservation and packaging of cores varies depending upon the type of core (consolidated versus unconsolidated), the core analysis measurements required, and the length of time the core is stored before testing.

Core preservation methods are typically either "dry" or "wet." Dry methods enclose the core in a material that prevents evaporation of formation fluids. Wet methods of preservation involve submerging the core in a brine or other fluid that preserves core wettability.. 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.

Table 1. Summary of dry and wet core preservation methods

Method	Alternatives
Dry	Sealing in air tight metal cans
	Sealing in rubber, plastic, aluminum, steel, or fiberglass tubes
	Sealing in plastic bags
	Wrapping in plastic wrap and aluminum foil and coating with wax or plastic
	Sealing in laminated, heat-sealable packages
	Freezing with dry ice
Wet	Sealing in anaerobic jars or polycarbonate, steel, glass, or PVC containers with brine, oil, or other fluids

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 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). 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.

Table 2. Chemical reactivity of Barex and Saran Wrap

Reactive Liquid	Weight loss after 30 days exposure at 100°F (%)
	Barex	Saran Wrap
Heptane	1.2	3.1
Cyclohexane	0.1	20
Gasoline	01.	2.0
Benzene	1.1	2.3
Toluene	0.2	1.9
Alaska Crude	0.3	2.3
Arctic Diesel	0.4	8.2
Oil phase drilling mud	0.6	1.4

• Then wrap the core in two or three layers of heavy duty aluminum foil. The edges should be crimped. The aluminum foil acts as a vapor barrier (Table 3).

Table 3. Transmissivity of seal and wrap materials

	Oxygen (cm3 x mil/100 in.2 x D x atm)	Water vapor (g x mil/100 in.2 x D x atm)	Carbon dioxide (cm3 x mil/100 in.2 x D x atm)
B-60 wax	3015	122	--
Coreseal(R)	Too high to measure	2-13	--
Aluminum foil	0	0	0
Saran Wrap (R)	1.52	0.18	1.0

• Double dip the wrapped core in melted wax or plastic. String should be used to dip the core, not wire, because wire can rip the aluminum foil. The string should be cut off and the ends also dipped in the wax or plastic. This wax or plastic coating protects the core and the aluminum foil during shipping and storage.

Note that wax and plastic are permeable and do not serve as barriers to oxygen or water vapor. However, CoreSeal® is relatively impermeable to water vapor (Table 3), as are several common polymers (Table 4).

Table 4. Permeation rates for various polymers.

	Oxygen (cm3 x mil/100 in.2 x D x atm)	Water vapor (g x mil/100 in.2 x D x atm)	Carbon dioxide (cm3 x mil/100 in.2 x D x atm)

Barrier foil laminate

The most common barrier foil laminate is ProtecCore. This laminate consists of aluminum foil--the major moisture and oxygen barrier--between several layers of bonded plastic. The innermost layer, Barex, is inert and heat sealable. The outer two plastic layers, polyethylene and polyester, provide strength and rigidity. The properties of the various components of ProtecCore are given in Table 4. The steps to preserve a core using this method are as follows:

1. Wrap the core in three or four layers of Barex film to prevent the core from puncturing the ProtecCore laminated material.
2. Slip the prewrapped core into the ProtecCore laminate tube. One end of the package is sealed with a heat sealer. The air space within the package is minimized by flattening it out as much as possible before heat sealing the other end of the package. To reduce free space around the core even further, a small hole can be left in a corner and a vacuum pulled on the package. This pulls the packaging down tight on the core and removes most of the air. The hole is then heat sealed.
3. 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. However, the packaging is fragile and can be easily ripped or punctured and is subject to pinholes and cracks. Careful handling can minimize this type of damage.

Freezing with dry ice

Freezing core is often done to minimize the loss of the volatile hydrocarbons, to preserve the fabric and structure of unconsolidated cores, and to immobilize the fluids in pressure cores. 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. 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.

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. 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.

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