Changes

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.

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., 1988<ref name=Basan_etal_1988 \>; American Petroleum Institute, 1960).

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., 1988<ref name=Basan_etal_1988 />; American Petroleum Institute, 1960).

Filtrate invasion can be minimized several ways (Basan et al., 1988<ref name=Basan_etal_1988 \>; Keelan and Donohue, 1985<ref name=Keelan_etal_1985>Keelan, D. K., and D. A. T. Donohue, 1985, Core analysis: Boston, MA, IHRDC Video Library for Exploration and Production Specialists, n. PE405, 186 p.</ref>):

Filtrate invasion can be minimized several ways (Basan et al., 1988<ref name=Basan_etal_1988 />; Keelan and Donohue, 1985<ref name=Keelan_etal_1985>Keelan, D. K., and D. A. T. Donohue, 1985, Core analysis: Boston, MA, IHRDC Video Library for Exploration and Production Specialists, n. PE405, 186 p.</ref>):

* Select a bit that directs the drilling fluid away from the core rather than toward it.

* 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.

* Increase the coring speed. The faster the core enters the core barrel, the less time there is for invasion to occur.

===Fluid expansion and expulsion===

===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, 1985<ref name=Keelan_etal_1985 \>). 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.

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, 1985<ref name=Keelan_etal_1985 />). 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. (see the chapter on "Conventional Coring" in Part 3).

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. (see the chapter on "Conventional Coring" in Part 3).

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:

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&reg; 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)<ref name=Hunt_etal_1988>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.</ref>. Barex&reg; 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)<ref name=Hunt_etal_1988 \>.

# 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&reg; 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)<ref name=Hunt_etal_1988>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.</ref>. Barex&reg; 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)<ref name=Hunt_etal_1988 />.

# 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).

# 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).

# 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.

# 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&reg; is relatively impermeable to water vapor (Table 3) (Bajsarowicz, unpubl. data), as are several common polymers (Table 4) (Hunt and Cobb, 1988)<ref name=Hunt_etal_1988 \>.

Note that wax and plastic are permeable and do not serve as barriers to oxygen or water vapor. However, CoreSeal&reg; is relatively impermeable to water vapor (Table 3) (Bajsarowicz, unpubl. data), as are several common polymers (Table 4) (Hunt and Cobb, 1988)<ref name=Hunt_etal_1988 />.

===Barrier foil laminate===

===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 (Hunt and Cobb, 1988)<ref name=Hunt_etal_1988 \>. 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:

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 (Hunt and Cobb, 1988)<ref name=Hunt_etal_1988 />. 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:

# Wrap the core in three or four layers of Barex film to prevent the core from puncturing the ProtecCore laminated material.

# Wrap the core in three or four layers of Barex film to prevent the core from puncturing the ProtecCore laminated material.

# 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.

# 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.

# For protection during shipment, the individual pieces should be wrapped in pads or bubble wrap.

# 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, 1989<ref name=Auman_1989 \>; Hunt and Cobb, 1988<ref name=Hunt_etal_1988 \>). However, the packaging is fragile and can be easily ripped or punctured and is subject to pinholes and cracks (Basan et al., 1988<ref name=Basan_etal_1988 \>; Whitebay, 1986<ref name=Whitebay_1986 \>). Careful handling can minimize this type of damage.

This preservation method provides a good vapor barrier, superior to the hot wax or strippable plastic method (Auman, 1989<ref name=Auman_1989 />; Hunt and Cobb, 1988<ref name=Hunt_etal_1988 />). However, the packaging is fragile and can be easily ripped or punctured and is subject to pinholes and cracks (Basan et al., 1988<ref name=Basan_etal_1988 />; Whitebay, 1986<ref name=Whitebay_1986 />). Careful handling can minimize this type of damage.

===Freezing with dry ice===

===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 (Torsaeter, 1985)<ref name=Torsaeter_1985>Torsaeter, O., 1985, The effect of freezing of slightly consolidated cores: [[spe:14300|SPE paper 14300]], 60th Annual Technical Conference and Exhibition, Las Vegas, NV, Sept. 22-25.</ref>. 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&deg;C]]), thus liquid nitrogen temperatures ([[temperature::-195.8&deg;C]]) are required.

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 (Torsaeter, 1985)<ref name=Torsaeter_1985>Torsaeter, O., 1985, The effect of freezing of slightly consolidated cores: [[spe:14300|SPE paper 14300]], 60th Annual Technical Conference and Exhibition, Las Vegas, NV, Sept. 22-25.</ref>. 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&deg;C]]), thus liquid nitrogen temperatures ([[temperature::-195.8&deg;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<ref name=Wisenbaker_1947>Wisenbaker, J. D., 1947, Quick freezing of cores preserves fluid contents: Oil Weekly, v. 124, n. 9, p. 42-46.</ref>; Lebeaux, 1952<ref name=Lebeaux_1952>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.</ref>; Kelton, 1953<ref name=Kelton_1953>Kelton, F. C., 1953, Effect of quick-freezing versus saturation of oil well cores: Petroleum Transactions, AIME, v. 198, p. 312-314.</ref>; 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.

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<ref name=Wisenbaker_1947>Wisenbaker, J. D., 1947, Quick freezing of cores preserves fluid contents: Oil Weekly, v. 124, n. 9, p. 42-46.</ref>; Lebeaux, 1952<ref name=Lebeaux_1952>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.</ref>; Kelton, 1953<ref name=Kelton_1953>Kelton, F. C., 1953, Effect of quick-freezing versus saturation of oil well cores: Petroleum Transactions, AIME, v. 198, p. 312-314.</ref>; 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.

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===

===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 \>.

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.

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.

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.