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.<ref name=Basan_etal_1988 />

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.<ref name=Basan_etal_1988 />

Filtrate invasion can be minimized several ways:<ref name=Basan_etal_1988 /> <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:<ref name=Basan_etal_1988 /><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.

* Establish a low pressure differential between the drilling fluid and the reservoir.

* Establish a low pressure differential between the drilling fluid and the reservoir.

===Fluid expansion and expulsion===

===Fluid expansion and expulsion===

[[File:Core-alteration-and-preservation fig1.png|thumbnail|{{figure number|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.)]]

[[File:Core-alteration-and-preservation fig1.png|thumbnail|500px|{{figure number|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.<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 [[:File:Core-alteration-and-preservation fig1.png|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.<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 [[:File:Core-alteration-and-preservation fig1.png|Figure 1]].

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.

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.<ref name=Basan_etal_1988>Basan, P., J. R. Hook, and K. Hughes, 1988, Measuring porosity, saturation, and permeability from cores: The Technical Review, v. 36, n. 4, p. 22-36.</ref>. A variety of dry and wet preservation methods used by the industry are summarized in [[:File:Table_rose_time-value-of-money_1.jpg|Table 1]]. Note that none of these methods provide an ideal solution to core preservation.

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]].<ref name=Basan_etal_1988>Basan, P., J. R. Hook, and K. Hughes, 1988, Measuring porosity, saturation, and permeability from cores: The Technical Review, v. 36, n. 4, p. 22-36.</ref>. A variety of dry and wet preservation methods used by the industry are summarized in [[:File:Table_rose_time-value-of-money_1.jpg|Table 1]]. Note that none of these methods provide an ideal solution to core preservation.

{| class="wikitable"

{| class="wikitable"

|+ Table 1. Summary of dry and wet core preservation methods<ref name=API>American Petroleum Institute, 1960.</ref><ref name=Basan>Basan et al, 1988.>

|+ Table 1. Summary of dry and wet core preservation methods<ref name=API>American Petroleum Institute, 1960.</ref><ref name=Basan_etal_1988 />

|-  

|-  

! Method || Alternatives

! Method || Alternatives

|-

|-

| rowspan=2 | Dry || Sealing in air tight metal cans

| rowspan=6 | Dry || Sealing in air tight metal cans

|-

|-

| Sealing in rubber, plastic, aluminum, steel, or fiberglass tubes

| Sealing in rubber, plastic, aluminum, steel, or fiberglass tubes

|}

|}

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:

[[File:Table rose time-value-of-money 1.jpg|thumbnail|'''Table 2.''' Chemical reactivity of Barex and Saran Wrap]]

* 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 ([[:Image:Table rose time-value-of-money 1.jpg|Table 2]]).<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.<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 ([[:Image:Table rose time-value-of-money 1.jpg|Table 3]]).

|+ Table 2. Chemical reactivity of Barex and Saran Wrap

| Heptane || 1.2 || 3.1

|-

|-

|-

| Benzene || 1.1 || 2.3

| Alaska [[Crude oil|Crude]] || 0.3 || 2.3

| Oil phase drilling mud || 0.6 || 1.4

[[File:Table rose time-value-of-money 1.jpg|thumbnail|'''Table 3. Transmissivity of seal and wrap materials.]]

* 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 ([[:Image:Table rose time-value-of-money 1.jpg|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 ([[:Image:Table rose time-value-of-money 1.jpg|Table 3]]) (Bajsarowicz, unpubl. data), as are several common polymers ([[:Image:Table rose time-value-of-money 1.jpg|Table 4]]).<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),<ref name=Bajsarowicz /> as are several common polymers (Table 4).<ref name=Hunt_etal_1988 />

[[File:Table rose time-value-of-money 1.jpg|thumbnail|'''Table 4. Permeation rates for various polymers.]]

|+ Table 4. Permeation rates for various polymers.<ref name=Hunt_etal_1988 />

|-

|-

|-

| Low-density polyethylene || 500 || 1.0 || 1900

| High-density polyethylene || 200 || 0.4 || 500

| Polystyrene || 350 || 6.5 || 900

| Polypropylene || 85 || 0.26 || 300

===Barrier foil laminate===

===Barrier foil laminate===

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.<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.<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.<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> <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> <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> <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.<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> <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> <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> <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.

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.<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.<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.

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.

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.

[[Category:Wellsite methods]] [[Category:Test content]][[Category:Pages with unformatted tables]]

[[Category:Wellsite methods]] [[Category:Test content]][[Category:Pages with unformatted tables]]