Difference between revisions of "Core alteration and preservation"
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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" | |
+ | |+ 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.> | ||
+ | |- | ||
+ | ! Method || Alternatives | ||
+ | |- | ||
+ | | rowspan=2 | Dry || Sealing in air tight metal cans | ||
+ | |- | ||
+ | | Sealing in rubber, plastic, aluminum, steel, or fiberglass tubes | ||
+ | |} | ||
===Dry core preservation methods=== | ===Dry core preservation methods=== |
Revision as of 13:53, 10 March 2014
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.[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.[2]
Filtrate invasion can be minimized several ways:[2] [3]
- 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.[3] 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.[2]. 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.