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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)<ref name=Keelan_1985>Keelan, D. K., 1985, Coring Part 1--Why it's done: World Oil, v. 200, n. 4, p. 83-90.</ref>. 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.

Considerable resources are invested in core analysis programs designed to furnish information on geological and petrophysical rock properties and on engineering and completion data.<ref name=Keelan_1985>Keelan, D. K., 1985, Coring Part 1--Why it's done: World Oil, v. 200, n. 4, p. 83-90.</ref> 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==

==Core alteration during recovery==

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:<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.

* Increase the diameter of the core cut to increase the area of uninvaded central core.

* 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.

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===

===Fluid expansion and expulsion===