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.<ref name=Basan_etal_1988 />

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>

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.