Changes

===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|300px|{{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]].