Unlike conventional techniques, which measure expulsed fluid volumes and give bulk Sw estimates, GIT-CAP measures pore fluids in situ. The figure below illustrates how the known radius of the centrifuge can be used to calculate the capillary force on a core plug. This information, coupled with the NMR-measured water saturation at known locations, allows the generation of fast, accurate, and non-destructive capillary pressure measurements.

S₁₀₀ is the saturation distribution of a fully saturated plug. Sw shows the saturation after being centrifuged. Dividing the Swx curves by Sw₁₀₀ curve removes the effect of porosity variations.

COMPARISON STUDIES
The GIT-CAP technique has been tested on more than a hundred different plugs of varying rock types to determine its accuracy. On the right is a plot of air/brine capillary pressure data using two different centrifuge speeds. The solid line is the capillary pressure curve calculated using the porous plate technique. GIT-CAP’s impressive accuracy is obtained in a fraction of the time necessary for porous plate or traditional centrifuge techniques. Instead of one capillary pressure point per centrifuge spin, up to forty points are obtained by measuring saturation values along the pressure gradient.

REDISTRIBUTION
The graph on the left shows water saturation distribution measured with GIT Systems software as a function of time. There is no noticeable redistribution of fluids with the rock, even after 1.5 days! Although water will spontaneously imbibe into rocks, water that is in the pores does not redistribute quickly within the rock.

This rock had a porosity of 14.5% and permeability of 0.91 mD. The dark green line represents 100% saturation and the lighter green lines are successive scans over a 36 hour period.

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