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Measuring capillary pressure relationships

Most methods for measuring capillary pressure may be grouped under three headings:

Mercury methods

Porous-plate methods

Centrifuge methods

Mercury methods

In the mercury method, a sample of rock is evacuated, and the volume of mercury that enters

the sample at increasing pressures is measured.

Mercury methods are especially suited for samples of irregular shape, such as those found in

drill cuttings. Mercury methods are useful for investigating the porous structure of the sample.

Complete mercury capillary pressure curves can be determined within an hour or so, depending

on the permeability of the sample

Porous-plate methods

The porous-plate method can yield very accurate capillary pressure relationships.

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Consider a cylindrical rock sample that is first saturated with water. A flat face of the sample is

then pressed against a flat porous plate (or membrane) in a chamber filled with gas

The porous plate is also saturated with water. Often, a moist tissue is placed between the

sample and the plate to produce good capillary contact. Then, pressure in the gas phase above

the porous plate is increased by a small step, forcing gas to displace the water from the sample

through the plate. When the displacement ceases, the difference in pressure between the gas

surrounding the sample and the water on the lower side of the plate is the capillary pressure

corresponding to the saturation of water remaining in the sample. After a measurement is

completed, the pressure in the gas is increased again, forcing more gas into the sample. This

process is repeated, increasing the capillary pressure in a series of steps, yielding the capillary

pressure relationship for decreasing water saturation.

Centrifuge methods

Centrifuge methods are increasingly favored for measuring capillary pressures. Although not as

quick as mercury measurements, centrifuge measurements are much faster than porous-plate

methods. To measure a gas/oil capillary pressure relationship with the centrifuge method, a

cylindrical sample is first saturated with oil; next, it is mounted in a centrifuge and is spun in

steps of increasing spin rate.

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The centrifugal forces throw oil from the sample while pulling surrounding gas into the sample.

The duration of each spin step must be sufficient for production of oil to cease. The average

saturation of oil in the sample at each spin rate may be calculated from the volume of oil that is

produced to the collector relative to the porous volume of the sample. Because 4 to 24 hours

are needed to reach equilibrium at each spin rate, most centrifuge data sets consist of eight orfewer spin rates.