This paper documents that it is possible to measure the pressure of water inside a hydrocarbon reservoir. The pressure difference between oil and water could provide valuable information about the hydrocarbon column height of a new discovery. Detecting changes in the hydrocarbon – water pressure difference could work as an early warning system for water approaching a field in production. Knowledge about the hydrocarbon column height could speed up the appraisal process, or guide infill drilling, saving time and money. The environmental impact will be reduced because fewer wells will have to be drilled.

In most hydrocarbon reservoirs the water phase is likely to be continuous even at a very low water saturation and at a significant hydrocarbon overpressure. This is because of the polar nature of the water molecule and the polarity in the surface of the minerals in the reservoir: Water sticks to the surface of the reservoir grains. Given enough time the pressure gradient through this thin film of water becomes the same as if the water was in bulk. By measuring both the water and the hydrocarbon pressure at a given depth inside a hydrocarbon reservoir it is possible to estimate the vertical distance down to the free-water level. The only additional information needed are the densities of the fluids.

The water has lower pressure and lower mobility than the hydrocarbon phase, therefore specialized equipment will be needed to capture the water pressure. The pressure measuring probe must have a hydrophilic filter to prevent invasion of the hydrocarbon phase.

The pressure measurement of the thin water film should be made in an undisturbed part of the reservoir, i.e. it normally cannot be made at the wall of a well. A hole through the wall of the well and a short distance into the reservoir should be made for the probe to secure a representative water pressure measurement.

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