Changes

Sometimes a new well will find an oil-water contact significantly shallower than the common oil-water contact established in the wells drilled so far in the field, yet there is evidence for pressure communication between the new and old wells ([[:File:M91FG30.JPG|Figure 7]]). The shallower oil-water contact may be a perched oil-water contact. These are common features yet are hardly ever mentioned in the literature. Perched oil-water contacts result from the trapping of small to moderate volumes of water when the oil initially migrated into the reservoir. Normally, the water will be displaced down and sideways as the oil enters. However, if a barrier prevents the water from being moved out of the way, the water will remain where it is.

Sometimes a new well will find an oil-water contact significantly shallower than the common oil-water contact established in the wells drilled so far in the field, yet there is evidence for pressure communication between the new and old wells ([[:File:M91FG30.JPG|Figure 7]]). The shallower oil-water contact may be a perched oil-water contact. These are common features yet are hardly ever mentioned in the literature. Perched oil-water contacts result from the trapping of small to moderate volumes of water when the oil initially migrated into the reservoir. Normally, the water will be displaced down and sideways as the oil enters. However, if a barrier prevents the water from being moved out of the way, the water will remain where it is.

Sometimes a localized perched oil-water contact can be found more than 50 m (164 ft) higher than the established oil-water contact. For example, in the Fulmar field, UK North Sea, a perched oil-water contact in the north of the field was found at 3228 m (10,590 ft) true vertical depth subsea (TVDSS), 73 m (240 ft) higher than the main oil-water contact at 3301 m (10830 ft) TVDSS. Pressure and production data indicate communication within the oil column between the two areas (Stockbridge and Gray, 1991).

Sometimes a localized perched oil-water contact can be found more than 50 m (164 ft) higher than the established oil-water contact. For example, in the Fulmar field, UK North Sea, a perched oil-water contact in the north of the field was found at 3228 m (10,590 ft) true vertical depth subsea (TVDSS), 73 m (240 ft) higher than the main oil-water contact at 3301 m (10830 ft) TVDSS. Pressure and production data indicate communication within the oil column between the two areas.<ref>Stockbridge, C. P., and D. I. Gray, 1991, The Fulmar field, Blocks 30/16 and 30/11b, UK North Sea, in I. L. Abbots, ed., United Kingdom oil and gas fields, 25 years commemorative volume: Geological Society (London) Memoir 14, p. 309–316.</ref>

The most common type of perched oil-water contact is a downwarped thin reservoir with the downward flow of water blocked by a sand pinch-out or a sealing fault. Local synclinal areas flanked by sealing faults can also retain water (Weber, 1995).

The most common type of perched oil-water contact is a downwarped thin reservoir with the downward flow of water blocked by a sand pinch-out or a sealing fault. Local synclinal areas flanked by sealing faults can also retain water.<ref>Weber, K. J., 1995, Perched hydrocarbon-water contacts—A common but poorly understood phenomenon: Abstracts of the 57th EAGE Conference, Glasgow, F035.</ref>

==References==

==References==

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