25/8-12 A

General

Well 25/8-12 A is a sidetrack from well 25/8-12 S, which encountered oil in Lower Jurassic, Upper Statfjord Formation sandstones. It was drilled to appraise the southern extension of the 25/8-11 Ringhorn discovery. In the Ringhorn structure, the BCU is also the base of the chalk and represents the overall top of the Lower Jurassic reservoir interval. The primary objective for well 25/8-12 A was Lower Statfjord Formation sandstones, in an eroded horst, fault block.

Operations and results

Appraisal well 25/8-12 A was drilled with the semi-submersible installation "Mærsk Jutlander". It was kicked of from 1014 m in the 8 1/2" section of 25/8-12 S on June 9, 1999 and drilled to TD at 2156 m in the Triassic Smith Bank Formation. The wellbore was drilled with oil based "Environment" mud. Top Statfjord Formation was found at 1945.7 m (1863.1 m TVD SS). Oil was encountered in Lower Statfjord Formation sandstones, as prognosed. Pressures and fluid sampling was undertaken with Schlumberger's Modular Dynamic Tester (MDT). Pressure measurements gave an oil gradient of 0.0719 bars/m (0.318 psi/ft), consistent with the oil gradient in the original 25/8-12 S wellbore (0.0749 bars/m) and in the 25/8-11 well (0.0741 bars/m). The oil and water gradients intersect to give an OWC at 1917.5 m TVD SS (2005 m MD RKB), in agreement with the Free Water Level defined in the 25/8-11. The deepest sands in the wellbore, the water bearing sands below 1970 m TVD SS, appear to record a separate, higher reservoir gradient. Four oil samples were collected in 450 cc MDT chambers at 1975 m, within the main Jurassic sand reservoir. No draw down was seen during the sampling. Temperature (measured, not static) ranged from 78.4 deg C to 79.3 deg C during the sampling. In addition to the 450 cc chambers, a single one-gallon chamber was filled with oil at the same depth. A one-gallon water sample was obtained at 2008.2 m. One core was cut in the interval 1951 - 1988.5 in