The Gjallar well is being drilled at the Voering Plateau, 400 km northwest of Sandnessjoen. Water depth is 1,324 meters, the deepest on the Norwegian continental shelf to be drilled to date. By definition, the well is a wildcat, as the nearest well is 75 km.
The cementing operation was a major focus during the planning phase because the Gjallar well contains over 600 meters of soft, low strength, low density biogenetic siliceous and carbonaceous ooze sediments. These sediments are microfossil deposits with a water content of up to 50-70%. Measured density of the ooze sediments, using sonic MWD, varied from 1.40 SG to 1.60 SG. Due to the massive layers of these ooze sediments, the cementing of 30" and 20" casings were planned in detail with special emphasis on how to achieve a successful cementing operation without losing circulation. In addition to the low formation strength, possible shallow gas and shallow water flow was taken into account when preparing the cementing program. A major challenge for the 30-inch cement job was the low temperatures at the seabed. Setting time was critical, as firm cement was required prior to continuing on to the next hole section.
Installation of the Conductor
On 7 June 1999, the 36 x 30-inch conductor was successfully cemented from the shoe at 1.477 meters TVD RKB to the seabed at 1.377 meters TVD RKB (1.352 meters MSL). Seventy-two cubic meters of 1,45 SG gas tight slurry were pumped through an inner-string around the shoe with full returns. BJ's Fluorescein tracer was observed by the ROV. The company's Deepset slurry system was used to obtain early compressive strength in combination with low slurry weight and very low temperature. The temperature at seabed was –2,3C.
Just a few hours following this first stage, a planned top-up job was performed using the TITUS integrated top-up tool. Ten cubic meters of the same slurry was pumped with full return. Firm cement was observed while drilling out the shoe. The wellhead had a stick up of three meters and the angle was 0.5 degrees when landing the BOP. No slumping of the conductor was observed and a successful cementing operation was achieved.
Installation of the Surface Casing
On 10 June 1999, the 20-inch surface casing was successfully cemented from the shoe at 2.162 meters TVD RKB to seabed. A one-plug sub-sea wiperplug was used to displace 187 cubic meters of lead and 33 cubic meters of tail cement. Again, full returns to seabed were achieved at all times, as observed by the ROV. Because of expected low leak-off and the potential for shallow waterflow in the ooze-formation, low density slurries were used. The lead was 1,40 SG, and the tail was 1,60 SG. Both slurries had gastight capabilities.
The 20-inch shoe was drilled out following installation of the BOP and riser. A good lead-off test was performed just below the shoe, therefore no squeezing was necessary.