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Exploration wellbore: 1/5-3 S

General information

Wellbore history

General

Block 1/5 is situated in the Norwegian Central Trough at the transition of the Feda Graben and the Breiflabb Basin. Well 1/5-3 S was planned as an exploration well with TD at 2910 m in the Tor Chalk Formation. The well was positioned in a seismically defined "gas chimney" on the crest of a salt induced diapir and was the first well drilled on this diapir. Similar cases have been drilled successfully by STATOIL on the Tommeliten Discovery 1/9-2 and 1/9-3 wells. The primary objective of well 1/5-3 S was to test the presence of moveable hydrocarbons in fractured, reservoir quality chalk of the Ekofisk and Tor formations along the southwestern flank of the diapir. A secondary potential objective was in the Paleocene Rogaland Group. A total depth of 1566 m was reached in the 12 1/4" hole section on June 29, 1998 before deciding to permanently abandon the well due to increasing pore pressure, without fulfilling any of the well objectives.

Operations and results

Exploration 1/5-3 S well was spudded with the semi-submersible "Byford Dolphin" on 10 June 1998 and drilled to TD at 1566 m in rocks of Late Miocene age (undifferentiated Nordland Group). The well was drilled with seawater and hi-vis pills down to 792 m and with Baroid "BARASILC" silicate / KCl glycol enhanced ("GEM GP") mud from 792 m to TD. Due to possible shallow gas hazard at 466 m, a 9 7/8" pilot hole was drilled below the 30" conductor to 780 m. The 9 7/8" hole was opened up to 26" at 792 m prior to setting 20" casing at 785 m. No shallow gas was observed from the MWD resistivity in this hole section.

Record setting overpressures were experienced in the 17 1/2" hole section in well 1/5-3 S. Abnormal pressures were indicated first at 700 - 800 m. Pore pressures built quickly to 1.4 g/cc due to gas just below 1000 m in. Having passed that depth, the hole drilled without problems until below 1200 m where it again became gassy. Mud weight was increased to 1.52 g/cc, thus reducing 30% gas to 5-10%. This weight was sufficient until below 1400 m when gas again increased. By 1450 m, the DXC was beginning to show signs of increasing pore pressure, as was the MWD resistivity. Below 1500 m, gas went off scale and an oil kick to 1.70 BMW was taken at 1544 m. Pressures of this magnitude were not forecast at all. Lost circulation was experienced during well control operations, which eventually lead to cementing of the BHA, and plugging back to sidetrack around the fish. The 17 l/2" hole was re-drilled as 1/5-3 S T2 from 1246 m to a revised 13 3/8" casing point at 1412 m. Although re-drilled with 1.60 g/cc mud weight versus the original 1.52 g/cc mud, the hole drilled nearly as gassy as the original hole. Following the 13 3/8" casing, an excellent leak-off was tested to nearly overburden gradient at 1.98 BMW.

The 12 1/4" drilling was done with Statoil's Tommeliten method which emphasized ignoring gas in favour of other pressure parameters while minimizing mud weight builds but this proved to be unsuccessful for 1/5-3S. After drilling out with 1.76 g/cc mud weight, the hole became so gassy (up to 50%) from limestone stringers oozing oil that it had to be circulated clean at 1494 m, and 1.80 g/cc mud was circulated around. This should have balanced the 1.7 BMW kick zone coming up at 1544 m, as well as leading to increased confidence, as gas and cuttings size would diminish. Although the cuttings remained small until growing to 7 cm splinters near TD, gas was again off scale. By 1566 m, only 22 m beyond the second kick zone, the well was shut-in. 1.86 g/cc mud weight was required to balance the formation, and 1.90 g/cc mud weight was eventually circulated around on a dead well. Well 1/5-3 S T2 had transitioned from a pore pressure of 1.7 EMW at 1544 m to 1.86 EMW at 1566 m in only 22m of new hole. At this point the decision was taken to plug and abandon the well.

Three kicks taken were regional records for both overpressure magnitude and shallowness of depth. Statoil's Tommeliten Field in block 1/9 did not see anywhere near the overpressure magnitude and shallow onset; mud weight was able to control mud gas far more successfully on Tommeliten and multiple hydrocarbon kicks were not experienced. Conoco's 1/6-5 crestal diapir well also exhibited a lesser overpressure profile. In hindsight the most important methods to monitor the pressure during drilling were the MWD resistivity and the cuttings shape and size. Gas in the mud was carefully monitored and plotted in units of percent methane in air. Gas was commonly 5% in the claystones, some of which showed bleeding gas at the surface, and ran 30-50% and higher in the carbonate stringers, which bled oil at the surface. The gas chimney section drilled with high gas background all the way from the top of overpressure to the terminal kicks below 1500 m. While the mud gas gave a general indication of overpressure, the high background levels actually obscured both of the final two kicks.

Good trace of crude oil in the mud was observed from 1498m. At 1544 m, a kick was taken which resulted in crude oil being circulated up to the rig. Circulating gas varied between 40-100%, with peaks way above 100% caused by large amount of hydrocarbons. The crude oil collected at surface was dark yellowish brown and had a density of 0.84 g/cc (37 API) measured with a pressurised mud balance. Later laboratory analysis onshore gave a density of 0.80 g/cc (35.1 API).

No conventional or sidewall cores were taken in this well. The well was permanently abandoned as a junked well with minor oil on 6 August 1998.

Testing

No drill stem test was performed.

Cuttings available at the NPD

Interval [m]: 800.00 - 1560.00

Cores available at the NPD

No cores in this wellbore.

Scanned core photos

No scanned core photos available

Palynological slides available at the NPD

No palynological slides in this wellbore.

Oil samples available at the NPD

No oil samples available.

Casing and leak-off tests

Mud

Drill stem test

No drill stem test in this wellbore.

Available logs

Lithostratigraphy

Composite log

Composite log (Format:PDF. Size: 0.201 Mb)
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Geochemical information

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Documents reported by the licencees (period for duty of secrecy expired)

No documents presently available.