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Exploration wellbore: 2/7-21 S

General information

Wellbore history

General

Well 2/7-21 S was drilled to appraise the "South Eldfisk structure", now known as the Embla Field, located in the Central Graben of the North Sea. It was the third well drilled on the structure, a pre-Cretaceous fault block forming the boundary between the Grensen Nose to the west and the Feda Graben to the east. The primary objective was to test the sandstones that tested oil in the 2/7-9 and 2/7-20 wells. It was anticipated that the proposed location would encounter reservoir quality sands similar in nature, thickness, and depth as in the 2/7-20 well. Closure of the structure is provided by normal down faulting to the North, South and West. Closure to the east is by the Lindesnes Ridge reverse fault. Vertical sealing is provided by the Early Cretaceous Shales. No shallow gas was expected in this area, and no major obstacles other than high formation pressure in the reservoir. If successful, the well was planned to constitute a second drainage point for future field development.

Operations and results

Appraisal well 2/7-21 S was drilled deviated from a three-slot template located at the 2/7-20 well to a location 1020 m to the southeast. The well was spudded on the 21st June 1989 and drilled using the semi-submersible installation Ross Isle to a Total Depth of 5039 m (4706 m TVD RKB) in rhyolitic igneous rocks of probable Early Devonian age. The well was drilled without significant problems, except for MWD failures. Shallow gas was encountered at 586.7 m but caused no problems. The well was drilled without problem using SOLTEX Actaflow water based mud down to 4232 m at the base of the Cretaceous section, where 9 5/8" casing was set. The remaining 8 1/2" and 5 7/8" hole sections in the well were drilled using Invermul oil based mud.

Oil-bearing reservoir quality sands were encountered at 4313 m. No definitive oil-water contact could be seen. The section consisted of undefined lithostratigraphy of pre-Jurassic age. It could be broadly divided into ten intervals as follows:

Interval 1, 4299.5 - 4313 m, is an uncored, relatively muddy sequence of unknown affinity.

Interval 2, 4313 - 4339 m, is a partly cored, possible alluvial fan/braid plain sequence which is probably faulted at its base.

Interval 3, 4339 - 4386 m, is a sequence of fractured and brecciated, variably silty and sandy mudstones with thin sandstones. The upper part of this was cored.

Interval 4, 4386- 4453 m, is a broadly coarsening-upward sequence consisting of mudstones with thin interbedded sandstones at its base overlain by more abundant and thicker, cleaner sandstones towards the top. This interval is uncored.

Interval 5, 4453 - 4540 m, is an interval of conglomerates and pebbly sandstones locally interbedded with finer grained sandstones and mudstones. The upper part of this interval was cored.

Interval 6, 4540 - 4577 m, consists of interbedded sandstones and basic igneous rocks, which are either extrusive or shallow intrusive in nature. Only the lower part of this interval was cored.

Interval 7, 4577 - 4638 m, is a broadly coarsening-upward sequence of which only the uppermost part was cored.

Interval 8, 4638 - 4662 m, is a partly cored interval of highly fractured and brecciated sedimentary and igneous rocks. The igneous rocks have a broadly rhyolitic nature and are similar to those which occur below 4837. It is not clear whether this represents a genuinely interstratified sequence with extrusive volcanics, whether the igneous rocks are intrusive or whether it represents a complicated fault slice.

Interval 9, 4662 - 4837 m, is a further, broadly coarsening-upward sequence which was cored in its upper part. The lower part of the interval is poorly defined because of non-operation of wire line logs. Argillaceous ditch cuttings yielded a Frasnian (lower Late Devonian) age, similar to the lowermost part of the sandstone sequence in Auk and Argyll fields.

Interval 10, 4837 - 5039 m (TD) a partly cored, thick sequence of porphyritic, very finely crystalline rhyolitic igneous rocks which are fractured and brecciated and have been extensively altered.

The basal rhyolites are most likely to be Early Devonian in age. These are overlain by Late Devonian (Frasnian) sediments. This stratigraphic pattern differs from that observed in Auk and Argyll fields (where there is a Middle Devonian limestone), but is similar to the stratigraphic pattern of the Midland Valley of Scotland. Overlying the Devonian sediments is a sequence of basic volcanics or shallow intrusives and sediments. These are most likely to represent either Latest Devonian - Early Carboniferous, or Early Permian rocks. These basic igneous rocks are highly altered and yield a mean K-Ar age of 60.7 Myr. This probably reflects a phase of hydrothermal alteration associated with Late Cretaceous -Early Tertiary inversion of the Lindesnes ridge. The overlying sedimentary package (Intervals 1-5) is most likely to be either Rotliegendes Group or Lower Carboniferous. K-Ar radiometric dating of illitic concentrates provides Late Triassic ages. This clearly rules out a Jurassic age of these rocks. Given that these concentrates represent a mixture of detrital and diagenetic illite, the radiometric date is only a minimum age.

There was a prominent oil show around 1602 m in the Miocene with a good trace of free oil in the mud and a strong petroleum odour in cuttings. Shows were seen with gas peaks through most of the sectionfrom 3228 to 4206 m in the Shetland Group. No odour or visible stain was noticed here. The pre-Jurassic sandstone from 4325 to 4337 showed a poor to fair yellow gold fluorescence, with a poor to fair dull white instant cut, followed by a slow streaming to blooming cut. There was a slight brown oil stain on quartz grains and a strong petroleum odour. There were occasional poor shows in the sectionfrom 4337 to 5039 m (TD). Some of the section displayed a trace of moderate to dull yellow fluorescence, with a very slow grey to bright white blooming, hazy cut, but no stain or odour. Samples from core eleven had distinct petroleum odour when heated. In the lower part of the section below 4877 m samples contained up to 30% black carbonaceous matter, resembling coal. This was interpreted as bitumen, based on cores ten and eleven, which consisted mostly of heavily brecciated quartzites. Later rock-eval analysis revealed up to 3.2% TOC in samples from this interval, and it is believed to represent an early emplacement of a petroleum that pre-dates the tested live oil from the well.

Eleven cores were cut within the reservoir and underlying basement sections. Although difficulties were experienced with tool sticking, a nearly complete and reliable wire line data set was acquired. A total of thirty sidewall cores were attempted, whereof fifteen were retrieved. The RFT tool was run to acquire pressure data and fluid samples. Fluid samples were taken at 4322.4 m, 4322.7 m and at 4323 m. The two first of these were contaminated with mud due to seal failure. The one from 4323 m gave a GOR of 321 Sm3/Sm3 and an oil gravity of 43 deg API.

The well was temporary abandoned on 9 January 1990, suitable for later re-entry and possible tie-back to production facilities. It is classified as an oil appraisal well

Testing

A well testing program was designed based on RFT, log and core data to investigate three separate sand bodies in the pre-Jurassic section.

DST #1 was designed to test the interval 4577 to 4612 m (4313.8 to 4346.8 m TVD) in the deepest unit, a poor quality sand that displayed limited but anomalous RFT results suggesting a different fluid from the higher zones. The interval produced in the order of 600 STBOPD (95.4 Sm3/day) on a 14/64" choke, but was unexpectedly accompanied by 20-25 ppm H2S. The DST string was not rated for H2S and the test was thus abandoned. The GOR in this test was ca 1600 SCF/STB (285 Sm3/Sm3), the oil density was 0.8 g/cm3, and the gas gravity was 0.78 (air = 1) with 5.5% CO2. The test bottom hole temperature (BHT) measured at 4530.9 m (4271 m TVD) was 160 deg C.

DST #2 was completed over the main reservoir section in the interval 4446 to 4519 m (4192.4 to 4259.6 m TVD). Up to 5000 STBOPD (794.9 Sm3/day) was produced on a 30/64" choke with a GOR of 1900 SCF/STB (338 Sm3/Sm3). The oil density was 0.816 g/cm3, and the gas gravity was 0.8 (air = 1) with 5% CO2 and 40 ppm H2S. The test BHT measured at 4251.4 m (4010.8 m TVD) was 153.9 deg C.

DST #3 was a commingled test over the DST #2 interval plus the interval 4308 to 4338 m (4063.6 to 4091.0 m TVD). It produced 8000 STBOPD (1271.9 Sm3/day) on a 48/64" choke with a GOR of 1850 SCF/STB (329.5 Sm3/Sm3). The oil density was 0.81 g/cm3, and the gas gravity was 0.8 (air = 1) with 5% CO2 and 20-25 ppm H2S. The test BHT measured at 4250.9 m (4010.3 m TVD) was 153.9 deg C.

Upon completion of the test, a PLT log was run over most of the perforated section. Results from the comingled test plus the PLT strongly indicated that killing the well with drilling mud after DST #2 had damaged the formation, and that the results of DST #3 were consequently poorer than expected.

In general, all tested fluids could be characterized as highly under-saturated volatile oil.

Cuttings available at the NPD

Interval [m]: 1073.00 - 5038.00

Cores available at the NPD

Core length [m]: 115.40

Scanned core photos

14175-14190ft	3547-3555m	14190-14196ft	3547-3555m	14196-14211ft
3547-3555m	14211-14226ft	3547-3555m	14226-14241ft	3547-3555m
14241-14251ft	14256-14271ft	3547-3555m	14271-14278ft	3547-3555m
14621-14636ft	3547-3555m	14636-14649ft	3547-3555m	14649-14664ft
3547-3555m	14664-14679ft	3547-3555m	14679-14684ft	3547-3555m
3547-3555m	3547-3555m	3589-3592m	3589-3592m	3589-3592m
3589-3592m	3589-3592m

Palynological slides available at the NPD

No palynological slides in this wellbore.

Oil samples available at the NPD

Casing and leak-off tests

Mud

Drill stem test (intervals and pressures)

Drill Stem Test (recovery)

Available logs

Lithostratigraphy

Composite log

No composite log presently available.

Geochemical information

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

No documents presently available.

Old NPD WDSS reports, NPD papers and other related documents

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