Mobil North Sea reaps a significant portion of its income from its equity share of gas from 17 Southern Basin producing fields, on six of which it is operator. This year they will account for 44% of all Mobil's production revenue and 72% of after-tax income.
On peak days the fields can supply 1 bcf/d of gas to UK customers via the gas terminals at Bacton and Theddlethorpe. The six operated fields can deliver around 300 mcf/d.
Mobil sells its share of the gas to Mobil Gas Marketing (377 mcf/d); British Gas (168 mcf/d); and PowerGen (23 mcf/d). Mobil Gas Marketing is one of Britain's leading independent gas marketers, whose customers include major players in power generation, industry and commerce.
In fact, Mobil regards its Southern Basin gasfields as one of the planks that form the firm basis of its business. As one spokesman put it: "They are our real bread and butter - they bring in the cash. We could not do our West of Shetlands activities without them."
The market for gas is growing and the domestic market is opening up to competition. New fields are needed so that Mobil can maintain its position. John Forrest, SGB business area coordinator, says: "Our role is to find and develop new gas resources to maintain our strong market position.
"The easy fields were developed in the 1960s and 1970s and more recently we've had considerable success with smaller, more difficult accumulations. Our task now is to use our technological and engineering expertise to develop even smaller and more challenging gas reservoirs."
Technology of which Mobil has made extensive use includes horizontal and extended reach drilling; multi-lateral wells; hydraulic fractures; 3D seismic interpretation; and geostatistics.
Yet it was only seven years ago that Mobil's first wholly-owned North Sea field came onstream. In fact, by 1985 some of Mobil's Southern Basin acreage was set to be traded for other exploration opportunities. It was felt that the smaller discoveries could not sustain economic production rates with available technology.
Two things altered this situation: British Gas lost its monopoly right to purchase all UK-produced gas and technological developments made it easier to find and develop smaller accumulations. Thus re-assessment of the acreage led to the drilling of well 53/1a-5 in May 1987, which discovered the Camelot Field. The unmanned platform was designed, constructed and commissioned in two years. Production began in October 1989.
This was Mobil's first solo venture in the Southern Gas Basin. It was the first in the cluster of fields which have become known as Mobil's `offshore Arthurian kingdom', as they are all named after places and characters from the Arthurian legend.
Camelot is linked by pipeline to the Amoco-operated Leman Field. This assured Mobil that smaller accumulations could become profitable with the new generation of unmanned production facilities, using existing infrastructure.
In 1992, the wholly Mobil-owned, 28 bcf Tristan Field came onstream. This was discovered in block 49/29b in 1976, but was too small to justify development at that time. But a reappraisal of the geology in 1992 changed all that. Tristan was developed through one well drilled from the Arco-operated Welland unmanned platform, 3.5 km away, in which Mobil has a 55% interest.
Also that year, a second Camelot platform was installed. The following year two more fields came onstream: Lancelot, designed as the hub for a core area, and Guinevere. Success with horizontal drilling meant that no wells were dropped from the Lancelot programme and about £ 18 million was saved from the overall budget. The wells began producing about 30% more gas than anticipated.
Both fields have a four-legged steel jacket platform, not normally manned and remotely controlled from Phillips' Bacton terminal. A small accumulation, Cador, has also been drilled from the Camelot platform.
Excalibur then came onstream in 1994, followed by Gawain and Galahad in 1995. Galahad, in blocks 48/12a and 48/13b, was one of the fastest projects eve completed in the UK North Sea incorporating a production platform. From the initial sanction date of December 1994, Mobil was able to achieve first gas in 11 months.
In November 1995, Galahad produced its first gas, 11 months after sanction. Recoverable reserves are 153 bcf of gas and peak production can be 120 mcf/d. Production flows 16 km through a 12-in. pipeline to a tie-in point on the existing Lancelot export line and joins gas from Excalibur and Guinevere heading to the Phillips plant at Bacton.
Galahad was discovered in 1975 with well 48/12-2, but remained undeveloped until advances in horizontal drilling made it an attractive proposition. The field's potential was confirmed by an innovative multi-lateral horizontal well in 1994, which was converted to a producer. This allowed the project to proceed without the need for a third and final well.
The speed of the development was due to a combination of innovative drilling - including a long-reach well with a 5,000 ft horizontal section, in addition to the multi-lateral well - and the application of Crine cost-reduction principles in platform design and management.
The unmanned monopod platform, with four production slots, incorporates separation, metering and power generation facilities. The design is essentially the same as that developed by Amoco for the Davy/Bessemer projects, and was built by the same fabricator (Barmac, Nigg). "This recycling of design information is a good example of two companies working together to reduce development costs," says Mobil.
Development of the latest field, Mordred, a satellite to Galahd, has run into problems as it has proven difficult to get the 19,500 ft development well to flow at commercial rates. The well was drilled by jack-up Rowan Halifax from the Galahad platform, with a horizontal section of 4,800 ft.
Attempts have been made to frac the well to stimulate production. Forrest says: "Frac-cing a horizontal well presents a real challenge. We have certainly learnt a lot. But we are still determined to develop the reservoir." Earlier this year Brown & Root AOC undertook offshore construction and commissioning work in parallel with the drilling activity and was responsible for modifications to the Galahad system for the tie-back of the well.
The next in line for development is Belvedere, a separate structure to Lancelot, where an appraisal well will be drilled next year. Production, whether from a subsea manifold or wellhead platform, will flow into the Lancelot-Bacton pipeline via a T-junction.
According to Forrest: "The real challenge is to continue our growth in this gas-prone area. The core area has become mature: it is more difficult to find new prospects and there are very few blocks left to licence."
In its search, Mobil runs an aggressive drilling programme both in and outside the core. This year it has drilled around Camelot and Mordred and in block 52/5b, south of Hewett. Roaming farther afield, it completed exploration well 42/10b2 on the northern flanks of the Southern Gas Basin. This was done to earn a 50% interest in the Total-operated block.
Mobil also farmed into Total block 43/28a, east of BHP's producing gasfield Johnston, in the process becoming the biggest stake-holder in the block with 35%. In February this year jack-up Glomar Labrador 1 spudded well 43/28a3, which was completed in the Carboniferous as a tight hole in April.
Next year, Mobil plans to drill numerous wells in the hope of establishing another core area alongside the Carboniferous hopes in Quadrants 42 and 43. There will probably be ten wells, drilled to depths of around 6,000 ft, each lasting 30 days - 40, if tested.
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