Buchan oil field

Infobox Oil field
name = Buchan oil field
region = Central North Sea, South Halibut Basin
country =
latd =
latm =
lats =
latNS =
longd =
longm =
longs =
longEW =
locblock =
offonshore = Offshore
operators = Talisman Energy
partners =


caption =
discovery = August 1974
startofproduction = May 1981
peakofproduction = 1983
expectedabandonment = 2018
currentproductionoil =
currentproductiongas =
estimatedstoiipoil =
estimatedstoiipgas =
producingformations =

The Buchan Oil field is located in the Central North Sea approximately convert|120|mi|km north east of Aberdeen, Scotland, in an area known as the South Halibut Basin (Wood Mackenzie 2002). It was first located in August 1972, two years after the issue date for those blocks. The field is spread out over two licensing blocks, 20/5A(E) and 21/1A . The majority of the field is located though in the latter block. The field is principally an oil field with small gas reserves.

The field was discovered by the Transworld Petroleum (UK) Ltd and Texaco . After drilling some initial appraisals wells to delineate the field was farmed out to BP who became the operator. Buchan was initial described as a “small field of uncertain reserve potential”(Hill 1979). Early estimates were for reserves of approximately convert|50|Moilbbl|m3 of oil. Some estimates though suggested upwards of convert|120|Moilbbl|m3 of oil might be extractable depending on further capital expenditure and the ability of the reservoir. Production of the field began in May 1981 and reached it peak production capacity in May 1983 when it was producing convert|32000|oilbbl/d|m3/d.

Initially the oil was transported to the mainland via tanker every day. In late 1986 a pipeline between the Buchan Oil platform and the Forties Charlie platform, also owned by BP at the time, was completed. This pipeline then continues to Cruden Bay. This pipeline added a further convert|15|Moilbbl|m3 to the reserves.

BP continued to be the principle operator of the field alongside with Texaco which had been awarded the 20/5(a) block. This continued until August 1996 when the field was sold along with the Beatrice Oil Field and Clyde Oil Field fields to Talisman Energy a Canadian oil company that specialises in fields that are nearing there abandonment stage. As of 2002 there is thought to still be a further convert|37|Moilbbl|m3 oil in the field still recoverable. Due to advancements in technology and better understanding of the field the final Recoverable Reserves (p+p) is thought to be convert|155|Moilbbl|m3 oil.

Exploration

As the initial licence block holder Transworld Petroleum (UK) with Texaco made the first well into the field in Aug 1974. In the next two years a further three appraisal wells were drilled in the area. However this proved to very difficult area to extract from as test eventually revealed an extremely complex and fractured field. This resulted in two of the appraisal drills eventually being lost. BP was then farmed in to become the operator of the field 1977. Further fields have been discovered in the Buchan field area, with the Buchan North Satellite field being discovered shortly afterwards and more recently the Hannay Field in 1996 which makes use of the Buchan Alpha rig.

Geology

The central horst of the field is approximately 400 meters thick on an East to West orientation. This central horst is made up of Old Red Sandstone from the Devonian and Lower Carboniferous periods (Edwards 1991). The horst is surrounded by middle to late Jurassic sands which are fluvial and Aeolian interbedded with siltstones (Hill 1979). The quality of these sands though varies both vertically and laterally (fig 2). The central horst is thought to contain convert|370|Moilbbl|m3 oil while the surrounding Jurassic sands are expected to contain significantly less, approximately convert|38|Moilbbl|m3 oil (Wood Mackenzie 2002).

The most important factor of this field though is the overpressure of the central horst. This overpressure has been created by what is though to be an uplift of 2500 m of which 1000 m occurred during the Lower Cretaceous Period (Hill 1979). The pressure data that was collected from the appraisal wells show that there is an overpressure of 3,200 lbf/in² (22 MPa). More importantly the transition zone from normal pressure to this overpressure happen over some extremely thin Lower Cretaceous layers, which in parts are only 30 m thick.

These initial combinations of data suggested that this would be a marginal field which would be difficult to extract from. This data also suggested that extraction would be limited due to a low porosity and permeability. However as with all fields as more was investigated from production data, new facts about the geology where revealed. In this case, the Buchan field is in fact fairly fractured due to its location on an old fault line. Therefore there are numerous fissures in the reservoir which improved the potential of it through its increased porosity and permeability.

The overpressure, the low porosity and permeability of the reservoir where not the only problems that this field suffered from. Three other major problems of the field’s geology would have to be overcome during the drilling stage. (Hill 1979)Initially there were problems with the geophysical data of the field. As mentioned the field is highly fractured due to its location. This hammered the quality of the sesmesic data giving poor reflections. As a result initial depth estimates were out by as much as 133 m when defining the top depth of the reservoir. Because of this the geophysics could not be used to accurately define the top of the reservoir.

The pore pressures of the layers over the reservoir were also very dangerous as it varies greatly from one to the next. There is normal pressure in the Upper Cretaceous Limestone layers however between this normal pressure in the limestone and the over-pressured reservoir there is only a small layer of Lower Cretaceous Shale. In parts the Cretaceous Shale is only 30 m thick separating the above layer and the over-pressured reservoir. As a result pore pressure in the shale at the top was close to normal but rapidly approaches 3200 psi (22 MPa) as it comes in contact with the reservoir. This circumstance was to prove difficult when selecting the appropriate mud weights during drilling. Furthermore the Shale layer did not have even layer depth throughout varying from 30 to 200 m so making it difficult to extrapolate pore pressure in different sections (Hill 1979).

During the drilling of appraisal wells in the reservoir two major loss zones were discovered. These occurred during the drilling of wells 21/1-2 and 21/1-4. Fractured zones such as those encountered around the Buchan field can lead to a complete or serious loss of the entire mud stream as it is absorbed by the formation. Both cases involved the penetration of the reservoir at a much shallower depth than originally considered. An unfortunate product of fuzzy geotechnical data. The first case resulted in considerable mud losses and a significant section of the hole had to be abandoned so the drilling could take a different path. Unfortunately the second occurrence of this in well 21/1-4 was not so lucky and the hole had to be abandoned eventually.

Further more the geological makeup of the matrix of the reservoir that was expected to contribute to the production of the field did not happen to the extent that was hoped. It was initially assumed that full production would peak at convert|72000|oilbbl/d|m3/d|abbr=on and have an average production rate of convert|48000|oilbbl/d|m3/d|abbr=on. The field though only ever reached a peak of convert|32000|oilbbl/d|m3/d|abbr=on and the field average was significantly lower than that.

Drilling

The uniqueness and difficulties of this field required BP to employ sophisticated measures to counter act the possible problems from the geology. Enough so that a paper in the Society of Petroleum Engineer [http://www.spe.org] was published by the two senior engineers involved on the project. They faced problems with the project timing, the operational drilling of the well as with the completion phase (Pinchbeck 1979).

The lack of suitable geophysical data on the reservoir required that serious precautions were taken with the well design with numerous backs up put in place. These were to ensure that if the reservoir top was penetrated accidentally that the casing strings would be able to handle the overpressure. There was also the problem of the encountering other loss zones such as those experienced by the appraisal wells.

These problems were overcome through a combination of strict control of the drilling and its location, along with careful analysis of the various geological variables. This ensured that the correct sized wells were drilled through the loss zones and that the over-pressured shales were avoided by careful monitoring of the pore pressure. In total nine wells were drilled for production, seven central wells and two satellite wells.

Development

The initial time life of this field was 5 years with a production of convert|50|Moilbbl|m3 oil so to maximize potential profit the development had to proceed very quickly and all capital costs kept to a minimum (Mieras 1983). For this reason a lot of the development was done in parallel so that the wells, pipelines, the offshore loading buoy were all completed in time. Since the drilling had been fully completed and then capped the previous autumn, when the rig was available to come on line it was almost able to reach full reservoir production capacity immediately. The biggest delay came from the conversion of a semi-submersible drilling rig, Drillmaster, to a production rig which ended up 20 months late and 40% costlier than first envisioned (Mieras 1983). This was initially done as it was seen as cost effective and stable platform. However essential retrospective upgrades had to be made to the structure after a similar sister rig collapse and killed 100 people earlier.

Production of the Buchan Oil field came on line in May 1981. With the first batches of oil exports leaving in June 1983. Initially oil was transported daily from the rig however in 1986 a pipeline was built connecting with the Forties pipeline. Production peaked soon in 1983 with an average of convert|32000|oilbbl/d|m3/d|abbr=on, however in late 1984 production was declining the rig went under major maintenance program along with an addition of gas lift facilities. Unlike most fitting of gas lifts causing long delays in production, the need for a gas lift had been anticipated and so was able to be completed via wireline intervention (Pinchbeck 1979). This addition of the gas life allowed production to reach convert|20000|oilbbl/d|m3/d|abbr=on for several years before beginning a slow steady decline in levels in 1989. Any remaining gas is flared.

Reserves for the field though were continuingly being upgraded from the cautious prediction of 50 mbo in 1979 to 134 mbo in 2002. This was put down to increased knowledge of the field as well as improvement in extraction methods. The latest jump in the eventual reserve predictions has been since Talisman has become the principle operator of the 21/1A block in 1996.

The field was sold to Talisman by BP along with the fields Beatrice and Clyde as part of a shift from operating fields as they become mature. (See fig 3 for production profile of the field)

Future plans

Talisman is a Canadian oil company specializes in acquiring mature fields and then extracting significant amount of reserves from fields by using advanced drilling techniques to improve the production and tap additional reserves in the field.

One such method being tested as of 2001 is for an underbalanced drilling and coil tubing drilling programme which at the time had not been tried before from an FSPO (CCNMathews 2001). This would sidetrack an existing hole and deliver an increase in production. This did indeed increase production by convert|5500|oilbbl/d|m3/d|abbr=on raising the daily average to convert|13000|oilbbl/d|m3/d|abbr=on. Since the success of this type of drilling a further two wells will under go this treatment.

The Buchan field play an integral part in Talismans long term strategy as they hope to continue to produce from this field long past the expected closing date of 2015 for the field. This continued success with new technology in extracting oil has put confidence in the hope extracting a much larger part of the total convert|400|Moilbbl|m3 in place.

Conclusion

Initially considered a very risky field both commercially and operationally, that was to be finished with after 5 years; Buchan has instead proved to be a much more profitable field than ever imagined. This success has come from a continuing relationship of using cutting edge technology and forward planning on the field. This success has not only been limited to the continuing increase in recoverable reserves in the field but also the geological hurdles that had to be overcome at the very beginning of this development. Overall it is thought that this field will continue to play an important part in the North Sea and that break throughs in drilling and extraction technology will allow the continued production of this field to at least 2018.

ee also

*Energy policy of the United Kingdom
*Energy use and conservation in the United Kingdom

References

*Edwards, C. W. 1991. The Buchan Field, Blocks 20/5a, 21/1a, UK North Sea. In "United Kingdom Oil and Gas Fields 25 Years Commemorative Volume". London, The Geological Society, Memoir 14,253-259.
*Hill, P.J. and Smith. G., Geological aspects of the drilling of the Buchan field, SPE No. 8153, Offshore Europe Conference, 1979. [http://www.spe.org]
*Pinchbeck, R.H. and Powell, S.E., Drilling and completion of Buchan Field, SPE No. 8154, Offshore Europe Conference, 1979. [http://www.spe.org]
*Mieras, A. A., Operational Performance of the Buchan Oil Field Floating Production and Offshore Loading System, SPE 12433, Offshore South East Asia Conference, Feb 1983. [http://www.spe.org]
*CCNMathews., Talisman News Release, Aug 20 2001. Accessed 8 Oct 2004 at [http://www2.cdn-news.com/scripts/ccn-release.pl?/2001/08/20/0820017n.html]
*Wood Mackenzie,. Fife Area UK Upstream Service-Central North Sea, 1980 [http://www.woodmacresearch.com/cgi-bin/corp/portal/corp/corpPortal.jsp]
*Wood Mackenzie,. Fife Area UK Upstream Service-Central North Sea, 1984 [http://www.woodmacresearch.com/cgi-bin/corp/portal/corp/corpPortal.jsp]
*Wood Mackenzie,. Fife Area UK Upstream Service-Central North Sea, 2002 [http://www.woodmacresearch.com/cgi-bin/corp/portal/corp/corpPortal.jsp]