The Buchan oil field is a small oil field with small
gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
reserves in the central
North Sea
The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...
. It lies in an area known as the South Halibut Basin, approximately northeast of
Aberdeen
Aberdeen (; sco, Aiberdeen ; gd, Obar Dheathain ; la, Aberdonia) is a city in North East Scotland, and is the third most populous city in the country. Aberdeen is one of Scotland's 32 local government council areas (as Aberdeen City), and ...
,
Scotland
Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to the ...
, and is located mainly in license block 21/1A, extending into block 20/5A(E). The field was discovered in August 1974, two years after the issue date for those blocks. It is named after Buchan, an area of N.E. Scotland with its main town being Peterhead.
Buchan was initially considered a very risky field, both commercially and operationally, and was expected to be finished with after five years. The field's geology created significant difficulties at the start of its development, but the development of new drilling and extraction technology has made possible a continuing increase in its recoverable reserves. It is thought that Buchan will continue to play an important part in the North Sea and that further technological breakthroughs will allow continued production of this field to at least 2018.
Exploration
As the initial licence block holder, Transworld Petroleum (UK) with
Texaco
Texaco, Inc. ("The Texas Company") is an American Petroleum, oil brand owned and operated by Chevron Corporation. Its flagship product is its Gasoline, fuel "Texaco with Techron". It also owned the Havoline motor oil brand. Texaco was an Indepe ...
drilled the first well in the field in Aug 1974. In the next two years a further three appraisal wells were drilled in the area. However, this proved to be very difficult area to extract from: tests revealed an extremely complex and fractured field, and two of the appraisal drills were eventually lost. BP was then farmed in to become the operator of the field in 1977.
Buchan was initial described as a “small field of uncertain reserve potential” (Hill 1979). Early estimates were for reserves of approximately of oil. Some estimates though suggested upwards of of oil might be extractable depending on further capital expenditure and the ability of the
reservoir
A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation.
Reservoirs can be created in a number of ways, including contro ...
. Production of the field began in May 1981 and reached peak production capacity in May 1983, when the field was producing .
Further fields have been discovered in the Buchan field area. The Buchan North Satellite field was discovered shortly after the main field. The nearby Hannay Field, discovered in 1996, is tied back to 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
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...
and Lower
Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
periods (Edwards 1991). The horst is surrounded by Middle to Late Jurassic sands which are
fluvial
In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...
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 oil while the surrounding Jurassic sands are expected to contain significantly less, approximately oil (Wood Mackenzie 2002).
The most important factor of this field is the overpressure of the central horst. This overpressure has been created by what is thought 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
The pound per square inch or, more accurately, pound-force per square inch (symbol: lbf/in2; abbreviation: psi) is a unit of pressure or of stress based on avoirdupois units. It is the pressure resulting from a force of one pound-force applied t ...
(22 MPa). More importantly the transition zone from normal pressure to this overpressure happens over some extremely thin Lower Cretaceous layers, which in parts are only 30 m thick.
These initial findings 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
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
and permeability. However, as with all fields, as more was investigated from production data, new facts about the geology were revealed. In this case, the Buchan field is fairly fractured due to its location on an old fault line. Therefore, there are numerous fissures in the reservoir which increase its porosity and permeability.
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 lowered the quality of the seismic data, giving poor reflections. As a result, geophysics could not be used to accurately define the top depth of the reservoir; initial depth estimates were out by as much as 133 m.
The pore pressures of the layers over the reservoir are also very dangerous, as they vary greatly from one to the next. There is normal pressure in the Upper Cretaceous limestone layers, but between this normal pressure in the limestone and the over-pressured reservoir there is only a small layer of Lower Cretaceous
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
. In parts this 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 is 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
A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation.
Reservoirs can be created in a number of ways, including contro ...
two major loss zones were discovered. 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 due to vague geotechnical data. The first case, in well 21/1-2, resulted in considerable mud losses and a significant section of the hole had to be abandoned so the drilling could take a different path. This option was not available in the second case, in well 21/1-4, and the hole had to be abandoned eventually.
Furthermore, the geological makeup of the matrix of the reservoir did not contribute to the production of the field to the extent that was hoped. It was initially assumed that full production would peak at and have an average production rate of . However, the field only reached a peak of and the field average was significantly lower than that.
Drilling
BP had to employ sophisticated measures to overcome the field's geological difficulties. These problems were significant enough that the project's two senior engineers published a paper about them in the
Society of Petroleum Engineers
The Society of Petroleum Engineers (SPE) is a 501(c)(3) not-for-profit professional organization whose stated mission is "to collect, disseminate, and exchange technical knowledge concerning the exploration, development and production of oil and ...
. They faced problems with the project timing, with the operational drilling of the well, and with the completion phase (Pinchbeck 1979).
The lack of suitable geophysical data on the reservoir required serious precautions. The well was designed to have numerous backups 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, which was 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 lifetime of this field was five years with a production of oil, so to maximize profit the development had to proceed very quickly and all capital costs be kept to a minimum (Mieras 1983). For this reason a lot of the development was done in parallel so that the wells, pipelines, and 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). These retrospective upgrades were necessary after a similar rig, ''Alexander L. Keilland'', collapsed and killed 100 people.
The topsides for Buchan Alpha were designed by Matthew Hall EngineeringMatthew Hall Engineering publicity brochure n.d. but c. 1990 which was also responsible for procurement, project management, construction management and offshore installation services. They were awarded the contract in October 1977. Initially there were facilities for eight oil production wells. The production capacity was 70,000
barrels
A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden staves and bound by wooden or metal hoops. The word vat is often used for large containers for liquids, u ...
of oil per day and 600,000 standard cubic metres of gas per day. There was a single production train of three stages of separation with the first stage operating at a pressure of 10 barg. Electricity generation was powered by four 2 MW diesel engines. The topside accommodation was for 80 people.
Production of the Buchan Oil field came on-line in May 1981, and oil export initiated in June 1981. Initially oil was offloaded from the platform to a tanker. However in 1986 a pipeline was built which connects with the
Forties pipeline system
The Forties pipeline system (FPS) is a major pipeline transport network in the North Sea. It is owned and operated by Ineos and carries 30% of the UK's oil, or about of oil per day, to shore. It carries liquids production from 85 fields in the ...
and then to Cruden Bay. This pipeline added a further to the recoverable reserves.
Production peaked soon in 1983 with an average of , but in late 1984 production was declining, and the rig underwent a major maintenance programme 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). The addition of the gas lift allowed production to reach for several years before beginning a slow steady decline in 1989. Any remaining gas is flared.
Reserves for the field, though, were continually upgraded from the cautious prediction of 50 million barrels of oil in 1979 to 134 mbo in 2002. This was put down to increased knowledge of the field as well as improvement in extraction methods.
BP continued to be the principal operator of the field alongside 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
Talisman Energy Inc. was a Canadian independent petroleum company that existed between 1993 and 2015. The company was created from the assets of BP Canada after British Petroleum divested its 57 percent stake in June 1992. It was one of Canada' ...
, a Canadian oil company that specialises in fields that are nearing their abandonment stage. As of 2002 the field was thought to still have a further of recoverable oil. Due to advancements in technology and better understanding of the field, the final size of the recoverable reserves is thought to be oil.
Future
Talisman specialises in extracting significant amount of reserves from mature 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
Underbalanced drilling, or UBD, is a procedure used to drill oil and gas wells where the pressure in the wellbore is kept lower than the static pressure of the formation being drilled. As the well is being drilled, formation fluid flows into the w ...
and
coiled tubing drilling Coiled Tubing Drilling (CTD) is a drilling method that combines coiled tubing and directional drilling.FSPO (CCNMathews 2001). This would sidetrack an existing hole and increase production. This did indeed increase production by , raising the daily average to . Since the success of this type of drilling, a further two wells will undergo this treatment.
The Buchan field plays an integral part in Talisman's long-term strategy as they hope to continue to produce from this field long past the expected closing date of 2015. This continued success with new technology has inspired hope that Talisman can extract a much larger part of the total in place.
Latest update from the HSE is that the Buchan field will cease production in September 2017, despite extensive surveying of the mechanical structure of the Buchan Alpha drilling platform discovering no signs of metal fatigue or weakness within the installation. It could, technically, go on for years but it is extremely unlikely that this will be allowed.
See also
*
Energy policy of the United Kingdom
The energy policy of the United Kingdom refers to the United Kingdom's efforts towards reducing energy intensity, reducing energy poverty, and maintaining energy supply reliability. The United Kingdom has had success in this, though energy int ...
{{reflist
*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 *Pinchbeck, R.H. and Powell, S.E., Drilling and completion of Buchan Field, SPE No. 8154, Offshore Europe Conference, 1979 *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 *CCNMathews., Talisman News Release, Aug 20 2001. Accessed 8 Oct 2004 a *Wood Mackenzie,. Fife Area UK Upstream Service-Central North Sea, 198 *Wood Mackenzie,. Fife Area UK Upstream Service-Central North Sea, 198 *Wood Mackenzie,. Fife Area UK Upstream Service-Central North Sea, 200 North Sea oil fieldsOil fields of ScotlandTexaco