Essay: The Clair field

The Clair field, had disappointing outcomes on its earliest studies when British Petroleum (BP) failed to confirm profitability due to the poor understanding of the reservoir properties such as significant faulting and fracturing [2]. However, later studies proved the presence of economically recoverable hydrocarbons [3].
Clair field is located 75 km west of Shetland. The complexity of the field, the bad quality of the seismic studies by the time of its discovery and the unknown impact of the fractures were highly reflected of its 28 year appraisal.
Discovery and Initial Exploration Data
The Clair Field, UK’s largest naturally fractured oil reservoir (> 4 billion bbl STOIIP) [1], was initially discovered in 1977 by BP with the drilling of two exploration wells; the two wells were drilled in June and December of that same year. The first well 206/8-1A discovered oil that tested at 25o API at 1500 barrels per day (bpd) and the second well, 206/7-1, testing at 23 o API oil at a maximum flow rate of 960 barrels of oil per day. The second well produced oil that came entirely from fractures [6][2]. However, this was not considered commercially viable at the time because of the difficulties of producing in the North Sea due to technological limits and the challenging nature of the reservoir characteristics [1].
Between 1977 and 1985 ten more wells were drilled, however, the initial success of the 206/8-1a and 206/7-1 wells was never reached and the commercial expectations were poor [5]. These wells did however provide valuable information about the field, the appraisal drilling proved the Clair field contained a large oil resource which was estimated to be between 5 – 7 billion barrels [2]. This discovery made the Clair Field the largest undeveloped hydrocarbon resource on the United Kingdom Continental Shelf (UKCS) [1] and thus Europe’s largest oilfield; the field however, remained undeveloped until 2005. Due to the complexity of the geology and the poor image resolution during the seismic tests it was not until 1991 and 1992 when other wells were drilled due to acquisition of 3D seismic data [2].
In 1991, two new wells were drilled, this time however, BP made use of horizontal wells in an attempt to achieve more accurate well test data. The first one, after acid wash simulation, tested 2100 barrels of oil per day. The second one, obtained values of 3000 barrels of oil per day from two zones, with both, natural fractures and hydraulic fractures. In 1992, two more wells were drilled with the 3D seismic acquisition as the appraisal development of the field. [5]
Despite the fact that the new seismic data showed that the well would now be deemed commercially viable, its true potential however was not confirmed until 1996 when they performed tests on wells 206/8-9z and 206/8-10z. An outstretched well test was performed on the last one and a production rate of 500,000 stb of oil was obtained and proved to the presence of 500 million stb of oil more [7]. This changed the perspective of the reservoir proving a sustainable delivery.
Exploration Stages
The success of the 1990’s appraisal programme caused the implementation of a re-appraisal programme which culminated into a pilot development in 2005 named Phase 1 [2]. Phase 1 saw the first major development of the Clair Field since its discovery in 1977, as the hydrocarbon resource was finally deemed economically viable to produce by BP [1]. A steel jacket platform with 15 production wells and 8 water injection wells were deployed during Phase 1 [1]. The production area originally included in the pilot development of Phase 1 has since produced 98 million barrels of oil, 870 million m3 of natural gas and achieved a maximum production rate of over 57,000 bpd; thus making the Clair field an economic success [3].
Production History of the Clair Field. Data is provided by the UK Government’s Department of Energy & Climate Change (DECC). Please note that units in the table are represented as cubic metres per month [3].
The Phase 1 of the Clair Field development started in 2001, as a fixed platform [1] that targeted the reserves in the Core, Horst and Graben segments located in the southern part of the overall Clair reservoir, and is believed to have an average of 1.5 million stb of oil [7]. The production did not start until February of 2005 and the initial step was the recompletion of the well 206/8-10z, as a production well. After this, 22 subsequent wells were drilled over a period of 3 years. Water-flooding was the primary mechanism utilised and at the end of this phase, 250 million barrels could be recovered with an estimated total of oil in place of 1.75 billion barrels [7].
Although there are uncertainties regarding how much actual hydrocarbons can be recovered from Clair field due to its complexity, the facilities were made to last until 2028 indicating good prospects as until 2015, around 98 million barrels of oil have been produced [8]. Though the production and acquisition of 98 million barrels of oil is adequate to deem Phase 1 a success, this represents only 1.5% of the hydrocarbon resource available in the Clair Field reservoir [3].
Fig. 1: Clair Field Phase 1 Area [1]
A multi-well appraisal programme was put into place after the ridge segments were found to be the most prospective. This programme included an extension of the original ocean bottom cable seismic survey from the Phase 1 area. In December 2005, in order to confirm hydrocarbon presence, its properties and the potential of the segment 4 of the Ridge area reservoir, well 206/8-13Y was drilled 8 km from the previous Clair platform [7]. The data received from the previously monitoring well, confirmed that there is connectivity between Clair Phase 1 and Clair Ridge, thus proving that the Clair Phase 1 area and the Clair Ridge area form one large reservoir because by definition, a reservoir cannot be interconnected to another.
Current State
Currently the Clair Field is in construction of the second phase of development named Clair Ridge; the implementation of this development phase was spurred on by the success of Phase 1 and the increased knowledge base acquired on the reservoir. Phase 2 sees the deployment of two more steel jacket platforms with a combined capacity of 120,000 bpd and an aim to produce 640 million barrels of oil (MMbbl) over a period of 40 years until 2050, with a peak production rate of over 100,000 barrels of oil per day. [1][2]
The Clair Ridge sits north of Clair Phase 1, with a now-estimated 8 billion barrels of oil in place, it still remains the largest undeveloped hydrocarbon resource on the UKCS. In order to target the 640 million barrels of recoverable resources, BP has made a £4.5 billion investment into the construction of these two new bridge-linked platforms. The steel jackets (legs) of the platforms were installed in 2013 during the summer.
The quarters and utilities (QU) topside modules were installed safely in 2015 and construction was completed with the installation of the final topside modules in June 2016 [1]. This new phase also saw new pipelines that allow communication and storage with the facilities in Shetland [9]. The commissioning and hook-up of the platforms are now underway with first oil expected by the end of 2017. [1]
The Clair Ridge will also see the first time BP implements their new enhanced oil recovery technology LoSal; a water-flooding technique which uses low salinity water to increase production from oil reservoirs. The injection of fresh water into a reservoir can cause swelling in some clays, resulting in blockages in the pores that are holding oil. BP observed that a reduction in the water salinity could have a positive influence on pore-scale displacement, and therefore the recovery from the reservoir. [4]
On the 3th of October of 2016, Clair platform suffered a leakage into the sea that lasted less than an hour, of about 700 barrels of oil [11]. According to BP, the appraisal drilling programme in the Clair Field is being continued with the intention of defining a possible third phase of development. [1]