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Field of View pressure wells. “To end up with an 8.5in When we load up the mud to penetrate one of the main challenges with HPHT. hole that deep down you need a 36in the reservoir we have to know exactly “We have gone for a design with two hole at the top,” Urquhart says, hinting at where we are in the geological sequence fully-rated annuli to mitigate against well the scale of these wells. This means any and associated pressure regime.” integrity issues and gas migra- appraisal campaign is going to be costly, tion,” he says.

HPHT heritage therefore has to be has to be well thought Once the gas and conden- through and effective. Maersk Oil has had the beneft sate reaches the wellhead, the

One of the biggest challenges of HPHT of others having already been pressure is quickly choked drilling is the pore pressure and the working in North Sea HPHT for down from around 11,500psi fracture gradient, Urquhart says, result- some years. Total and Shell led to ~90-100 bar. But to manage ing in narrow drilling windows. The pore the way with the Elgin, Franklin this process, and the fact that, pressure on Culzean is high and very and Shearwater felds. Total was as a result of the temperatures

Martin Urquhart close to the fracture gradient. If, during challenged with up to 370°F and downhole, the wellhead, pro- drilling, the mud weights are too highly 16,000psi on Elgin, brought on stream in vided by Cameron on Culzean, has been over-balanced, the reservoir can fracture, 2000, with the neighboring Franklin feld designed to go up 500mm in height from leading to mud-loss and loss of well having similar fgures and Shearwater cold, spring supports have been used, control. To make matters even more inter- having lower pressures but higher tem- this together with fowline design and esting, these central North Sea reservoirs peratures. Shearwater and Franklin also stress analysis ensures all these forces can have complex geological histories, came onstream in 2000. are controlled and dissipated. The pipe making predictability diffcult. Back then, as with today, the operators valves, manifold and pipe connected to “Your drilling window is narrow and worked together on solutions for these the tree are all API 15,000psi – which can that is a feature of most HPHT reser- tricky reservoirs. Maersk Oil’s Culzean be a tough call when there are a limited voirs,” Urquhart says. “Generally, they Well Delivery Manager is also co-chair of number of manufacturers producing are very close to blown traps. What this Oil & Gas UK’s HPHT Workgroup, which certain 15,000psi-rated valves.

means is we have to calculate the weight meets to discuss HPHT well life cycle Having had others come before has of the oil-based mud very carefully. issues and solutions. “The industry meant Maersk has also been able to ben- shares knowledge and experience in this eft from existing supply chain materials area because of the risk profle,” Urquhart qualifcation, at levels slightly higher uHPHT allowance hub means that the area shouldn’t just says. “We have also been able to build than it actually needs, adding its own

Culzean is the frst development to see new developments, but also an on the legacy and experience of previ- specifcation, within industry standards, beneft from a new ultra high-pressure, increase in exploration and appraisal. ous HPHT developments and develop where it needs to. In order to meet some high-temperature (uHPHT) cluster tax “With the introduction of a tax break an extremely robust design, for virgin criteria needed for Culzean specifc allowance on the UK Continental Shelf. for uHPHT felds of the UK, along with pressure drilling and completion and for compositions, hot isostatic pressing (HIP)

The allowance was designed to encour- the simultaneous advances in technol- the later wells depleted drilling.” In the has been used for some components, age the development of uHPHT felds ogy, now is a better time than ever for wells space, the company has also used including the manifold. HIP manufactur- that are economic, but not commercially operators to develop HPHT assets and

Rushmore Reviews, now part of IHS, to ing – frst developed to produce synthetic viable because of the current 62% tax to continue exploration and appraisal in benchmark well design performance. diamonds and nuclear fuel elements in imposed on normal felds.

nearby areas,” she says. “Maybe, since

One of the key learnings has been the 1950s - reduces the porosity of metals “The project may signal a turning point the introduction of the tax allowance, around annuli management, especially and creates a homogenous crystalline for the crisis facing the UK North Sea the UK will see more and more uHPHT following the 2012 Elgin platform leak structure, at the same time reducing where projects continue to be delayed or developments ofshore, and that could (OE: July 2014), which Urquhart says is machining or welding. put on hold,” says Rebecca Edgill, IHS. be what sets the area apart from other “And the allowance including a cluster regions.” jackup rig. This was Maersk Oil’s frst UK

HPHT exploration well. The discovery well found gas and condensate within the target reservoirs of Triassic and Jurassic age. Further appraisal was then carried out during 2009-12, with three, 5km-long wells.

It was always likely to be a HPHT reservoir at that depth, says Martin

Urquhart, project director. “The pres- sure is equivalent to being under 9km of water and the temperature to what you roast your Sunday dinner at,” he says. “Because of the reservoirs we were target- ing, at the depths we were targeting, you know it’s likely going to be HPHT.”

A Maersk Oil artist’s impression

The appraisal campaign had to be eff- of the Culzean facilities.

cient, due to the high costs involved in drilling such deep, high-temperature and oedigital.com December 2015 | OE 19 018_OE1215_field of View.indd 19 11/21/15 12:33 AM