Taming Culzean

Mastering high-pressure, high-temperature reservoirs has been a UK North Sea challenge for more than a decade. Maersk Oil is taking lessons learned by others and applying them to the Culzean field.

The Culzean wellhead jacket under fabrication. Images from Maersk Oil.

With few major upstream projects reaching final investment decision in the current climate, Maersk Oil’s Culzean is one of the UK North Sea’s bright sparks.

Approved in August 2015, the US$4.7 billion, 270 MMboe plus project is not just a significant project, signaling contracts and jobs, it is also complex, with high-pressure and high-temperature (HPHT) reservoirs, offering challenges around materials selection, drilling, well completion design, and field drainage philosophy, to name but a few.

Maersk Oil’s development plan for Culzean, which could provide about 5% of the UK’s total gas consumption by 2020-21, is a new three-platform installation, consisting a 12-slot wellhead platform, central processing facility and a utilities and living quarters platform, construction on which is now under way.

The reservoir, in about 90m water depth, is approximately 4300m below sea level, reaching some 348°F or 176°C, and around 13,500psi, some 240km from Aberdeen. It is not far from the Elgin, Franklin and Shearwater HPHT fields, operated by Total and Shell, respectively, in the East Central Graben area of the central North Sea.

Maersk expects first gas in 2019, and plateau production to be in the range of 300-450 MMscf/d and 25,000 b/d condensate.

Beginnings

Although the area was tested in 1986, Culzean itself was discovered in late 2008, with discovery well (22/25a-9z), in Block 22/25, using the Ensco 101 heavy duty jackup rig. This was Maersk Oil’s first 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 pressure 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 targeting, at the depths we were targeting, you know it’s likely going to be HPHT.”

Martin Urquhart

The appraisal campaign had to be efficient, due to the high costs involved in drilling such deep, high-temperature and pressure wells. “To end up with an 8.5in hole that deep down you need a 36in hole at the top,” Urquhart says, hinting at the scale of these wells. This means any appraisal campaign is going to be costly, therefore has to be has to be well thought through and effective.

One of the biggest challenges of HPHT drilling is the pore pressure and the fracture gradient, Urquhart says, resulting in narrow drilling windows. The pore pressure on Culzean is high and very close to the fracture gradient. If, during drilling, the mud weights are too highly over-balanced, the reservoir can fracture, leading to mud-loss and loss of well control. To make matters even more interesting, these central North Sea reservoirs can have complex geological histories, making predictability difficult.

“Your drilling window is narrow and that is a feature of most HPHT reservoirs,” Urquhart says. “Generally, they are very close to blown traps. What this means is we have to calculate the weight of the oil-based mud very carefully. When we load up the mud to penetrate the reservoir we have to know exactly where we are in the geological sequence and associated pressure regime.”

A Maersk Oil artist’s impression of the Culzean facilities.

HPHT heritage

Maersk Oil has had the benefit of others having already been working in North Sea HPHT for some years. Total and Shell led the way with the Elgin, Franklin and Shearwater fields. Total was challenged with up to 370°F and 16,000psi on Elgin, brought on stream in 2000, with the neighboring Franklin field having similar figures and Shearwater having lower pressures but higher temperatures. Shearwater and Franklin also came onstream in 2000.

Back then, as with today, the operators worked together on solutions for these tricky reservoirs. Maersk Oil’s Culzean Well Delivery Manager is also co-chair of Oil & Gas UK’s HPHT Workgroup, which meets to discuss HPHT well life cycle issues and solutions. “The industry shares knowledge and experience in this area because of the risk profile,” Urquhart says. “We have also been able to build on the legacy and experience of previous HPHT developments and develop an extremely robust design, for virgin pressure drilling and completion and for the later wells depleted drilling.” In the wells space, the company has also used Rushmore Reviews, now part of IHS, to benchmark well design performance.

One of the key learnings has been around annuli management, especially following the 2012 Elgin platform leak (OE: July 2014), which Urquhart says is one of the main challenges with HPHT. “We have gone for a design with two fully-rated annuli to mitigate against well integrity issues and gas migration,” he says.

Once the gas and condensate reaches the wellhead, the pressure is quickly choked down from around 11,500psi to ~90-100 bar. But to manage this process, and the fact that, as a result of the temperatures downhole, the wellhead, provided by Cameron on Culzean, has been designed to go up 500mm in height from cold, spring supports have been used, this together with flowline design and stress analysis ensures all these forces are controlled and dissipated. The pipe valves, manifold and pipe connected to the tree are all API 15,000psi – which can be a tough call when there are a limited number of manufacturers producing certain 15,000psi-rated valves.

Having had others come before has meant Maersk has also been able to benefit from existing supply chain materials qualification, at levels slightly higher than it actually needs, adding its own specification, within industry standards, where it needs to. In order to meet some criteria needed for Culzean specific compositions, hot isostatic pressing (HIP) has been used for some components, including the manifold. HIP manufacturing – first developed to produce synthetic diamonds and nuclear fuel elements in the 1950s - reduces the porosity of metals and creates a homogenous crystalline structure, at the same time reducing machining or welding.

“Given the challenges of welding such heavy wall material we have elected, where possible, to make sections utilizing the HIP process and connecting them together with Grayloc 15,000psi-rated hubs,” Urquhart says, “This eliminates the need to weld completely.”

Fully rated

The Culzean wellhead jacket under fabrication.

While the pressure of the production stream at surface is dropped at the choke, the entire wellhead platform has been fully rated to 15,000psi, with an over pressure system, comprising four, fast-acting 15,000psi gate valves from Bel Valves, each weighing 6-tonne, compared to just 500kg for an equivalent normal pressure, normal temperature valve. By taking the decision to have three separate facilities linked by bridges, Maersk has also been able to achieve a temporary refuge impairment frequency of zero on the utilities and living quarters platform.

Exotic materials, primarily corrosion resistant alloys and titanium are prevalent throughout the process piping systems, separators, coolers, pumps and valves due to the nature of the Culzean gas and fluid properties and the temperature and pressure that the systems are required to operate at.

Maersk also has another challenge – dealing with some 12% wax in the condensate and the associated flow assurance issues was the reason why the field needed a floating storage and offloading vessel (FSO) instead of being able to produce as a tie-in to existing liquid export infrastructure.

Japan’s MODEC is building the floating storage and offloading vessel for the project (the company’s first UK North Sea project), at Sembcorp Marine’s Tuas and Tanjong Kling facilities. It will have 350,000 bbl storage capacity, and an internal turret system supplied by MODEC subsidiary SOFEC.

“This small rate of waxy condensate has a big impact on the design of our liquid process facilities,” Urquhart says. “We have four stages of separation to get it to a point where it can be exported to the tanker. Because the separation and stabilization process takes the pressure so low we then have off gas streams, which need to be compressed all the way back up to a much higher pressure in order to export into the CATS transmission system.”

Maersk also wants to minimize manning on the facility, by using an advanced collaborative environment onshore with real time data from the process and safety equipment, via fiber optic link. The living quarters has room for 140 people, but only half of that will be used in the early years, until later in field life when the balance is taken up by well intervention teams.

Future proofing

Due to the way HPHT fields deplete, the drainage strategy is also important on Culzean. Maersk is pre-drilling three of the six development wells, which will come on stream sequentially at first gas effectively before virgin pressure drops. “We want to put as many wells in the ground ready to produce as the challenges of depletion drilling after production have to be factored in to the development,” Urquhart says. Maersk will be using a newbuild heavy duty, JU-2000E jackup rig supplied by Hercules Offshore.

As well as accounting for the extra people on board needed for future intervention work, the facility has also been designed to take on intervention equipment. “Mid-life, most HPHT fields need some kind of intervention, such as scale squeeze or downhole safety valves checks,” Urquhart says. “Once the drilling rig has gone, the top deck is designed to have a heavy duty coil well intervention spread so that once the reservoir comes down to 10,000psi, we can run heavy duty well interventions.”

Construction underway

With first production planned for 2019, most of the key contracts have been awarded. Sembcorp Marine subsidiary SMOE is building the development’s three topsides, the utilities and living quarters platform, the central processing facility including flare, the wellhead platform and finally the two connecting bridges.

The wellhead platform jacket and heavy duty jackup are on schedule for installation in Spring/Summer 2016, to start drilling the Culzean development wells. The jacket is being fabricated by Heerema in Vlissingen, the well access deck is being completed by Heerema Hartlepool. The heavy duty jackup will be on contract from Hercules Offshore, which is fabricating a new-build with Sembcorp Marine at their Tanjong Kling facility in Singapore.

Late October, the three topsides were in the kick off and ramp up phase, with Sembcorp Marine subsidiary SMOE. These will be assembled in the Admiralty yard facility in Singapore. SMOE have subcontracted the power generation module, bridges and flare to their UK facility at Lowestoft.

The two remaining jackets have been awarded to Heerema and they will be fabricated in Vlissingen with six pile clusters manufactured in Hartlepool.

Going to press, KBR was awarded a detailed design and engineering and follow-on services contract by SMOE for the topsides facilities on Culzean. Specifically, KBR will deliver the living quarters, central processing facility and wellhead decks for the three Culzean platforms.

uHPHT allowance

Culzean is the first development to benefit from a new ultra high-pressure, high-temperature (uHPHT) cluster tax allowance on the UK Continental Shelf. The allowance was designed to encourage the development of uHPHT fields that are economic, but not commercially viable because of the current 62% tax imposed on normal fields.

“The project may signal a turning point for the crisis facing the UK North Sea where projects continue to be delayed or put on hold,” says Rebecca Edgill, IHS. “And the allowance including a cluster hub means that the area shouldn’t just see new developments, but also an increase in exploration and appraisal.

“With the introduction of a tax break for uHPHT fields off the UK, along with the simultaneous advances in technology, now is a better time than ever for operators to develop HPHT assets and to continue exploration and appraisal in nearby areas,” she says. “Maybe, since the introduction of the tax allowance, the UK will see more and more uHPHT developments offshore, and that could be what sets the area apart from other regions.”