Constructing Mariner

The Mariner heavy oil discovery on the East Shetland Platform in UK block 9/11 will be one of the largest offshore developments on the UK Continental Shelf in more than a decade. Meg Chesshyre checks on its progress.

Chris Andrew (left) vice president for asset management for Statoil in Aberdeen and Gunnar Breivik (right) MD for Statoil production UK and head of the Aberdeen office, in front of the Mariner jacket under construction at Dragados Offshore yard in Cadiz, Spain. Images from Statoil.

Construction for Mariner, one of the largest recent UK North Sea offshore developments, is in full flow. Some four different yards are all producing different elements of what will become the Mariner facilities, sit­ting in license 335, on the East Shetland Platform of the UK North Sea, approxi­mately 150km east of the Shetland Isles.

The project is on schedule for first oil in 2017, and is expected to be in pro­duction for more than 30 years. Mariner was first identified on 2D seismic data acquired in 1977, which led to Union Oil picking up the license in 1980, and drilling the 9/11-1 well in 1981, discover­ing oil in both the Heimdal and Maureen sand members.

Since Mariner was first discovered, the field has been subject to a number of development studies and concepts by the different operators.

Since the P335 licence was awarded (1980) and the 9/11-1 discovery, five seis­mic surveys have been acquired, a further 18 wells have been drilled and the license has seen four changes in operator. The license equity history lists 12 companies that have been involved in the project with up to three partners at any one time. Statoil assumed operatorship from Chevron in 2007, with partners JX Nippon joining the group in 2012 and Dyas in 2013.

Statoil is the first company to put forward a development concept that addresses the complexities of the field, in particular imaging the Heimdal reservoir, reservoir management, recovery rates and project execution.

The field develop­ment concept includes a production, drilling and living quarters (PDQ) platform with a floating storage unit, together with a jackup rig assist­ing initially. In total this means a gross invest­ment of more than US$7 billion for the project. The development will contribute to more than 250 MMbbl reserves and provide a long term cash flow over a 30-year field life. Improved oil recovery together with additional near field volumes, for example Mariner East and near field exploration, aim to extend the development beyond 30 years.

The 22,400-tonne Mariner jacket under construction at Dragados Offshore’s yard in Cadiz, Spain.

Construction

Schematic of Mariner PDQ and FSU.

The load-out, transportation and instal­lation of the 22,400-tonne jacket from Dragados’ yard in Cadiz, Spain, in May/ June will be one of the major milestones this year. Subsea, umbilicals, risers and flowlines installation will also take place this year.

Last summer, first steel was cut on the floating storage unit at Samsung and on the 36,000-tonne platform topsides at Daewoo Shipbuilding & Marine Engineering in Korea. Sailaway and installation of the topsides is planned in 2016. The Cat J jackup, designed to Statoil’s specifications, to be owned and operated by Noble, is under construction at Sembcorp Marine’s Jurong yard in Singapore.

Tackling Heimdal

The main, deeper (at about 1492m) Maureen reservoir is currently planned to have 17 producers and five water injectors, with first oil from the Maureen reservoir in 2017. The Maureen reservoir has 14º API gravity oil and an oil viscos­ity of 65 cP, with excellent reservoir properties. The reservoir itself is very well understood; it has a clear seismic response, can be correlated between wells and has good core recovery.

Schematic of Statoil’s Cat J design rig currently under construction at Sembcorp Marine’s Jurong yard in Singapore for use on Mariner.

However, the shallower (1227m) Heimdal reservoir has long been con­sidered the main challenge at Mariner and could, at least until recently, not be mapped seismically. The challenging nature of vintage seismic led towards a stochastic reservoir model. The Heimdal has 12º API gravity oil and an oil viscosity of over 500 cP, again with excellent reservoir properties. The cur­rent development strategy based on the stochastic model is pattern drilling using an inverted nine-spot pattern, which is planned to have 39 dual multi-lateral production wells and 32 water injectors. First oil from the Heimdal reservoir will follow on from the Maureen plateau to optimize plateau production.

In 2012, a full field broadband seismic survey was acquired, with final processed products were available in 1Q 2014. This applies the newest advances in acquisition and processing technology to obtain better imaging of the Heimdal sands. There has subsequently been a focus on integrating the new seismic data into a more deter­ministic reservoir model. A more deter­ministic model would greatly improve well planning, geo-steering and therefore optimize the Heimdal development, mov­ing away from pattern drilling towards more target driven drilling. The improved image of the Heimdal sands and their distribution across the Mariner licenses on the new seismic data will ultimately be tested when drilling of the Maureen production wells starts in 2017.

130-well program

Over the field’s lifetime, as many as 130 well targets are planned. This will be a major factory, deliver­ing one well per month in the first years of operation. Submersible pumps will be needed to bring the oil to the surface. Statoil’s experi­ence from its own heavy oil projects, like Grane on the Norwegian Continental Shelf and Peregrino in Brazil, has made a significant contri­bution here. The develop­ment of directional drilling, long horizontal wells and multi-laterals was crucial for Mariner’s development, allowing increased exposure to the reservoir and the ability to reach more well targets. This technology was not available when Mariner was first discovered.

Drilling is planned to start in 2016. Odfjell has the platform drilling contract. The main Mariner platform will include a drilling rig and a well intervention and completion unit. The intervention and completion unit is an innovation that will allow Statoil to run completions and set submersible pumps without using the drilling rig. The wells will have sophis­ticated valves that automatically shut out reservoir zones with high water cut. In addition, the newbuild jackup rig will be located next to the Mariner installation, working through well slots on the platform for the first 4-5 years.

A contract for integrated drilling and well services on the Mariner field was awarded to Schlumberger Oilfield UK in December 2014. A total 22 drilling and well services are included in the scope, including a logistics support responsibility that goes beyond the normal scope for similar Statoil contracts. The contract started in January 2015 and runs for four years, with options to be extended several times for further four-year periods. The contract with Schlumberger Oilfield also includes options for Bressay, another Statoil oper­ated field on the UKCS, currently in the concept evaluation phase.

“Mariner is by all accounts a large and challenging project,” says Gunnar Breivik, managing director of Statoil Production UK and head of the Aberdeen office. “When Mariner starts producing in 2017, it will be 40 years after the first seismic that identified the structure. The discovery well was drilled already in 1981. Several operators have worked hard on this project before us. We are proud to be the operator able to bring this field on stream, but we are building on decades of good work by many others.”

Statoil is the operator of the Mariner field with 65.11% equity. Co-venturers are JX Nippon Exploration and Production (UK) (28.89%) and Dyas (6%).