BP has been making strides in LowSal enhanced oil recovery techniques and Bright Water. It now has its sights set on polymerized water for its Quad 204 development. Elaine Maslin found out more.
Onshore, enhanced oil recovery (EOR) using polymers is an established practice. Offshore it is a different ball game and, until now, not quite mastered by the industry. But, after a string of projects, mostly pilots, operators are getting to grips with offshore polymer EOR.
EOR technologies are attractive because they can help operators increase their reserves – without having to spin the exploration bit. According to France’s Total, global conventional recovery rates are about 32% of oil in place; improving this performance by 5% could add some 300 billion of additional reserves.
Polymer EOR is like water flood, but it involves increasing the viscosity of the water using polymers to stabilize the flood front and help divert water from high-permeability zones (potentially already swept) to improve sweep efficiency and aid additional oil recovery. However, it is also expensive and typically most cost-effective on more viscous oilfields or, where oil in place is large, on moderately viscous fields.
To date, polymer use for EOR offshore has been limited to a handful of projects. Since 2003, CNOOC has been using polymer from platforms on its heavy oil fields in Bohai Bay, offshore China, first as a pilot and then extending it across 27 wells on three oilfields by 2010*. Total was first to take polymers on a floating production, storage and offloading (FPSO) vessel deep offshore with its polymer EOR pilot on the Camelia field offshore Angola in 2010-11, with a skid-mounted injection pilot on the deck of the Dalia FPSO. More recently, Chevron, in the UK North Sea, has been trialling polymer EOR on the Captain heavy oil field, a project which has led to plans for a new bridge-linked platform on which it can store, mix and pump polymer.
Others have been considering offshore schemes, with Statoil working on pilots at Heidrun, Norway, and Peregrino, Brazil. BP now looks set to join the polymer party.
Further to its multi-billion pound redevelopment of the Schiehallion and Loyal fields, known as the Quad 204 project, West of Shetland, BP has been considering introducing polymer EOR. The Quad 204 project involves a newbuild FPSO, the 270m-long Glen Lyon, which will become the North Sea’s largest unit once operational, onto which BP is planning to pre-invest in key polymer injection facilities. If the subsequent polymer project goes ahead, it will be the first deepwater subsea polymer EOR project.
BP’s Glen Lyon FPSO taking shape in South Korea. Photo from BP.
Quad 204 is 175km West of Shetland. Schiehallion was discovered in 1993, and Loyal a year later, with production starting in 1998 from the Schiehallion FPSO, which the Glen Lyon replaces. Schiehallion and Loyal together had more than a billion barrels in place and recovery to date had been about 15%.
Recognizing the greater potential of the area – BP has produced ~400 MMbo to date and thinks there’s a similar amount to be had – and both the falling efficiency and limitations of the original Schiehallion FPSO, the operator, with partners Shell and OMV, decided to redevelop the field through the Quad 204 project, investing in the new, higher capacity Glen Lyon FPSO, subsea infrastructure renewal, a seven-year drilling program, with at least 20 new wells planned, as well as pre-investment in key polymer EOR facilities. As a result, it hopes it can maximize recovery to above 30%.
Key to the project has been improving liquids handling capability. The Glen Lyon, due onstream in 2016, will have 320,000 b/d liquids capacity. It will be needed. Some 380,000 b/d of water injection capacity will be installed to sweep the field and maintain pressure, with the vessel having a capacity to produce 130,000 bo/d.
For Quad 204, which had been under a water flood scheme since 1998 (with 50%+ water cut in places), a large part of the benefit of polymer is that, through reducing the mobility of the water, less water will be produced, freeing up valuable plant capacity for more infill wells or tieback opportunities.
“When you inject regular water in to the field, the water fingers through the reservoir and leaves oil that has not been swept, which can get left behind,” says Scott Thomson, area subsurface manager, BP. “If you use polymer, instead of creating fingers, the polymerized water moves in a more piston-like and efficient manner. That is the traditional industry application of polymer flood.
“Quad 204 is a bit different,” Thomson says. “Our new wells will benefit from the traditional mechanism of piston-like sweep, but because we have already produced several millions of barrels from the field and we have been injecting water for some time, there are huge chunks of the field where the water has already broken through at the producers.
“We are less interested in delaying water breakthrough in these areas and more interested in capturing some of the oil that has been left behind via the mechanism of viscous cross flow,” he says. Viscous cross flow happens when pressure gradients created by the polymer-flood force water into parts of the reservoir that were previously bypassed by fingering.
Using polymer will also have another, significant, beneficial effect. Despite being set to be the biggest FPSO in the North Sea, the Glen Lyon will still have limitations, i.e. there’s only so much water you can produce. This would mean that production has to be choked back. “We want to reduce the fractional flow of water into our facility and polymer helps us do this,” Thomson says. “The polymer effectively slows down the water in the reservoir and means the water cut is reduced at the surface. With less water coming to the surface, we can open up more wells and accelerate production.”
Powder or emulsion?
An intense subsea campaign is ongoing West of Shetland, ahead of the FPSO’s arrival. Photo by chrisallanphotohraphy.com.
Once a decision has been made to use polymer, the next step is looking at how it is deployed, including in what form it is supplied, and overcoming the challenges around deploying it.
There are two ways polymer can be supplied, in powder form and, more recently, as an emulsion. In most onshore deployments, polymer is supplied in a powder form, which requires a mixing process before it is injected downhole. The risk with having the polymer supplied as powder, especially offshore, is inefficient powder wetting, creating inconsistencies in the mix, which can adversely impact well injectivity.
As an emulsion, the polymer molecular chain is delivered coiled up within a water droplet, within an oil main phase. Water is then added to the emulsion, which allows the water droplets to join, enabling the polymer chains to unfurl and link, causing the water to viscosify. This is called “inverting” the emulsion. “We believe the emulsion will be easier for us to handle and to prepare for injection in the relatively harsh environment, deepwater, West of Shetland,” Thomson says.
The next challenge is deployment. Many large offshore fields that could be suitable for polymer EOR may be viewed as having limited application because they have been developed using subsea wells. Offshore, because the polymer is injected from topsides, it has to go through various restrictions, such as the choke, as it flows into the wells. Onshore, injection could be downstream of the choke, which can be prohibitively expensive for subsea wells.
“Every restriction poses an opportunity to damage the polymer,” Thomson says. “The polymer unfurls in the dilution process to the specified viscosity. If the polymer chain is then stretched and sheared as it passes through a restriction, it loses some of that viscosity. You could overdose the polymerized water, but all the wells are choked at different rates, so the polymer will shear differently in each well. We are looking at ways to get the target viscosity to the reservoir without shearing the polymer. We have ideas how we can overcome these problems and improve commercial performance.”
BP is also working with vendors to test polymers for this application. Getting it right could be a game changer for the vendors, as well as BP, Thomson says. A successful EOR project in the Schiehallion and Loyal fields will create a large demand for polymer, the production of which may require new manufacturing facilities. This in turn would make polymer for EOR more widely available to the broader industry.
Although the Quad EOR project is still only in concept development with several technical and commercial hurdles to overcome, BP and its partners are working hard to make polymer deployment in the North Sea a reality.
*SPE paper SPE-144932-MS, presented at the SPE Enhanced Oil Recovery Conference, Kuala Lumpur, 2011.