Production from subsea wells is growing in importance in many operator portfolios. As easily accessible oil is depleted, subsea processing offers a compelling solution to economically produce in a variety of challenging conditions. Worldwide, there are more than 4000 subsea wells onstream today and FMC projects that this number will grow to nearly 7000 by 2017. In fact, analysts expect the number of subsea wells coming onstream between 2013 and 2017 to increase by 68% over the previous five-year period.
In 2007, the Tordis SSBI was installed in 650ft of water.For operators, extracting as much of the original oil-in-place as possible is critical.
While conventional oilfields report typical recovery rates between 30-60%, subsea wells often fall below that mark, reporting recovery rates less than 40%. Just a 1% increase in subsea recovery can deliver an additional 10mmbo of production from a typical large field. This additional production can dramatically affect project economics, improving the viability of greenfield projects or ensuring that brownfield installations continue operating profitably.
For greenfield applications such as long distance step-outs in deepwater, in the Arctic or challenging reservoirs with lowenergy or heavy oil, the increased production rates and recovery can be moving forward. The Arctic holds more than 90 billion bbl of undiscovered oil.
For brownfields, increased oil recovery (IOR) can be critical to keep the field operating economically despite depleting reservoir energy and increasing water cut.
To improve recovery rates, operators often turn to IOR techniques. These are typically deployed during, or after, primary depletion and can be considered basic reservoir management techniques to improve sweep and accelerate production from the reservoir. These techniques often include:
To achieve these results, a number of technologies are required:
Subsea separation has been effective in delivering more efficient liquid boosting and cost-efficient hydrate management, and is a potential enabling technology for longer step-outs where cold-flow concepts can be employed.
Globally, five subsea separation systems are currently installed. Separation systems for two-phase (liquid/gas), three-phase (oil/water/ gas), and four-phase (oil/water/ gas/sand) separation employ a variety of separator types including horizontal, vertical, and caisson separators.
Operators have been using seabed pumps for many years to reduce back pressure on the reservoir; this increases flow rates and total recoverable reserves. Whether the pumps are used for boosting or injection, they are widely believed to be the most mature subsea processing solution. A variety of pump types, including rotordynamic, positive displacement, and centrifugal, have been used in different applications. The most powerful subsea, multiphase pumps available today provide more than 3MW of boosting power at the seabed.
Statoil expects to increase ultimate oil recovery at the Tordis field from 49% to 55% of original oil-in-place, resulting in an additional 35mmbo of oil.
Subsea gas compression represents a major technological leap for the E&P industry. Gas compression offers a number of advantages as it enables development of gas fields without the need for topside facilities, manages flow assurance issues such as slugging, and contributes to improved production and recovery. The solution requires a system that includes compressors, pumps, scrubbers, coolers, and controls.
Using compressors upstream of LNG process facilities adds value through delay of production decline, due to poor turndown capability of LNG processes. Statoil will be the first operator to deploy this technology to increase recovery rates from its Asgard field in the North Sea with anticipated start up in 2015.
Today, subsea compression is being used for long-offset subseato- beach situations, where flow rates are high and the differential pressure is sufficient to overcome the friction losses in the transport pipeline. The use of gas compression could be broadened to include lower rates and sufficient pressure rise to enable gas injection.
Additionally, future technology developments are required for power distribution, instrumentation, automation, and control. These advancements are critical to make IOR possible in more remote and difficult to access environments such as the Arctic, where ice can cover the surface for much of the year.
The Tordis seperator is designed to handle almost 190,000b/d of liquids operating at around 500psi. It can remove as much as 100,000b/d of water to a standard of less than 1000ppm of oil.
Increased recovery in greenfield
FMC Technologies has been closely involved with gas/liquid separation and boosting since 2005 when several projects were in their appraise-to-select phase. Today, three projects use gas liquid separation and pumping, including Shell’s Perdido and Parques de Conchas (BC-10), and Total’s Pazflor.
Gas/liquid separation projects aim to achieve a number of advantages over multiphase pumping alone. The primary advantage of deploying a separation and pump system is the improved hydraulic efficiency of the pump. Additional benefits include increased pressure rise through the pump and improved flow assurance.
Application of gas/liquid separation to a full field development from day one can be an effective solution because of impressive economic benefits. Many of the competing solutions such as multiphase pumping and gas-lift may not deliver the rate and recovery offered by the processing option. It is well known that gas/ liquid separation offers the lowest possible back pressure on the reservoir. When coupled with the great operational flexibility (ie ability to manage changes in production index, gas-oil ratio and water cut), and the system’s ability to mitigate hydrate and slugging issues, the application of gas/ liquid separation and pumping to a full-field solution delivers many operational benefits.
Increased brownfield recovery
FMC Technologies supplied the world’s first full-scale commercial subsea separation, boosting, and injection system (SSBI) to Statoil in 2007 for its Tordis IOR project. As the key component, the subsea separation, boosting, and injection system provides Statoil with a cost effective means to increase ultimate oil recovery to 55%, adding more than 35mmbo reserves.
North Sea oil output now depends heavily on IOR techniques. Projects typically must overcome challenges involving diminished reservoir pressure and large amounts of water in the production stream. These challenges have been particularly difficult at Tordis. Without adequate reservoir pressure, the oil and gas cannot flow from the subsea wells to the surface processing facility. Large water volumes reduce overall production efficiency and may exceed processing facility waterhandling capabilities. When these problems cannot be overcome at a reasonable cost, fields can be abandoned.
The subsea separation, boosting and injection system gave Statoil the solution needed to overcome these challenges at Tordis, where the water depth is 650ft. The separator at the SSBI station is designed to handle almost 190,000b/d of liquids operating at around 500psi. It can remove as much as 100,000b/d of water to a rated cleanliness standard of less than 1000ppm of oil. A single-phase subsea pump boosts the water to 1900psi for injection into a low pressure aquifer.
As much as 35mmcf/d of gas are separated from the oil and water, most of it by a cyclone at the separator inlet. Incorporating the cyclone minimizes the size of the separator vessel, facilitating its ability to be installed or retrieved to the surface. The gas is routed through bypass piping and then recombined with the de-watered oil. An electric-powered, multiphase pump boosts the oil and gas mixture to 1000psi for the 16-mile (25.7km) trip to the Gullfaks field for processing. The separator is also designed to remove up to 1100lb/ day of sand from the separator. This is mixed with the separated water for injection downstream of the injection pump.
The subsea separation, boosting, and injection system enhances production by reducing back pressure on the Tordis wells and thereby facilitating fluid flow from the reservoir. Accelerated production from Tordis results in too much water for the production facilities at Gullfaks C to manage. Separation and disposal of the water subsea enables an increased production rate.
Increased oil recovery from subsea fields represents a huge opportunity to enable production from greenfield developments and to ensure that brownfields can continue to operate profitably. To improve recovery rates in subsea fields, a number of key technologies are required in the areas of subsea separation, pumping and compression. Using these technologies will enable operators to increase recovery by 3-10% of original hydrocarbons in place and can extend a field’s productive life by several years. OE
Chris Shaw is presently the field development manager – IOR Systems for FMC Technologies in Houston, Texas. Shaw joined FMC in 2007 to lead subsea processing technology commercialization initiatives in the western region.