Subsea separation system a key technology for TORDIS IOR project
FMC supplied the world’s first full-scale, commercial subsea separation, boosting, and injection system (SSBI) to Statoil in 2007 for its Tordis field Improved Oil Recovery (IOR) project. As the key IOR component, the subsea separation, boosting and injection system has the potential to provide Statoil with a cost-effective means to increase ultimate oil recovery from Tordis from 49 percent to 55 percent of oil originally in place, a gain of some 19 million barrels.
IOR project challenges
North Sea oil output now depends heavily on IOR techniques. These projects typically must overcome challenges involving diminished reservoir pressure and large amounts of water in the production stream. These challenges have been particularly difficult for Tordis. Without sufficient 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 water-handling capabilities. When these problems cannot be overcome at a reasonable cost, the field will be abandoned.
Subsea separation, boosting and injection provides the solution
FMC’s subsea separation, boosting and injection system gave Statoil the solution it needed to meet these challenges at Tordis, where the water depth is 650 feet. The separator at the subsea separation, boosting and injection system station is designed to handle almost 190,000 barrels per day (bpd) of liquids operating at around 500 psi. It can remove as much as 100,000 bpd of water to a cleanliness standard of less than 1,000 ppm of oil. A standard electrical motor subsea pump boosts the water to 1,900 psi for injection into a low pressure aquifer.
As much as 35 million cubic feet per day 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 retrieval to the surface. The gas is routed through bypass piping and then recombined with the dewatered oil. An electric powered multiphase pump boosts the oil and gas mixture to 1, 000 psi for the 16-mile trip to the Gullfaks field for processing. The separator is also designed to remove up to 1,100 pounds of sand per day from the separator. This is mixed with the separated water for injection downstream of the injection pump.
Multiple benefits and options
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 Gulfaks C to manage. Separation and disposal of the water subsea enables the increase in production rate to be realized.
Subsea separation and boosting with injection of the removed water comprise a technology package that gives offshore operators a rich set of options and great flexibility for both IOR projects and new field developments.