During OTC, the company confirmed that the unit made by FMC Technologies is running in Brazil’s Marlim field. Work is still needed to scale up this pilot project and other devices under evaluation that are designed to handle a significant part of the output from the aging but still prolific Marlim. Based on a Petrobras paper (OTC 24161) delivered at the conference, the goal is to have a subsea processing unit ready for field use 3 years from today.
For now, the FMC system is separating the relatively heavy crude from the water, which makes up about 70% of the flow from a single well. The technology uses a pipe separator to split the oil and water at a depth of nearly 3,000 ft.
Industry interest in subsea oil/water separation is strong because the process could extend the life of fields where rising water cuts increase the energy needed to lift the crude to the surface and require costly upgrades to water processing facilities. Often the cost of dealing with these problems is prohibitively high in aging fields.
Oil/water separation is one of many efforts by Petrobras to expand what it can do subsea. Technical sessions at OTC also featured one program to use multiple electric submersible pumps for cost-effective multiphase pumping over long distances, and a second seeking to create a subsea gas compression system able to inject sour gas into reservoirs in water more than 8,000 ft deep.
A presentation about the water/oil separation program by Fabio Alves Albuquerque, an equipment engineer at Petrobras, described the current stage of the subsea water processing program as “technical studies.” The company is also considering alternatives, such as a unit developed by Cameron using electrostatic separation to remove the oil from the water. Petrobras will need a compact system, both in terms of the size of the unit that must be handled by offshore construction vessels and its output per pound. As depths increase, weight becomes a greater challenge because rising pressure levels demand ever-thicker steel pipe.
Engineering challenges extend beyond the processing units. Albuquerque said a big problem is “shearing in the subsea production chokes” in the manifold, which can homogenize the oil/water mix, making it harder to separate. A simplified explanation for how shearing can affect an oil/water mix is: “It is really easy to turn heavy oil into mayonnaise,” said John Byeseda, a senior principal engineer at OneSubsea, a joint venture recently formed between Schlumberger and Cameron.
On its first test, Petrobras avoided a choke by running the flow from a single well through the FMC unit. Its goal is a unit processing a stream of oil from multiple wells that would need to flow through a manifold.
On Petrobras’ list of technology gaps to fill is a valve design that does not cause shearing. It has tested two alternatives, including one developed in Brazil, which the OTC paper said showed “better performance” compared with conventional alternatives. When Albuquerque was asked by an FMC representative whether his company unit was working well, the Petrobras engineer said, “In my opinion, the contribution of the choke to shearing is enormous.”
