Petrobras takes novel subsea approach to boost reservoir productivity

Brazilian energy giant Petrobras has taken a well-known onshore pumping technology to the bottom of the sea to boost oil production in deepwater fields. The company's seafloor electrical submersible pump (ESP) without preseparation is a world first, and promises to more than double the oil production in some fields.

Geraldo Spinelli, Flow and Artificial Lift corporate manager for Petrobras, recently discussed his company's rationale for developing subsea-suitable centrifuge-pumping systems, and the progress made to date.

"The technological challenge is simple to describe: to operate a pump in a place where it is not supposed to be, at the seafloor. It is far more difficult to actually accomplish this in reality.

"Centrifuge-pumping systems and other types of ESPs are not new, of course, and have been used widely inside the well onshore and in offshore systems with dry completions," Spinelli continued. "They are very effective there, and in the event that they fail and need to be replaced, they are relatively cheap to replace."

Although there are some ESP applications in wells with wet Christmas trees, in subsea completions, a failed pump can be a logistics and production nightmare. "The intervention costs associated with a failed subsea centrifuge pump are large," Spinelli continued, "because you have to bring in a very expensive rig, and these are usually not readily available. Not only are you forced to pay for a replacement pump and high rig rates, while you are waiting for a rig, you are not producing."

Deepwater fields provide first testing grounds

Petrobras began examining the possibility of using ESPs along with subsea completions in the early 1990s, in large part because of the increasing number of deepwater finds it was making offshore Brazil. "The theory was that if ESPs could be made subsea-compatible, they would make these new fields commercially viable," Spinelli said.

Petrobras partnered with several vendors in the 1990s, most notably Schlumberger's REDA and later with Baker Centrilift, to develop ESPs for wells with subsea Christmas trees. The first field trial for this technology took place in the mid 90s in the shallow-water Carapeba field of the resource-rich Campos basin (which accounts for 80% of Brazil's output). "This marked the first ESP installed in a wet completion system," Spinelli confirmed, "and there was a great deal of planning and preinstallation testing involved to ensure as long a life as possible for this pump." This included stack-up tests, used to ensure that the pump and motor were well conditioned and ready for installation.

"We had some positive results with this pump in the Carapeba field, and with those data in hand, we took the next step of placing an ESP in a deeper well, in the deeper waters of the Campos basin's Albacora field." Spinelli explained that this ESP worked successfully downhole in a wet completion well for more than 3 years without issue. "When we shut down the well after 3 years, it was because of reservoir constraints, not a problem with the pump. The gas/oil ratio was increasing too quickly, most likely due to some gas coning taking place. While the ESP was able to bear relatively large amounts of gas, we did see a slight drop in pumping efficiency. But we turned off the pump for reservoir strategy reasons."

The first Petrobras application of an ESP outside of the production well was the Vertical Annular Separation and Pumping System (VASPS), which was developed in a joint effort with Petrobras, Eni-Agip, and ExxonMobil. The first VASPS prototype was installed in 2001 in the Marimba field, on the seafloor and at a depth of 395 m. In this application, the liquid production stream was separated from the gas and pumped through the ESP to the platform, while the separated gas was ventilated to the same platform. This boosted the oil production to approximately 1,000 m³/d, up from the 800 m³/d possible with gas lift.

Heavy-oil fields provide next challenge

At the beginning of 2000, Petrobras faced a new challenge in that most of the new fields being discovered were not only in deepwater, but also contained heavy-oil reserves (17ºAPI or less). In light of this, and the fact that Brazil has a keen interest in maintaining its oil self-sufficiency, the Heavy Oil Technological Program (PROPES) was initiated. PROPES has focused much research around developing subsea ESP technology, since the challenges involved in pumping heavy oil are great.

"With the subsea ESP work we had done in lighter-oil wells, we figured out that ESP technology could actually be quite economically competitive compared to gas lift, which was our usual lift method for lighter oils," Spinelli said. "In fact, we found that in some high-productivity heavy-oil wells, you could actually achieve greater flow rates with ESPs than with gas lift."

Spinelli continued to explain that many of these wells are in fairly shallow reservoirs, where the water depth is large compared to the depth of the reservoir. "In these situations, typically heavy-oil reservoirs in Brazil, you may have low gas fractions in the production flow. You may be at a gas fraction that is tolerable to ESPs, and you can place the pump at the seafloor."