Producing Heavy Oil from Offshore Platforms at the Santa Ynez Unit
The API gravity of the produced oil from the Monterey reservoir of the Santa Ynez Unit (offshore California) has decreased over the last several years as the lighter oil has been depleted and new discoveries have come on line. Monterey crude oil generally varies from 10° to 23°. This low API gravity oil is already viscous (20 cp) at reservoir temperature (200°F or higher); however, as oil flows to the surface its temperature decreases significantly (especially while it flows inside the wellbore through the cold Pacific Ocean). At the surface, the temperature of the oil is ~100° F and its viscosity is ~9000 cp. In addition, a very viscous emulsion forms when water flows with oil at 30% to 70% watercut, making the fluids even more difficult to produce via gaslift. Several technologies including Vacuum Insulated Tubing (VIT) and downhole diluent injection are employed to enable us to flow this heavy oil at economic rates.
VIT is tubing in which the annulus between the inner and outer parts of the tubing has been evacuated. The evacuated annulus greatly decreases heat transfer between the fluid traveling through the tubing and the environment outside the tubing. This allows the fluid to retain its heat as it moves up the tubing, which reduces the viscosity of the fluid, and results in higher rates. Extensive analytical modeling has yielded recommended guidelines for use.
Downhole diluent injection increases fluid rates in highly deviated wells producing viscous oil. It has become an important technology for producing heavy oil at SYU. A light oil or condensate is injected down tubing/casing annulus through a 1” injection line just above the production packer to commingle with the heavy crude oil. The commingled fluid has a lower viscosity and reduced friction in the tubing, thereby enhancing production rates.
AAPG Search and Discovery Article #90062©2006 AAPG Hedberg Research Conference, Veracruz, Mexico