Adoption of a Process to Prevent Souring Associated with Seawater Waterflooding in the Ursa-Princess Fields, Deepwater Gulf of Mexico

Abstract

Abstract A comprehensive evaluation of the potential for preventing reservoir souring through the use of Sulphate Rejection Membranes (SRM) was made for the Ursa-Princess Waterflood (UPWF), deepwater Gulf of Mexico. The operator, Shell Exploration and Production Company (SEPCO) has long believed that injection of seawater is a precursor to souring; the question is not 'if' but 'when' souring would occur.1 Metallurgy of the producer well tubing, casing, safety valves and topsides is not consistently sour safe and retrofitting would have rendered the waterflood project economics unviable. Therefore, prevention of souring by SRM was investigated, evaluated and adopted in order to avoid retrofitting the metallurgy. This is the first time that an SRM has been selected by a major operator for a seawater waterflood project based primarily on prevention of souring, while accepting SRM as proven for scale control. Introduction Field Description The Ursa Tension Leg Platform (TLP) produces from the Ursa and Princess Fields in the Mississippi Canyon area of the Gulf of Mexico, roughly 150 miles southeast of New Orleans. These fields comprise eight offshore continental shelf (OCS) blocks. The Ursa field was discovered in 1990. First oil through the Ursa platform was in 1999. The Princess Field was discovered in 2000. First oil from Princess, through a subsea tieback to the Ursa platform, was in December 2003. Total production from the Ursa platform peaked in the 2004 period at close to 150,000 BOPD. This phase of production was based on primary depletion driven by fluid expansion and natural compaction of the sedimentary rock with no appreciable aquifer support. Such compaction creates stability problems, including potential sand failures, and a range of drilling challenges. Following several years of primary production from these two fields, it was decided to install a seawater waterflood. The decision to waterflood was based on a number of factors including improving sand stability, maintaining reservoir pressure, improving the long-term sustainability of the production drive, and improving the long term economic benefits associated with increased ultimate recovery. The Ursa and Princess Fields have their main reservoirs in common and are in pressure communication. The Yellow reservoir is the largest and most productive. Waterflood is targeted exclusively for the Yellow reservoir which does not have natural aquifer support. It is an Upper Miocene turbidite reservoir of some 12,000 acres. Produced fluids are sweet and relatively light (average 27°API), with a bubble-point pressure of 5,900 psi at a reservoir temperature of 175 °F. Reservoir pressure had dropped significantly from an initial value of 11,800 psi. At the time that the decision was made to implement a waterflood the flowing bottom hole tubing pressure was already 5,500 psi.