Delay P&A Through Improvement in Oil Cut for Mature Offshore Facilities

Abstract

Enhanced oil recovery operations are less common offshore in comparison to their mature counterparts onshore. Large inter-well distances, facilities and weight constraints as well as capital expenditure limitations due to declining production greatly restrict choices for tertiary recovery strategies. Moreover, as oil cut continues to fall, maintaining economic production requires processing ever-increasing water volumes eventually requiring plugging and abandonment (P&A), even though typically 45 to 70% of the original oil remains in the formation. Ideally, the economic life of mature offshore facilities would be extended by incremental oil production at a reduced, per-barrel cost to lower the overall lifting expense. As fluid processing and pumping rates are usually already optimized at maximum rates, an ideal enhanced oil recovery method would, in addition to increasing the oil production, also improve the oil-to-water ratio and reduce the per-barrel operating expenditure, or OPEX. A biological enhanced oil recovery (EOR) solution initially deployed offshore in the North Sea has been validated with multiple onshore demonstrations and is now available for mature, offshore facilities to promote preferential oil flow in water-flooded sandstone reservoirs. The method makes use of the existing biology in the formation, which is activated from its dormant state by continuous addition of a low-dose (ca. 100 ppm) nutrient solution from a small footprint injection unit. The resulting biological growth reduces interfacial tension (IFT) and mobilizes otherwise trapped oil. This paper shows the rapid improvements of oil production and oil-water ratios over large injector- producer well distances in as little as 2-4 months following initiation of biological EOR. Recommendations are offered that are relevant for extracting otherwise trapped oil and recovering significantly more of the oil-in-place prior to P&A. Strategies are also offered for field-wide implementation to enhance oil recovery and reduce OPEX with minimal capital investment and equipment installation.