Recovery Improved by Subsea Sulfate Removal and Low-Salinity Water Injection
Chris Carpenter_
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 231579, “Improving Recovery With Subsea Sulfate Removal and Low-Salinity Water Injection,” by Ojonimi S. Haruna, SPE, and Bruno C. Kahn, NOV. The paper has not been peer-reviewed.
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Water injection remains the most widely applied method for maintaining reservoir pressure and improving oil recovery offshore. However, conventional topside water-treatment systems face performance limitations because of space, weight, reliability, chemical-handling, and operational issues. Recent advancements in subsea seawater treatment have enabled the development of fully integrated subsea sulfate-removal systems for disinfection, ultrafiltration (UF) technology for solids removal, and nanomembrane (NM) technology for sulfate removal. This paper discusses waterflooding and field-development implications of subsea seawater treatment for improved oil recovery.
Introduction
Experience from large deepwater carbonate fields shows realized water-injection (WI) rates fluctuating between 40 and 80% of planned volumes during early and middle field life, primarily for the following reasons:
- Limited topside space and weight margin for debottlenecking or expansion
- Filtration- and membrane-system fouling and frequent backwash disruptions
- Reliability issues in high-pressure pumps and drive systems
- Chemical handling and health, safety, and environment constraints for oxygen scavengers and other chemicals
- Operational conflicts between production and injection priorities
- Shutdown requirements for maintenance and modification campaigns
Subsea seawater treatment addresses these challenges by moving the injection-water-preparation process from the platform to the seabed. A subsea seawater treatment and WI architecture allows injection to be performed locally with minimal topside impact. This shift transforms reservoir management flexibility and unlocks improved-oil-recovery (IOR) potential that is difficult to realize using conventional topside-only approaches.
Waterflooding and Recovery Mechanism
Waterflooding is essential to sustain production and maximize ultimate recovery by providing pressure support and displacing oil toward producers. The key contributions of WI to IOR include the following:
- Maintaining reservoir pressure above bubblepoint and avoiding uncontrolled free-gas liberation
- Providing a displacement drive that improves macroscopic sweep efficiency
- Enhancing conformance through vertical and areal support, delaying localized depletion
- Maximizing incremental oil produced before water breakthrough and late-life decline
Even relatively short periods of underinjection can produce irreversible reservoir-energy deficits, accelerate gas breakthrough, and create bypassed-oil regions that are difficult to recover later. Injected-water quality has a direct effect on reservoir and well performance. To support long-term IOR, injection water must be compatible with the reservoir and completion materials while also maintaining injectivity.
Low-salinity water has been shown to enhance oil recovery in many sandstone and some carbonate systems through mechanisms such as wettability alteration, ion exchange, and reduced residual oil saturation. Tailoring injection-water chemistry—by adjusting sulfate content, total salinity, and ionic balance—can improve both macroscopic and microscopic displacement efficiency. This is particularly valuable when combined with chemical IOR in offshore environments where topside space for additional processing is limited.