The use of seawater for waterflood pressure maintenance offshore offers several advantages over alternative water sources, The presence of sulfate ions in seawater however, represents a potential scaling problem when combined with high concentrations of barium or strontium often found in formation waters, Scales such as barium sulfate and strontium, sulfate pose potentially enormous operational problems on floodfront breakthrough at producing wells.
Nanofiltration membrane technology has been successfully employed offshore to eliminate sulfate ions from seawater prior to injection, The first sulfate removal facility (SRF) was installed in 1988, on the Marathon Oil UK operated Brae "A" Platform, Marathon is currently installing a second seawater SRF, this time on a Marathon Oil Company operated platform in the Gulf of Mexico, This paper presents some of the design philosophy decisions and technical challenges in store for prospective waterflood treatment facility/SRF project teams.
Also reported are guidelines to assist in evaluation of the capital and operational costs to be expected when considering a SRF installation.
Though waterflood treatment facilities (WTF) vary widely from location to location, the choices for source water are generally limited to three: Drilled Source Well(s), Produced Water, and Seawater The water source selection is based largely on the natural and manmade resources available on site as well as the reservoir and water chemistries expected.
In cases where seawater is used as the waterflood medium, the barium (Ba++) and strontium (Sr++) content of the formation water could be critical. Seawater universally contains sulfate (S04) Ions which readily combine with formation water barium and strontium to precipitate barium sulfate and strontium sulfate scales.
A majority of formation waters contain sufficiently high barium concentrations to form damaging barium sulfate scale when mixed with seawater having normal concentrations of sulfate ion (herein referred to as HSS), Only 50 ppm of precipitation, from 10,000 BPD of produced water, equates to over 33 tons per year of tenacious, plugging scale. In addition, barium and strontium sulfate scales have very low solubilities and are frequently radioactive.
Well workover and mechanical removal of deposited scales are expensive, and the use of chemical scale inhibitors only postpone required remediation (and require constant monitoring). Removing the cause of the problem, i.e. the sulfate ions in injected seawater, virtually eliminates sulfate scale precipitation in the producing wells and platform topsides on floodfront breakthrough.
Installation of a SRF to remove sulfate ions from seawater (herein referred to as low sulfate seawater - LSS) for waterflood injection minimizes the economic and operational impact of scaling problems, but also complicates design of the entire seawater waterflood treatment facility.
There is considerable debate regarding the role that sulfate content plays in the souring of seawater waterflood reservoirs.(1-5) Such discussion is outside the scope of this paper.
