WaterSolutions Offshore Seawater Treatment for Improved Oil Recovery

Protection of injection wells and maintaining quality of oil produced are critical to offshore economics. SR-90 membranes are an industry standard for low sulfate injection water to avoid reservoir scale and souring. Sulfate Removal from Injected Water in Oilfield Operations.

DOW™ FILMTEC™ SR-90

The DOW™ FILMTEC™ SR-90 nanofiltration membrane (and the resultant sulfate removal technology) has been developed in a cooperative effort between The Dow Chemical Company and the Marathon Oil Company to selectively remove sulfate from seawater that is used for waterflood injection operations.

There are two major advantages in the removal of sulfate from injected seawater.

• Prevention of barium and strontium sulfate scale precipitation. When normal high sulfate seawater is injected into reservoirs which have formation water containing barium and strontium, mixing occurs forming a supersaturated barium and / or strontium sulfate solution. Upon pressure decreases in and around the production wells, the supersaturated barium and / or strontium sulfate solution is no longer stable and precipitation occurs. The result is scale formation in the production tubing and / or plugging of reservoir rock around the production well. Petroleum reserves are often lost. By removing sulfate from injected seawater, the potential for scaling is prevented. This is contrasted with traditional scale remediation treatments and procedures addressing scale problems after they occur. In deep water and other complex oil developments, sulfate removal, and the subsequent prevention of scale, provides significant cost advantages.
• Souring Control / Mitigation in reservoirs containing barium and strontium. By removing the sulfate in the injected seawater, there simply is a reduced source of sulfur that can be converted to hydrogen sulfide by thermophilic sulfate reducing bacteria. Consequently, the risk of well souring is reduced. Examples of sweet oilfields that became sour upon waterflood breakthrough following the injection of high sulfate seawater, include Prudhoe Bay, Kuparuk, and Gulfaks in the North Sea. By removing the sulfate from the injection water providing subsequent souring control, there is a reduced need for sour safe metallurgy, costly hydrogen sulfide removal equipment, and health and safety concerns.
• The nanofiltration membrane also removes all particles greater than one one-thousands of a micron resulting in high quality injection water free of silica and bacterial materials thereby insuring continued injection rates reflective of initial reservoir conditions.

Sulfate removal provides maximum benefits in oil developments exhibiting the following characteristics:

a) Deep water operations with subsea vs. dry well heads. Chemical squeeze inhibitor treatments generally cannot be applied when access to a dry well head is not available.
b) Horizontal or multilateral reservoir developments where there is difficulty / risk in the placement of chemical squeeze inhibitors.
c) Costly production wells where the loss of one well due to scaling could approximate the cost of a sulfate removal unit.
d) Mild / Moderate scaling potential in combination with costly and / or complex reservoir developments.

Sulfate removal is being accepted as the "default" scale control technique in most West African and Brazilian operations. Seven additional sulfate removal facilities are either in operation or under construction in the North Sea. The total sulfate removal capacity is > 6MM BWPD and with a proven performance of the fifty units that are now in operation or under construction.