Sulfate removal provides two benefits. First, when seawater, generally high in sulfate, is injected for reservoir pressure control it mixes with formation water that is often high in barium and strontium. A supersaturated solution of barium and strontium sulfate can occur. Under high pressure this solution is relatively stable, but as pressure is reduced around the production well the solution becomes unstable and precipitation can occur resulting in scale formation in production tubing or the formation rock around the well. Production can be severely affected. Removal of sulfate from injection water reduces the potential for scaling. Second, removal of sulfate significantly reduces the likelihood of reservoir souring. Sulfur is converted to hydrogen sulfide (H2S) by sulfate reducing bacteria (SRB). By removing a feedstock that can be converted to H2S, the need for costly metallurgy and H2S removal and handling systems is eliminated. Additionally, the worsening of health and safety conditions is avoided.
The process is based on nanofiltration technology. Using some of the technology from reverse osmosis (RO) for seawater desalination, membranes were developed that operated at significantly higher recovery rates and much lower operating pressures than conventional RO. Operating at pressures of 20 - 30 barg (300 - 435 psig), the spiral wound membrane allows monovalent ions to pass while rejecting divalent ions. Additionally, the membrane rejects particles greater than 0.001 micron including silica and bacteria.
Sulfate removal packages typically are configured with six membranes in series in a 2:1 array configuration. The reject from the first two modules feeds a single module where the reject is discarded overboard. Most of the current SRP installations are in the North Sea or offshore North Africa and Brazil in deep water production.
