Sulphate Removal
Tecchnical Profile
The objective of this process is the removal of sulfate ions from seawater prior to injection for enhanced recovery. Specific membranes are used to remove sulfate from injection water to prevent sulfate precipitation.
In a cooperative effort between Dow Chemical and Marathon Oil sulfate removal technology was developed. Dow developed membranes (nanofiltration) that selectively reject sulfate ions in seawater while allowing sodium and chloride ions to pass through. Sodium chloride provides salinity that is particularly useful in maintaining stable clay structure of the formation. Marathon Oil successfully used these membranes to remove sulfate from seawater for injection. Using this technology, sulfate concentration may be reduced from 2800 mg/l to less than 50 mg/l.
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.
