The sulfate level in the effluent of sulfate removal unit (SRU) is usually continuously monitored for quality assessment. SulfaVer 4 method of Hach Company is a commonly used technique for sulfate measurement, and it is based on turbidity measurement caused by barite formation. The residual scale inhibitors in the effluent of SRU were observed to interfere sulfate measurement by inhibiting barite formation. Twelve different types of scale inhibitors were investigated on their impact on sulfate measurement. It was found that phosphonate, phosphate ester, and certain poly-carboxylate scale inhibitors do not interfere sulfate measurement, but sulfonate co-polymer and phosphino-polycarboxylate scale inhibitors showed strong interferences, i.e., the measured sulfate levels were lower than the actual concentrations. Several pre-treatments for the test sample were attempted to remove the interference. One pre-treatment method, called "Fe2+ thermal aging", was found to completely removed scale inhibitor interference on sulfate measurements without causing any side effect. The method has been verified with both synthetic brine and field brine. This pre-treatment method can also help improving the accuracy of residual scale inhibitor analysis.
Seawater injection is commonly utilized for offshore wells to maintain or increase oil production; however, treatment for seawater before injection is always necessary to reduce or remove bacteria, dissolved oxygen, sulfate, and other impurities1. Seawater typically has >2000 mg/L sulfate. Without proper sulfate removal, such high levels of sulfate can cause not only barium sulfate, strontium sulfate, and calcium sulfate scales, but also reservoir souring and H2S corrosion in the presence of sulfate reducing bacteria (SRB) 2. Therefore, sulfate removal from seawater is critical before seawater injection into reservoir. Sulfate removal unit (SRU) is essentially composed of a series of ultrafiltration (UF) membranes and/or nanofiltration (NF) membranes2. The sulfate level in the effluent of SRU requires continuous monitoring for quality assessment.