Sulfate Ion Removal from Seawater for Hydraulic Fracturing by Barium Sulfate Precipitation

To face the worldwide shortage of fresh water resources and the increase demand of oil and gas, researchers have devoted to study on application of seawater as a base fluid for hydraulic fracturing. One of the primary challenges for seawater fracturing application is the scale precipitation tendency due to the incompatibility of high sulfate concentration with high calcium, barium or strontium concentration in formation water, which will lead to overall reduction in production capacity. This work aims to develop a chemical precipitation method, which is a low cost way to remove sulfate before injection to solve the scale problem. Barium chloride dihydrate was used to precipitate sulfate from seawater for hydraulic fracturing. The chemical dosage, working temperature and precipitation time were optimized. The sulfate concentration in treated water was determined using an inductively coupled plasma mass spectrometer (ICP-MS). The sedimentation speed to separate treated water and precipitates at different precipitation time was measured using an optical particle stability analyzer. The obtained precipitates were dried at 60?C, and the morphology was observed using scanning electron microscopy (SEM) and X-ray diffraction method (XRD). Experimental results showed the barium chloride dihydrate can reduce the sulfate concentration in seawater from more than 4,000 ppm to less than 200 ppm when the dosage barium is higher than 5,500 ppm. The reaction efficiency is not altered in the temperature range from 15ºC to 45ºC. It turned out the treated seawater could meet the requirement for hydraulic fracturing application. As to the separation of water and precipitates using sedimentation method, it showed the highest speed appeared when precipitation time was 5 mins. And the addition of flocculants cannot improve the sedimentation speed. In addition, SEM results showed the size of obtained precipitates was in nanometer range. Besides, XRD confirmed the composition of precipitates were barium sulfate with purity >90%. The characterization results demonstrated the precipitates could be used as additives in drilling fluid, which will greatly reduce the operation cost. The work has revealed that barium sulfate precipitation method is promising to remove sulfate in seawater for hydraulic fracturing. Besides, the obtained barium sulfate is a commercially valuable product used in drilling fluid. Comparing to nanofiltration methods, this method is low cost and has no energy input requirement, which is suitable for a low carbon economy.