Co-precipitation/Adsorption of Boron for Treatment of Produced Water at the Arroyo Grande Oil Field, California

Abstract: The goal of this Master’s thesis project is to develop a method for boron precipitation inproduced waters from the Arroyo Grande oil field outside San Luis Obispo in centralCalifornia. The current oil recovery is a closed system that pumps up to 1,500 barrelsoil/day. A new system is proposed to increase oil production three times andsimultaneously dewater half of the water in the oil formation during the time span of tenyears, which amounts to 55,000 barrels/day. The water will be treated and used forirrigation or discharged into a stream. The water contains high levels of boron (7-8mg/L), which will be removed with chemical precipitation/adsorption. The full treatmentsystem will include, besides precipitation, lime softening, walnut shell filter, strong-acidcation exchange, microfiltration, and reverse osmosis. All experiments were conducted at site-similar conditions, i.e. at water temperatures of70 ºC. Titrations were conducted with NaOH-solution and slaked lime (Ca(OH)2) toestablish the amount needed to increase the pH to levels needed in the precipitationexperiments. Softening to remove divalent ions (Ca2+ and Mg2+) and silica will be needed to protectcoming steps in the treatment, with the possible removal of some boron. The amount oflime, added as slaked lime, needed was determined experimentally to 500 mg/L. Limesoftening calculations were in good agreement with experimental results. Up to 20 %boron was removed by adding large amounts of lime, up to 2700 mg/L. The experiments were conducted varying the amount of magnesia (0-30 g/L), pH (9.7-11), and reaction temperature (50-90 °C), showing that 90 % boron can be removed whenadding 30 g/L magnesia, on the other hand, little or no boron was removed for additionslower than 5 g/L MgO. Increase in temperature and lowering pH was advantageous to theboron removal. The silica removal was strongly promoted by an increase in temperature.Magnesia was added to synthetic water, at low and high temperatures (50 and 80 °C), atlow and high pH, with sodium hydroxide and lime, confirming previous results.Magnesium chloride was added to the produced water, and compared to samples ofmagnesia with the same molar concentrations; magnesium chloride was more efficient atlow concentrations and less efficient at high concentrations, removing 31 % when adding5 g/L, and 11 % when adding 25 g/L. When studying the adsorption onto alumina, the amount was varied between 0 and 35 g/Land the pH was varied between 7 and 10.4. The boron removal increased, with theincreasing alumina, removing 38 % when adding 35 mg/L. Changing the pH did notimprove or worsen the removal.These results are important for determining a suitable boron removal process in thewastewater treatment plant at the Arroyo Grande oil field, though more studies must beconducted to reach optimum and realistic results.