Arsenic Decontamination: Exploring the Efficacy of Sodium Hydroxide in Altering Arsenic Solubility and Ensuring System Compatibility
Authors: Dararatana Naruphorn , Thammachart Matina , Punpruk Suchada , Srisawat Kornrawee , Yotapan Nattawut , Limthongkul Pimpa , Chianpairot Amnuaysak , Sombatmankhong Korakot 
Published: October 2024

Oil and natural gas reservoirs in the Gulf of Thailand are contaminated with mercury and arsenic. The arsenic contamination not only raises concerns related to health and safety but also significantly impacts the overall quality and market value of the crude product. The conventional filtration effectively removes only insoluble arsenic contaminants in the condensate, but not soluble arsenic species. Thus, posing the challenge of soluble arsenic removal by alternative approaches. This study aims to investigate the feasibility of employing sodium hydroxide (NaOH) as an active reagent in chemical treatment process particularly for removing soluble arsenic contaminants from condensate.

The experimental setup was designed to simulate the real operating conditions in the 1st and 2nd stage production separators in a simplified manner. This study involved selecting 100% and 50% condensate-to-produced water ratios and adjusting the reaction time for each condition to mimic real-life scenarios. A wide range of NaOH concentrations from 0-550 ppm was introduced in the experimentation process. The efficiency of arsenic removal was assessed by comparing the arsenic levels in samples before and after treated with NaOH. A comprehensive crude specification was conducted for quality evaluation. Furthermore, this study encompassed a compatibility test evaluating the interaction of NaOH with demulsifiers discern inhibitory effects on the chemicals employed. An immersion corrosion test was conducted on carbon steel and corrosion-resistant alloys (CRA) to verify the corrosion compatibility of NaOH with the materials constituting the system.

The predominant arsenic species in field A comprise triphenylarsine and triethylarsine which are soluble in condensate. The concentration screening test for suitable NaOH dosage demonstrated that higher NaOH dosage promoted higher arsenic removal efficiency. With the optimal NaOH dosage at 550 ppm, the arsenic removal efficiency ranged from 50% to 65% were obtained for both 100% and 50% condensate-in-produced water. The addition of NaOH increased transformed of arsenic to water-soluble forms. This led to a decrease in arsenic concentration detected in the condensate phase while increasing in the water phase. The adsorbent flow testing with NaOH-treated condensate indicated that the selected NaOH dosage did not adversely affect demulsifier activities. Additionally, crude specification confirmed a slight decrease in total chloride ion, while other crude properties exhibited no observable changes. Corrosion test results showed that the average corrosion rate of carbon steel in the condensate treated with NaOH was lower than the non-treated system after immersion for 14 days. No corrosion product was observed on CRA after 45 days of immersion testing in both NaOH-treated and non-treated conditions.

This study elucidates the arsenic decontamination process by altering its solubility in condensate using an alkaline chemical. The investigation provides a comprehensive feasibility analysis addressing a practical application concern encompassing the compatibility, integrity, and safety of the NaOH solution within the operational framework of the system.