Tackling the Operational Challenges of Sour Produced Water Treatment and Disposal

Treating produced water in Upstream Oil and Gas industry is challenging due to high H2S and salt content, presence of temperature-sensitive salts and acidic components. Conventional H2S stripping technologies like reboiled stripping used in downstream facilities need extensive review before applying them in Upstream due to propensity for scaling at high temperature. This paper discusses the several challenges faced in the newly installed Produced Water Treatment (PWT) Unit, to treat sour produced water to the specification of disposal into aquifers and how they were overcome to maintain Business continuity. Problems faced include severe corrosion in carbon steel piping and components of treated water circuit, hydrocyclones under-performance, frequent filter choking (present downstream of hydrocyclones), and salt/scale deposition in the high temperature reboiler section. This caused frequent shutdown of the system, reducing the unit's availability and impacting oil production target. To identify the root causes and recommend mitigation measures, design data was collected, and deviations were established with respect to design basis. Sampling and analysis for water characterization including pH, suspended solids, oil and oxygen content, bacteria, deposits, and metallurgical failures were carried out. Studies including process, corrosion were conducted by the Original Equipment Manufacturer (OEM). The severely corroded carbon steel sections were found to be due the Low pH (<4.5), against the design value of pH 6.5, caused by the Oxygen scavenger injected upstream and the presence of Volatile Fatty Acids (VFAs). To maintain the pH, Alkali dosing downstream the stripper is considered as a low-cost solution. The hydrocyclone under performance was due to high oil content (> 5000 mg/l) in feed, compared to design (2500 mg/l), low feed rate (below turn-down) and feed flow variation. These were overcome by adjusting interface levels in upstream separators, changing demulsifier and dosage rate, reducing flow variation through control loops tuning and providing recycle line. As for the frequent filter choking, the main reason is the higher oil content due to hydrocylone underperformance, biofilm formation and wrong cartridge selection (20-micron instead of 70-100 micron acceptable to downstream stripper/disposal well). Improvement in hydrocyclone performance and change in cartridge mesh size resolved the problem to manageable level. Salt/scale deposits were mainly due to high salinity of water and high temperature as a result of reboiled stripping. Mode of operation was changed to cold stripping with fuel gas taking the advantage of low pH in the system. To maintain the low pH in the system, acid was added whenever required along with caustic injection in treated water to avoid costly CRA material, which was found to be a cost-effective solution.