MANAGING THE OIL IN WATER CONTENT OF NSO OVERBOARD WATER

摘要

The North Sumatera Offshore (NSO) field is part of the ExxonMobil Oil Indonesia Inc. (EMOI) network of fields that provide gas to the P. T. Arun Liquefied Natural Gas (LNG) Plant in Aceh. The NSO field met the government regulatory threshold of 50 ppm oil content in overboard water (OIW) quality since the field first started up in 1998 until 2004. After a platform turnaround was done in 2004 to increase production capacity, the NSO OIW began to rise. EMOI project teams have conducted detailed studies and implemented new technologies to address this issue starting in 2005. This paper presents the study progression, results, implementation performance and future plans for improvement.The NSO field is a sour gas field located in the Malacca Straits, about 100 km from shore. The offshore facilities include preliminary separation units, dehydration units and a Produced Water Treatment Plant (PWTP). Gas and condensate are exported to the onshore plant for H_2S removal, and then sent to the PT Arun LNG plant. Produced water is treated on the platform before it is discharged overboard. The PWTP unit consists of Sour Water Flash Drum (SWFD), hydrocyclones, equalizing water storage drum and Close Drain Drum (CDD). Chemical treatment including demulsifier and clarifier, are also utilized to assist and improve separation efficiency. After reviewing the existing water handling process, the team laid out the execution plan into phases.The first phase of the project aimed to improve efficiency of existing facilities by ensuring smooth CDD operation, increasing hydrocyclone reject ratio, and optimizing chemical injection rate. However, significant improvements were not achieved from these methods.One way to improve the oil and water separation at hydrocyclones is to increase the oil droplet diameter. According to Stokes Law, oil rises at a rate dependent on the droplet diameter and the fluid viscosity. Smaller diameter droplets rise more slowly. Thus, the next phase's plan was to increase the performance of hydrocyclones by installing a Mare's tail droplet coalescer, upstream of the hydrocyclones. A Mare's tail unit consisted of a bundle of oleophilic fibers inside a cartridge. As the water and some fine particles from the sour flash drum traveled along the fibers, small oil droplets were attracted to the surface and coalesce with other droplets as they migrate towards the outlet. The installation of the Mare's tail was able to increase the hydrocyclone efficiency from 60% to 80%. However, the overboard water OIW content was high. Using hydrocyclones and Mare's Tail alone to remove the oil from water was unlikely to be successful since oil-coated fine solids of iron sulfide (FeS) were present.Tetrakis Hydroxymethyl Phosphonium Sulfate (THPS) is a new chemical that acts as a biocide and FeS dissolver. Thus, the next phase of the project was to conduct THPS chemical injection testing. The testing results showed a significant reduction of OIW content (500 ppm injection for three days resulted in 14 ppm OIW content). The THPS treatment has resulted in a lower overboard water OIW content with an optimum injection rate at 100 ppm.