Performance Appraisals of Gas/Oil Separation Plants

Abstract

Produced crude oil is typically processed at wet crude handling facility (WCHF) or a gas oil separation plant (GOSP). Most of the crude is commingled with water and produced in the form of emulsions that are separated at the GOSP using demulsifiers. In general, the emulsions at the inlet of Saudi Aramco GOSPs are tight and no free water is observed in the feed stream. In spite of the tightness of the emulsions, the demulsifier costs per barrel of produced oil in Saudi Aramco are one of the lowest in the world. This paper describes the performance appraisal of several Saudi Aramco GOSPs. It also discusses the evaluation of these facilities and the efforts undertaken to reduce and optimize the costs of oil-water separation. The most important effort was the characterization of emulsions from various fields. It shows the relative tightness of emulsions and the associated causes of tightness. It has become a diagnostic tool in the quest to understand the causes and treatment of emulsions. The inlet emulsions were characterized by conducting field tests at each of the GOSPs using fresh emulsion samples. The factors associated with the tightness of emulsions include the amount of fine solids such as asphaltenes and scales in the crude, watercuts, temperature, brine chemistry, and shear. Another aspect of oil-water separation investigated was the effect of processing equipment and vessel retrofitting. These included the conversion of two-phase (gas-liquid) separators into three phase (gas-oil-water) separators, installation of coalescer packs, demulsifier injection strategies and use of online analyzers. An innovative approach to optimizing oil-water separation is described. The paper provides invaluable operational experience and many ‘lessons learned’ are applicable to other crude oil treating facilities. Introduction Crude oil is seldom produced alone and is generally commingled with water. Produced water occurs in two ways: some of the water may be produced as free water, i.e. water that will settle out rapidly, and some of the water may be produced in the form of emulsions. Emulsions can be difficult to treat and cause a number of operational challenges such as tripping of electrical grids in dehydrators/desalters, GOSP upsets, production of off-spec crude oil, and creating high pressure drops in flow lines. Emulsions have to be treated to remove the dispersed water and associated inorganic salts in order to meet crude specification for transportation, storage and export and to reduce corrosion and catalyst poisoning in downstream processing facilities. A typical Saudi Aramco GOSP consists of a 2 or 3-stage oil-gas separation facility, with a 2 or 3-stage dehydrator/desalting train (Fig. 1). A typical GOSP processes ∼300 MBD of crude and ∼100 MBD of water. The GOSPs are generally designed to handle watercuts up to 30%, and some have been modified and retrofitted to handle higher watercuts. With increasing fluid throughputs, the GOSPs face a number of challenges mostly related to dehydration and desalting. Some of these include increased chemical consumption, possibility of increased water (and salt) carryover into the oil, oily effluent or disposal water, wide emulsion pads (rag layers), and electric grid shorting among others. These challenges can cause operational upsets and lead to higher operating costs. During crude oil processing at the GOSP, one of the most important variables that determines the efficiency of oil-watergas separation is the tightness of the incoming emulsion. The tighter the emulsion, the higher the dosage of demulsifier needed to break them. The performance of the GOSP is intimately tied to the characteristics of the feed emulsions. Therefore, one of the most important aspects of this study was to characterize the emulsions from every GOSP. Another aspect of GOSP performance is related to the process facilities (hardware) and process variables. The hardware includes the number and type of separators, dehydrators and desalters, water-oil separators or WOSEPs, and other hardware at the GOSP. Process variables include oil and water flow rates, temperatures, watercuts, and GOSP operating conditions. A higher residence time of fluids in the GOSP will generally lead to better separation and better performance, all other variables being constant. Besides the residence time, process retrofits in the vessels also tend to enhance performance.