West Seno Deepwater Development Case History - Production Chemistry

Abstract

Abstract The West Seno field is located in the Makassar Strait PSC offshore East Kalimantan, Indonesia about 60 km offshore in ~3,200 ft of water. Field development consists of a tension leg platform (TLP), a floating production unit (FPU), and two export pipelines tied back to existing onshore infrastructure. Production commenced in August 2003, and Phase 1 of the project has now ramped up to over 45,000 BOEPD. The field presents a number of production chemistry and flow assurance challenges, the most serious of which is an unusual emulsion. Organic acids in West Seno crude oils react with bicarbonate rich produced waters to form sodium and other metal carboxylates. These metal carboxylates are "soaps", which tend to linger at water/oil interfaces and afford highly stabilized brine-in-oil ‘forward’ emulsions. Conceptually, soap-stabilized emulsions should be easy to break, as one simply adds a demulsifier to reverse the soapforming reaction and allowing the phases to separate. Unfortunately, the emulsions are constantly undergoing shear, by various items of standard production process equipment, helping to re-emulsify partially separated fluids. Furthermore, in the deepwater environment, produced fluids tend to cool significantly, thereby losing the benefit of heat-induced demulsification and causing waxes to precipitate. Also, fine solids (formation fines, residual drilling mud solids, etc.) typically accompany production, further adding to emulsion stability. Although acid demulsifiers have been successfully applied at West Seno, due to the complex nature of the emulsions with multiple stabilizers identified, a finite residence time is still needed for phase separation, which creates a potential production bottleneck. Emulsions must not be over-treated since injection of acidic chemical induces corrosion, and the phosphoric acid portion of the demulsifier reacts with calcium in the brine to form hydroxyapatite and vivianite/polyphosphate scale solids. To destabilize the emulsions it is important to dehydrate the oil as early as possible with heating, minimize shearing and reduce the introduction of solids to the production stream by insuring an efficient mud displacement prior to well completion. Desander hydrocyclones are used on the FPU upstream of deoilers, and regular sand jetting of the main separator is necessary. Other important production chemistry/flow assurance issues that are being addressed include:gas hydrate inhibition,produced water treatment with reverse emulsion breakers, flocculants and solids wetting agents,microbiological control in FPU "off spec oil" and off spec water" surge tanks, andcompatibility of treatment chemicals and adverse effects on the soap emulsion. Introduction The West Seno deepwater field, discovered in 1998, is located approximately 60 km offshore East Kalimantan, Indonesia (Borneo) due east of the Bontang LNG terminal in 3,200 ft of water. Figure 1 is a map of the Unocal operating area of the Kutei Basin. The field was placed on production in August 2003. The field is being developed with a tension leg platform (TLP), a floating production unit (FPU), and associated drilling tender barges. The TLP consists of 28 dry tree wells. Produced fluids are routed to the FPU from the TLP using flexible jumpers. Thorough descriptions of the project and facilities are available.1,2 (See Figure 1)