Abstract
Abstract
Greenish-brown sludge is formed in significant quantities during production of oil and water from the Serang field, offshore East Kalimantan, Indonesia. This interfacial sludge is comprised of entrained free oil, water and solids, and is stabilized by material described as a "metallic soap." In the absence of fluid treatment, removal and disposal of the sludge is tedious, expensive, and represents significant un-recovered oil.
The sludge was carefully characterized to understand its formation mechanism, so that remedial actions could be taken to mitigate its deposition. The sludge was characterized by elemental analyses, x-ray diffraction, nuclear magnetic resonance, Fourier transform infra-red spectroscopy, gas chromatography-mass spectrometry, and scanning electron microscopy. The results of the characterization studies suggest that sludge filtered and dried at 100°C (to remove free oil and water) consists of ~70 wt% branched and straight chain hydrocarbons in the range of C14 – C28. About 20 wt% of the sludge consists of C28 – C30 carboxylate salts and <5 wt% is inorganic minerals. Emulsification of oil, water droplets and solids by carboxylate salts and bicarbonate ions from water is the most likely sludge formation mechanism.
Laboratory and field tests have demonstrated that the sludge can be dissolved by low dosages of commercially available sludge dissolving agents containing combinations of acids. As a result of dissolving the sludge, incremental oil is recovered, which offsets chemical treatment and sludge disposal costs. Although the sludge may be dissolved at the Santan onshore processing facility (end-of-pipe), sludge formation is best mitigated by treating the point-of-source at the Serang offshore platform.
Introduction
During production of oil from the Serang field, offshore East Kalimantan, Indonesia, a greenish-brown sludge accumulates in the bottom of onshore terminal crude oil storage tanks. Initially, this waxy sludge, exhibiting a consistency of toothpaste, is observed as a thick emulsion layer between separated oil and water sampled at the inlet to the on-shore oil-gas-water processing facility and at the arrival of the production sub-sea pipeline from the Serang platform to another centralized platform. Up to 100 barrels of the sludge may accumulate in onshore crude oil storage tanks per day, as conventional demulsification treatment with standard demulsifier chemicals does not remove this sludge layer. This sludge is difficult to remove from the tanks due to its high viscosity and pour point. The sludge is ultimately disposed at the terminal's bioremediation plant, where it undergoes very slow biodegradation.
Material similar to this Serang sludge has previously been observed in Southeast Asia oil fields, as well as in fields in West Africa and the North Sea. The sludge is believed to be a "metallic soap" consisting of oil, fatty or naphthenic acids with bicarbonate complexation.1 It is closely related to naphthenate sludge/scale that is observed in numerous oil fields around the world.2
This naphthenate/metallic soap material has been observed to occur in many forms, including viscous paste-like emulsion layers (sludge), dispersed particles not visible to the human eye, fibrous strands of several mm in length, and large hand specimen-sized accumulations of hard scale solids. Soap and scale can foul pipelines, valves, separator vessels and other oilfield processing equipment. Co-precipitation of naphthenate materials (sodium, calcium and iron naphthenates, with naphthenic acids of varying molecular weights) with other scale types has also been observed; e.g., with calcium carbonate and asphaltenes.3,4,5
The present study was conducted to characterize the Serang sludge and to determine methods to prevent its formation.