Calcium Naphthenate Solid Deposit Identification and Control in Offshore Nigerian Fields

Two operated fields in offshore West Africa began experiencing aggressive solid deposition challenges soon after the onset of water breakthrough for Field A and Field B. This led to significant process control and flow assurance issues in the crude processing plants. Problems experienced included: plugging of separator bridles, hydrocyclone liners and level control valves, solid loading in overboard water, poor oil-water interface and loss of retention time in separators and plugging of reject lines of the produced water treating vessels. An extensive solid deposit identification campaign was undertaken in order to establish the composition of the samples. Calcium naphthenate (CaN) scale was identified as the major component in the solids, the first major occurrence experienced in the Gulf of Guinea for the operators of fields A and B. A competitive screening of CaN inhibitors resulted in the preselection of a low dose non-acidic formulation. The CaN inhibitor formulation selected in the laboratory was then trialed in the two offshore fields, according to a well-defined set of key performance indicators (KPIs). Results from the field trials indicated significant reduction in the solid deposition rate. The flushing of separator sight glasses reduced from three times per shift to once in four days. Hydrocyclone liners and induced gas floatation units opened for inspection following five weeks of treatment showed no significant CaN scale present. Online oil and water filtration exercise showed downward trend in solid deposition across the FPSOs in the systems of both Field A and Field B. Produced water quality was not compromised. The CaN issue is now under control and subject to a surveillance campaign. This paper showcases an operational excellence approach to solving the CaN problem in the face of increasing solid deposition challenges in offshore operations as seen in the Gulf of Guinea and currently in other parts of the world. __________________________________________________________________________________________ Background. Calcium naphthenate (CaN) scale represents one of the many production chemistry challenges which result from the presence of naphthenic acids in crude oil production systems 1,2 . For the deposition of CaN to occur, so-called ARN acids (tetra-acids with molecular weight above 1200 Dalton) need to be present in the crude oil. Identification and structural studies of ARN acids have been discussed in other publications 3,4 . Both the ARN acids as well as the other naphthenic acids present in a produced crude oil, may react with the cations present in the water under a specific set of conditions. It is a known fact that naphthenic acid phase behavior is a function of produced water composition and pH 5 . If the pH of the produced water increases above pH 6 (due to depressurization) and if there is sufficient calcium, there will be a competition of the different naphthenic acids to precipitate as scale. Selectivity towards the divalent ions has been explained via cation size and hydrated water number 6 . The kinetics of the reaction between ARN and produced water cations is very fast 7 . ARN acids have up to four acid groups and are surface-active, and, thus they can react to form a network with the calcium ions in the produced water 7 . This network is very sticky, insoluble in oil or water, and settles out in the water leg of the separator and from there it is carried through the water treatment plant. Therefore within the operating asset, the precipitation and deposition of CaN has been reported as a serious threat to safeguarding stable production activities. Nevertheless, the management of the CaN problem has progressed significantly over the last 20 years since the first published field cases 8,9 . Practical control philosophies have been outlined and various chemical treatments available discussed 1,10 . In addition, better analytical and modeling tools are made available to study the production chemistry drivers for CaN deposition 11,12,13 . Where chemical treatment for CaN needs to be implemented in a producing asset, this comes with additional challenges including material