Calcium Naphthenate Solid Deposit Identification and Control in Offshore Nigerian Fields

Abstract

Abstract 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 pre- selection 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 flotation 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.