Inhibitor-Resistant Calcites Causing Solids Fouling Identified, Studied

Abstract

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 209475, “Unraveling the Mystery of Inhibitor-Resistant Calcite Affecting Onshore and Offshore Operations,” by Hugh M. Bourne, SPE, Adil A. Abbasov, and Ben E. Smith, BP. The paper has not been peer reviewed. _ Magnesian calcite has been identified as the main constituent of solids fouling in several of an operator’s facilities in the North Sea and Azerbaijan and in partner operations in other locations. These solids were, for many years, thought to be calcite scale. The complete paper describes how this “pseudoscale” was detected and its prevalence, using field case examples. The Discovery: Case 1 Soon after the start of operations in May 2017, fouling of the rundown coolers was observed on a North Sea floating production, storage, and offloading vessel. The increase in differential pressure across the coolers was believed to be the result of wax deposition. The cooler performance, however, progressively deteriorated with every dewaxing cycle to a point at which running the cooler without cooling medium had no effect on the differential pressure. One of the rundown coolers was removed and returned to the manufacturer for inspection and cleaning. This revealed that hard, scale-like solids were filling the space between the plates completely (Fig. 1). The retrieved solids dissolved in acid, were calcium-rich, and appeared visually and mechanically to be a scale; thus, they were assumed initially to be calcium carbonate. The first analysis of solids from the rundown cooler and other parts of the plant suggested that up to 33% of the composition of these solids was calcium, which would be equivalent to greater than 80% of these solids being calcium carbonate. Scaling in the rundown cooler was not expected and raised concerns about uncontrolled scaling in other parts of the plant. A wider review of plant performance was initiated, and more-detailed analysis of the solids collected in the rundown coolers was undertaken using X-ray diffraction spectroscopy (XRD). The rundown cooler solids analysis revealed an inorganic crystalline compound consisting of magnesian calcite (calcium/magnesium carbonate) that had the appearance of a compacted transported solid. The dewaxing frequency was increased to prevent this accumulation, which ultimately proved an effective mitigation strategy. The wider review of plant performance identified frequent fouling of the produced water-pump suction strainers and a gradual decrease in the performance of the crude-oil heaters. Analysis of the solids fouling the produced-water-pump suction strainers also was dominated by magnesian calcite, suggesting that the reservoir fines were transported in both the oil and the produced-water streams. Reservoir solids fouling is not uncommon, but, by the beginning of 2018, the decrease in the differential pressure trend across the crude-oil heaters became more prominent. By the end of the first quarter of 2018, the degradation was limiting the ability of the plant to meet temperature set points in the separators, potentially limiting production rates.