Factors that impact scale inhibitor mechanisms
Boak, Lorraine Scott
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The formation of mineral scales such as barium sulphate and calcium carbonate remains an issue for the oil industry, after many years of oil exploration. In the last 10 years, the difficulty in dealing with scale deposition has been accentuated by the appearance of more complex conditions, involving complicated well completions for deepwater or long sub-sea tiebacks. If scale control measures fail in these situations then long distances between the scale deposits and the production platform are present. Intervention into such systems is either impossible or extremely expensive. To combat such problems, the front end engineering design stage (FEED) now attempts to bring together multidisciplinary teams to provide a full risk assessment of all areas in which production chemistry problems might arise. Hence, benefits come from each discipline team having as much knowledge as possible available to them. This thesis aims to fuel this knowledge by developing a fundamental understanding of how various factors, conditions or environmental, impact scale inhibitor mechanisms, so that the results can be incorporated into the FEED process. Key areas affecting scale inhibitor operation were investigated. From these studies, a number of important findings can be highlighted. The presence of calcium was found to improve scale inhibitor (SI) performance, especially phosphonate types, whilst magnesium ions had little effect on polymeric performances and detrimentally affected the phosphonates’ inhibition efficiency (IE). These trends were related to the SI affinity for the divalent ions – polymer PPCA binds to calcium but shows incompatibility at [Ca2+] > 1000ppm - observed as low IE, whilst the phosphonate DETPMP binds with either ion but prefers calcium. Two inhibition mechanisms - nucleation and crystal growth blocking - were identified for different types of SI species and were illustrated using static IE tests relating IE to [SI] left in solution. High IE corresponds to high [SI] and similarly low IE with low [SI]. These initial results have since been investigated further in a additional study. An extensive range of phosphonate and polymeric scale inhibitor species can now be classified as i. either Type 1 or 2 (based on IE, Ca2+ and Mg2+ sensitivity ration and SI consumption tests) or ii. either Type A or B (based on compatibility/incompatibility with [Ca2+]= ~1000-2000ppm+). A requirement for both homogeneous and heterogeneous nucleation to be investigated for a scaling system was identified, as deposition kinetics can vary requiring different ii levels of SI. A [SI] falling below minimum inhibitor concentration (MIC), can promote surface scaling. Hence, scaling systems should be studied experimentally over a range of temperatures, to represent the conditions from sub-sea tiebacks to the production well. A model was developed from experimental data enabling the prediction of safe sulphate levels and mass of barite deposited. This model can be applied to un-seeded and seeded tests where, as expected, the foreign particles accelerated the reaction to equilibrium with the greatest deposition rate for barite over sand and for a higher surface area over a lower one. Both theoretical and experimental confirmation of each retention mechanism occurring in a porous medium was achieved. This adsorption/precipitation model has been incorporated into Squeeze VII, an in-house squeeze design software, to allow a better physical description of a squeeze treatment. The predictions of Squeeze VII have also been improved by using the more accurate data for the scale inhibitor return concentrations from core floods due to the better developed analysis techniques. The direct value of these improvements to industry is significant. These advances reduce OPEX costs and deferred oil production whilst giving the industry the opportunity of improved future lifetime predictions and operations.