The Impact of Dissolved Iron on the Performance of Scale Inhibitors Under Carbonate Scaling Conditions

Abstract

Abstract The laboratory determination of scale inhibitor (SI) performance under field specific conditions using dynamic or static scale inhibitor tests provides an important method for determining minimum inhibitor concentrations (MIC's) for the inhibition of scale growth. This paper will discuss the ability of small amounts of ferrous iron to dramatically reduce the ability of SI's to inhibit calcium carbonate scale formation under dynamic laboratory test conditions. It has been previously reported that the presence of ferrous ions has an inhibiting effect on calcium carbonate formation under dynamic test conditions, which was confirmed for the brine systems used in this study. However, despite the somewhat milder scaling regime in the presence of ferrous ions, addition of Fe2+ ions to test brines caused the observed MIC's of typical scale inhibitor chemicals to increase more than one hundred fold when tested against calcium carbonate scale. A much less dramatic reduction in SI performance in the presence of ferrous ions was observed for barium sulfate scale formation under dynamic test conditions. Such interference in inhibitor performance can have major implications for the field application of scale inhibitor chemicals, leading to unexpected decline in production. The presence of ferrous ions has been shown to adversely affect scale inhibition of a number of different classes of SI chemicals, including poly(vinylsulphonate) (PVS) which has previously been reported as being iron-tolerant. This paper will describe the results of laboratory studies into this phenomenon, in which the nature and scope of the interference were investigated. Introduction Inhibitor performance in terms of the minimum inhibitor concentration (MIC) or the threshold concentration required to prevent scale is one of the most important aspects for scale control additives, equalled only by the challenge of effective placement and deployment in today's ever more complex production environments. The laboratory test protocols adopted throughout the industry are very similar and are based upon static "bulk" inhibition performance tests and dynamic "tube blocking" inhibitor performance tests. The importance of appropriate laboratory test procedures has recently been discussed in detail.[1] The conventional static "bulk" or "jar" test procedures commonly adopted are related to that described in the NACE standards TM 0197–97[2] and TM 0374–2001.[3] Such tests have been described in many previous papers for both examination of the factors controlling inhibitor performance and for selecting scale inhibitor products prior to field applications. These tests are used routinely throughout the industry for scale inhibitor selection and optimisation studies.[1] In addition to static "jar" tests, dynamic "tube blocking" performance tests are also routinely used for scale inhibitor selection in oilfield environments. Dynamic tests complement static tests by allowing different facets of scale inhibitor activity to be examined. For example, dynamic tests examine activity under much shorter residence times than static tests, and so can be used to highlight differences between nucleation and crystal growth inhibition effects. Dynamic tests also allow for the impact of scale nucleation and growth on the walls of the micro-bore tubing to be assessed under laminar flow conditions. There are a number of benefits associated with dynamic performance tests, which have been described in a number of previous publications as follows[1]:Dynamic laminar flow system, as in oilfield productionExamines growth and blockage of microbore metal coilsSystems can be examined under pressure. This allows for routine testing under;higher temperature conditions (> 100°C),in the presence of bicarbonate ions without loss of pH control Finally, and of more importance for this study, the sealed nature of the dynamic flow tests means that:Examination of systems in the presence of dissolved iron can be more readily achievable than in static tests, provided that feed brines are adjusted accordingly to minimize potential oxidation of Fe(II) - > Fe(III).[4]