Crystal Growth Inhibition Studies for the Qualification of a Kinetic Hydrate Inhibitor under Flowing and Shut-In Conditions

One of the methods to control the formation of hydrates in oil and gas pipelines is the injection of kinetic hydrate inhibitors (KHIs). The accepted understanding is that KHIs slow down or interfere with hydrate nucleation, forcing an extended “induction time” (time to emergence of viable hydrate crystals) at a given subcooling. As a result, KHIs are commonly evaluated by measuring induction times in the laboratory. However, this experimental approach has some limitations, notably in that data can be stochastic due to the nucleation element, raising questions over reliability/transferability, with multiple repeats often required to establish clear trends. As KHIs also exhibit powerful growth inhibition properties, a new crystal growth inhibition (CGI) method for the evaluation of KHIs has been previously developed with the aim of providing a means to more rapidly evaluate KHIs in a robust manner. This method shows that KHIs induce a number of well-defined hydrate CGI regions with different growth rates as a function of subcooling, and these can be used to reliably evaluate inhibition performance on quite short time scales. In this work, we present the results of an experimental program for the qualification of a commercial KHI to be used in a greenfield development using this CGI method. The aim of the laboratory work was to determine required inhibitor dosage, investigate the effects of a corrosion inhibitor (CI) on KHI performance, and evaluate the potential for KHI inhibition during shut-in/restart, in addition to flowing conditions. The program focused on CGI methods for evaluation in addition to standard induction time measurements. A methodology to recreate pipeline flowing, shut-in, and restart conditions was also developed and used. The CGI approach was found to offer advantages in the speed of KHI assessment and provides a useful decision-making tool with respect to KHI field deployment. Data also correlate with and compliment traditional induction time results which still provide valuable information on the degree of “nucleation” inhibition offered on top of crystal growth inhibition. In addition to offering excellent hydrate inhibition under flowing conditions, results suggested the KHI could readily offer good protection for long periods of shut-in (e.g., >168 h at up to 15 °C subcooling) followed by restart, reducing or negating the need for depressurization procedures in the event of shut-in.