Development and application of a novel crystal growth inhibition (CGI) method for evaluation of kinetic hydrate inhibitors
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Gas hydrates can cause serious economic/safety concerns in oil and gas production operations. Recently, low dosage polymeric Kinetic Hydrate Inhibitors (KHIs) have seen increasing industry use as alternatives to traditional thermodynamic inhibitors (e.g. methanol, glycol). To date, KHIs have been primarily understood to work by delaying/interfering with the hydrate nucleation process, inhibiting the onset of hydrate growth for a significant ‘induction time’ (ti) period. If the induction time exceeds fluid residence time in the hydrate region, then hydrate formation/plugging is avoided. However, due to nucleation being probabilistic, induction time data measured in standard laboratory KHI evaluation studies are often highly stochastic, making KHI assessment problematic and time-consuming To address this problem, the primary aim of this project was to develop a crystal growth inhibition (CGI) based approach to KHI evaluation. In this technique gas hydrate growth and dissociation patterns in the presence of KHI polymers were carefully inspected to evaluate repeatability of features and the existence of any consistency between runs and transferability between set ups within KHI systems. Extensive studies using this method show that KHIs - rather than being solely ‘nucleation delayers’ - induce a number of highly repeatable, well-defined hydrate crystal growth inhibition regions as a function of subcooling, ranging from complete inhibition, through reduced growth rates to ultimate failure with increasing subcooling. These crystal growth inhibition properties, in addition to offering further protection against hydrate formation/plugging (e.g. if hydrate nucleation does occur), provide a means to evaluate formulations much more rapidly and reliably. These measured CGI regions have shown good correlation with traditional induction time data, meaning CGI methods can be used to both rapidly approximate ti patterns and support/confirm ti test results, speeding up the KHI evaluation process while giving greatly increased operator confidence in inhibitor performance. Furthermore in this project, the new approach has been applied for evaluating the performance of different types of kinetic hydrate inhibitors as well as assessing the influence of various other components (e.g. liquid hydrocarbons, salts and thermodynamic inhibitors) on KHI performance. Moreover, studies have been conducted on KHI evaluation in different hydrate structure systems (i.e., Structure I, structure II and structure H) systems in the presence of several different single, binary and multi-component gases. For this purpose in all experiments undertaken throughout this thesis, with the application of the newly developed CGI technique, crystal growth inhibition regions have been measured for different systems and from the extent of these regions, hydrate inhibition properties of each KHI system has been evaluated and analysed. Results of these studies proved that the pendant group of a polymer plays a major role on the KHI inhibition properties. Also investigations of different guest gas/hydrate structure systems using the new CGI technique indicated that guest/cage occupancy plays an important role in hydrate inhibition and different hydrate structure systems (e.g. s-I, s-II and s-H) are inhibited differently by the same KHI. For instance, PVCap performance was considerably superior in s-II and s-H forming systems compared to s-I forming systems (e.g. methane), supporting stronger polymer adsorption on s-II or s-H hydrate crystal surfaces. Also through the newly developed CGI studies, it was found that while the presence of NaCl enhances PVCap methane hydrate inhibition, a carbonate salt like K2CO3 can have a generally negative effect on PVCap performance. In addition to that, test on liquid hydrocarbons proved that the presence of these compounds can slightly deteriorate PVCap performance. Moreover, results indicated that the combination of thermodynamic inhibitors and PVCap show better performance than thermodynamic inhibitors alone although glycols generally acted as ‘top-up’ thermodynamic inhibitor with PVCap which was a much better compared to the performance of alcohols with PVCap.