Reliable and Repeatable Evaluation of Kinetic Hydrate Inhibitors Using a Method Based on Crystal Growth Inhibition

Abstract

Low dosage hydrate inhibitors (LDHIs)antiagglomerants (AAs) and kinetic hydrate inhibitors (KHIs)are increasingly used as an alternative to the traditional thermodynamic hydrate inhibitors for controlling gas hydrate problems in the petroleum industry. As nucleation inhibitors, KHIs induce an extended induction time (t i ) at particular subcoolings before nucleation of hydrate can proceed to growth. However, due to the stochastic nature of nucleation, gaining repeatable and transferrable induction time results can be very challenging and time-consuming. To overcome this problem, study on KHI crystal growth inhibition (CGI) has yielded a KHI evaluation technique which is considerably simpler than induction time measurements. Moreover, this study shows that KHIs create a number of well-defined hydrate CGI regions as a function of subcooling which are completely repeatable and transferrable between different setups. These regions range from complete inhibition (CIR), to slow growth region (SGR), and ultimately to rapid growth region (RGR) as subcooling increases, making KHI assessment more rapid and reliable. Furthermore, results revealed that a true induction time can only be measured for a relatively short-range of subcooling within the SGR and RGR. Likewise, hydrate nucleation induction time is impossible to measure in the CIR, supporting CGI studies which show that hydrate growth is completely/indefinitely inhibited in this subcooling range.

Experimental Determinations of the Complete Inhibition, the Slow Growth, and the Rapid Failure Regions of Methane Hydrate Formation in the Presence of Polyvinylpyrrolidone and Polyvinylcaprolactam Aqueous Solutions

“…This region is called the RFR. Conducting experimental tests with this procedure not only eliminates the stochastic nature of the gas hydrate nucleation step but also indicates the effect of KHI on the gas hydrate crystal growth region. − …”

“…These regions are the complete inhibition region (CIR), slow growth region (SGR), and rapid failure region (RFR). 27 Tohidi et al experimentally and theoretically studied the impacts of acid gases in gas mixtures on the performance of PVCap as a kinetic hydrate inhibitor. 28 Aminnaji et al conducted experimental measurements to examine the irregular dissociation for a variety of KHIs.…”

“…Mozaffar et al applied the CGI technique to define the CGI regions as a function of subcooling, that opposed to the induction time measurements, are completely repeatable and reliable. These regions are the complete inhibition region (CIR), slow growth region (SGR), and rapid failure region (RFR) . Tohidi et al experimentally and theoretically studied the impacts of acid gases in gas mixtures on the performance of PVCap as a kinetic hydrate inhibitor .…”

Hydrate Risk Management in Gas Transmission Lines

“…While the industrial deployment of KHIs has tended historically to focus on systems with limited, seasonal exposure to the hydrate region, a backup strategy of THI injection has continued to remain available. In these cases, an array of rocking cell tests are typically deployed to estimate KHI dosage requirements. − …”

High-Fidelity Evaluation of Hybrid Gas Hydrate Inhibition Strategies

“…This limits the number of repeated, independent hydrate formation events which can be measured in a practical time period and thus effectively precludes the rapid collection of large formation datasets in the presence of multiple inhibitors or inhibitor combinations. In an effort to increase reproducibility, some hydrate researchers have reduced the measured degree of stochasticity in KHI-containing systems by carrying out formation experiments in the presence of what are thought to be residual hydrate structures − (formed via an initial hydrate formation event followed by incomplete dissociation). However, these approaches do not allow for the characterization of a KHI’s ability to impede nucleation since hydrate structures already exist at the beginning of the measurement. , From an industrial standpoint, these techniques also do not allow for the accurate quantification of hydrate formation likelihood from the hydrate-free fluids that enter a flowline from a wellhead at elevated temperatures…”