High-Fidelity Evaluation of Hybrid Gas Hydrate Inhibition Strategies

Abstract

In subsea oil and gas production, a transition away from complete gas hydrate avoidance to riskbased hydrate management has the potential to offer cost savings and improved viability for new developments. Rigorous characterization of hydrate formation probability (via the measurement of statistically significant numbers of independent hydrate formation events) represents a critical step towards accurate quantification of hydrate blockage risk. Such characterization is especially pertinent when deploying low dosage kinetic hydrate inhibitors (KHIs) which, unlike thermodynamic hydrate inhibitors (THIs), affect hydrate formation kinetics rather than thermodynamic stability envelopes. Here we demonstrate the use of a 2nd generation, Peltiercooled, high pressure, stirred, automated lag time apparatus (HPS-ALTA) to efficiently measure hydrate formation under conditions simulating a methane dominant natural gas asset. Over 2,500 hydrate formation events were measured using a low salt content brine, enabling the production of smooth, high resolution hydrate probability distributions in the presence of three inhibitor chemical additives and combinations thereof (a corrosion inhibitor, a KHI and a conventional THI). Beyond enabling rapid, high fidelity testing of potential inhibitor interactions, the results explicitly demonstrate the ability to effectively manipulate formation probability boundaries via a 2 combination of thermodynamic and kinetic inhibition effects. Such hybrid inhibition strategies can be used to achieve long induction times at operationally relevant formation temperatures (over 2 days at 2.5 °C in this study) and may be more beneficial and/or cost-effective than strategies focused on complete hydrate avoidance.

Experimental solid–liquid equilibria and solid formation kinetics for carbon dioxide in methane for LNG processing

“…When treated as a single dataset, the data from the low concentration mixtures are referred to as 59 ppm (the arithmetic mean of these concentrations). Bin widths were selected such that the resultant histograms exhibit a monotonic increase and decrease on either side of the mean subcooling, based on previous work in which greater numbers of formation events were used to generate the histograms 36,46,47 . Accordingly, the choice of bin width, δ T f , varies slightly between each dataset: In Figure 6, (a) δ T f = 1.5 K (59 ppm), (b) δ T f = 1.25 K (100 ppm), and (c) δ T f = 1.75 K (500 ppm).…”

“…While nucleation rates at constant temperature can be determined by fitting induction time formation probabilities to Equation (), such measurements are slow and limited in their ability to efficiently probe how J varies with subcooling. The measurement of solid formation along constant‐cooling ramps can more efficiently provide information about the dependence of nucleation rate on subcooling if analyzed correctly 46 . Recently, Barwood et al 36 demonstrated an efficient method of extracting methane hydrate nucleation rates from formation data measured along constant cooling ramps that were equivalent to those determined by constant temperature experiments.…”

“…Bin widths were selected such that the resultant histograms exhibit a monotonic increase and decrease on either side of the mean subcooling, based on previous work in which greater numbers of formation events were used to generate the histograms. 36,46,47 Accordingly, the choice of bin width, δT f , varies slightly between each dataset: In Figure 6, In addition to the formation data measured along the constant cooling ramps, 34 solid carbon dioxide formation events at a constant subcooling of ΔT = (8.0 ± 1.0) K were recorded for the 500 ppm sample. A ramp rate of 2 KÁmin À1 was used to cool the system down to this constant temperature.…”

Hydrate Risk Management in Gas Transmission Lines

“…Over the past three years, an array of these HPS-ALTA cells have been used to record thousands of hydrate formation events, across the range of accessible shear rates and compositions. , Efforts by Metaxas et al configured these cells to study induction time, which enabled a new probabilistic input for field-scale simulation with the above-described mechanistic model; the most recent efforts by Lim and co-workers , and Barwood et al have extended this induction time approach to study various KHIs as a function of concentration, with suggestions that systems containing KHI may not follow classical nucleation theory, which the HPS-ALTA system successfully established for pure guest–water systems . Together, the unique experimental capability and mechanistic model provide a means by which both the probability and severity of a hydrate formation event may be calculated.…”

“…As introduced above, the development pathway of low-dosage hydrate inhibitorsKHIs and AAshas historically relied on rocking cell tests, − which are only able to provide binary assessments of inhibitor effectiveness (i.e., the hydrate was or was not sufficiently severe as to prevent the ball from traversing the volume); such a method constrains insight into how nuanced changes to inhibitor molecules affect performance, and consequently limits the resolution by which research laboratories can compare new chemical inhibitor designs or solvation packages. The HPS-ALTA approach described above is one such example of high-fidelity KHI assessment, enabling new insight as to the molecular mechanism(s) , responsible for KHI performance; this method further enables a new generation of structure–function assessment, whereby the effect of minor structural variations on KHI performance can be optimized against ecotoxicity, biodegradability, bioaccumulation, or other key metrics.…”