(114c) A New Application of Differential Scanning Calorimetry to Identify the Adsorption of Surfactants on Gas Hydrate Interfaces

Gas hydrates represent a critical risk in subsea flow assurance, where the aggregation or deposition of hydrate particles in the flowline may result in complete flowline blockage. In conventional designs, gas hydrates are avoided altogether by injecting thermodynamic chemical inhibitors, such as methanol. In the past decade, hydrate anti-agglomerants (AAs) have been developed to enable a new generation of subsea system design, where the risk of hydrate blockage is managed inside the hydrate stability region. The performance of each AA depends on both the chemistry and physical properties of the aqueous and hydrocarbon phases, which may traditionally be validated by qualitative autoclave or rocking cell tests. We present a new strategy for quantifying the stability of a hydrate-in-oil dispersion with and without AAs present, based on the use of a differential scanning calorimeter (DSC). The DSC cells are filled with a water-in-crude oil emulsion and pressurised to 1000 psi; cell temperature is cycled 10 times between 20 and -40 °C to repeatedly form and dissociate gas hydrate. As the dispersion destabilises, the amount of hydrate formed in each subsequent cycle decreases. This behaviour is quantified by a change in total heat flow, which is measured by the DSC. We have applied this measurement strategy to a water-in-crude oil emulsion containing representative industrial surfactants over a wide range of concentration. The DSC data provides evidence to quantify the maximum dispersion stability gained through the surfactant, as well as a relationship between the dispersion stability and chemical concentration. The minimum adsorption concentrations for the surfactant was successfully identified and compared to previously-reported estimates for hydrate systems. The results indicate that simple quaternary ammonium surfactants are active at the hydrate-oil interface at mass fractions above 0.001% in oil. A secondary suite of tests was performed to quantify the effect of brine salinity on dispersion stability. As the mass fraction of NaCl in water approached 10 wt%, the hydrate dispersion became stable; this result suggests the importance of using representative aqueous phase samples when evaluating chemical performance.