Impact of lighter alkanes on the formation and dissociation kinetics of methane hydrate in oil-in-water dispersions relevant for flow assurance

Abstract

Various crude oil fractions (saturates, aromatics, resins, and asphaltene) affect the hydrate formation cumulatively during the multiphase phase flow in offshore flowlines. This study investigates the impact of chain length of saturated hydrocarbons (nC5, nC6, nC7, and nC10) on methane hydrate formation and dissociation kinetics in oil-in-water dispersions at 275.15 K and 8 MPa. Sodium dodecyl sulfate (SDS) has been added (at 0, 500, 1000, and 1500 ppm concentration) to mimic the natural surfactants in the crude oil. Methane hydrate formation kinetics without surfactant does not correlate well with methane solubility trend in liquid alkanes (this study) due to narrow methane solubility difference along with poorly dispersed oil droplets without surfactant and stochastic hydrate induction behaviour. Increased SDS concentration (up to ∼1000 ppm) enhances the hydrate formation during initial stages due to enhanced oil–water interface following a decrease for higher concentration (∼1500 ppm) due to possible disruptions in the structure of local water by the hydrophilic surfactant part. At fixed surfactant concentration (∼1000 ppm), the hydrate formation is enhanced with increasing carbon number (nC6 < nC7 < nC10) during initial stages (∼10 h) due to lower methane solubility (increased methane availability in water) along with synergistic mechanism of alkane and SDS surfactant, before achieving similar formation kinetics during later stages. During dissociation, the gas release has been observably delayed at higher concentrations (∼1500 ppm) compared to lower (∼1000 ppm and below), possibly due to hydrate crystals trapping by surfactant molecules and methane adsorption on the hydrophobic surfactant part. A faster gas release has also been observed for systems with higher alkane at fixed surfactant concentration, possibly aided by lower methane solubility upon gaining enough thermal energy.