Combined effect of NaCl and sodium dodecyl sulfate on the mechanism and kinetics of methane hydrate formation in an unstirred system

Interest in studying an effect of the mixed NaCl + SDS additive on gas hydrate formation is associated with the direct use of saline waters and seawater in hydrate-based technologies for the storage and transportation of natural gases. We performed studies of methane hydrate formation without stirring using pressures of 8, 12 and 16 MPa for water, aqueous solutions of NaCl and SDS, and the mixed additive of NaCl + SDS. A 3 wt% NaCl solution, as a model for seawater, and a 0.1 wt% SDS solution were used. The mixed additive was 3 wt% NaCl+0.1 wt% SDS. Data on the methane solubility, induction time of methane hydrate formation, water-to-hydrate conversion and methane hydrate growth rate, as well as data on morphology of methane hydrate formed with and without additives have been presented. It was found that the mechanism of methane hydrate growth in NaCl solution under static conditions was determined by the morphology of hydrate film at the gas-liquid interface and was pressure dependent. A liquid penetrable hydrate film at the gas-liquid interface formed in the presence of NaCl at moderate pressures (8 and 12 MPa) that allowed capillary-driven hydrate growth with a high rate and water-to-hydrate conversion. At high pressures (16 MPa), methane hydrate growth was limited by formation of impenetrable hydrate film at the gas-liquid interface, that restricted hydrate growth. In contrast to the existing concepts that the addition of NaCl to SDS solutions enhances surfactant micellization and accelerate hydrate formation, we found that SDS micelles did not form under the studied methane hydrate-forming conditions with NaCl. Evidence for SDS hydrate formation in the NaCl + SDS solution have been presented. Mechanisms of gas hydrate nucleation and growth in NaCl + SDS solution taking into account SDS hydrate formation and permeability of gas hydrate film at the gas-liquid interface are discussed. • Addition of 3 wt% NaCl to water enhances methane hydrate formation kinetics. • Presence of micelles in NaCl + SDS solution has no effect on the methane solubility. • Evidence of SDS hydrate formation in SDS solution with NaCl is presented. • SDS hydrates influence on the methane hydrate induction time. • Efficiency of SDS as gas hydrate promoter reduces in the presence of NaCl.