Corrosion inhibitor interaction at hydrate–oil interfaces from differential scanning calorimetry measurements

The stabilities of water–oil emulsions and hydrate–oil dispersions are critical parameters for assessing the risk of hydrate plug formation. These stabilities may be affected by the injection of chemicals designed for hydrate inhibition, but little is known about the impact of corrosion inhibitors on the formation of hydrate plugs; some corrosion inhibitors are chemically similar to hydrate anti-agglomerants, and are often used in similar concentrations. A new experimental method for studying dispersion stability is presented in which successive cycles of hydrate formation and dissociation from water–oil emulsions are measured with a differential scanning calorimeter over a broad range of inhibitor (surfactant) concentration. As the emulsion is taken through successive temperature cycles in which hydrate is formed and dissociated, the amount of heat exchanged with the environment per cycle decreases as the emulsion destabilizes and average water droplet size increases. A surfactant that stabilizes the emulsion results in a smaller decrease in the measured heat exchange per cycle, and the extent of this reduction can be used to quantify the surfactantʹs efficacy in stabilizing the emulsion. For a water–oil emulsion with no surfactant added, the cumulative heat flow decrease over seven serial hydrate dissociation trials was approximately 70%. The addition of either centrylpyridium chloride (CPC) or cetyltrimethylammonium chloride (CTAC) surfactants at or above 1 × 10−5 mass fraction in the oil phase eliminated this heat flow decrease, indicating the emulsion remained stable through all hydrate formation and dissociation cycles. For mass fractions of either chemical below 1 × 10−5 the heat flow decrease was the same as for the baseline experiment. We propose that this threshold corresponds to the concentration in oil required to enable sufficient adsorption of the corrosion inhibitor to the water–oil and hydrate–oil interfaces for it to act as an anti-agglomerant.