Abstract
Gas hydrates typically form at high pressure and low temperature conditions where water and small gas molecules exist in sufficient concentrations. When these thermodynamic (temperature, pressure, composition) conditions are fulfilled in oil and gas production flowlines, gas hydrate plugs can form and greatly restrict flow, which can lead to severe safety, economic, and environmental consequences. One potential strategy towards hydrate management involves allowing hydrates to form, but mitigating their tendency to deposit on the flowline walls by deploying a low adhesion, protective surface coating on the interior of the flowline. Without deposition (and agglomeration), the hydrate slurry can be transported until it exits the thermodynamically stable region without impeding production.
In this study, two coatings were produced and evaluated to determine their effect on hydrate adhesion onto carbon steel surfaces. The coatings used in this study were a super-hydrophobic, anti-icing coating and an omniphobic and corrosion resistant coating. The coatings were applied to substrates in both pristine (no corrosion) and pre-corroded conditions, where the coupon was exposed to a 5 wt.% salt solution prior to coating. Hydrate/surface interactions were studied using micromechanical adhesion force measurements in both liquid and gas bulk phases. A model sII cyclopentane hydrate (forming the same hydrate structure that is typically formed in subsea flowlines) was utilized at atmospheric pressure for the liquid phase measurements, while the high pressure measurements were performed using methane/ethane mixed gas hydrate (also sII). Rocking cell tests were also conducted to evaluate the coatings at high pressure under dynamic conditions using methane/ethane mixed gas hydrates and a liquid loading of 70 vol.% (water + oil).
It was observed that applying the coating to a surface was effective at reducing the adhesive forces between hydrate particles and steel surfaces, especially in the presence of corrosion. This held for both the low and high pressure micromechanical force adhesion tests. Rocking cell tests additionally revealed a significant reduction in time to hydrate deposition at low water content under dynamic conditions. The initial results from these three apparatuses suggest that utilizing these coatings may be an effective hydrate mitigation strategy at low water content, and in flowlines which have undergone significant internal corrosion, but more research is necessary to characterize the hydrate/surface interaction when a higher water content is present.
