High Pressure Rheological Measurements of Water-in-Oil Systems to Evaluate Gas Hydrate Agglomeration and Plugging Tendency under Transient Shut-In and Restart Conditions

Characterizing the ability of oils to naturally transport gas hydrates through the action of surfactant-like properties is critical when considering gas hydrate management strategies in the field. Especially important is the characterization of viscosity profiles under simulated transient field operational conditions, which present the highest risk for gas hydrate plug formation and are, yet, poorly understood. Water-in-oil (w/o) emulsions using two oils were investigated in two high pressure rheometers (with differing impeller geometries) using a transient shut-in/restart method to determine important system properties, including viscosity, yield stress, gas hydrate volume fraction, and water conversion to hydrate as a function of time. Rheological properties, such as storage and loss modulus, were used as part of a method to determine the impact of gas hydrate formation and growth during a simulated shut-in period on the important operational fluid properties (viscosity and yield stress) in a subsequent restart scenario. This type of oscillatory measurement during the shut-in period has not been previously reported to detect gas hydrate formation and quantify growth. It was found that increasing the water content of the water-in-oil emulsion leads to a significant decrease in the transportability of the formed gas hydrate slurry. An increase in subcooling temperature was found to increase the yield stress (due to increased gas hydrate volume fraction) and formation rate of gas hydrates in the slurry while decreasing the overall bulk fluid relative viscosity (due to the lower amount of unconverted water at high subcooling). Viscosity profiles obtained in both flowable and nonflowable regimes allow for a gas hydrate agglomeration and breakup mechanism involving dynamic gas hydrate particle/water droplet interaction forces. Finally, it was shown that gas hydrate formation and growth during a shut-in period lead to increased risk of plugging upon restart due to a ∼2–5 times increase in viscosity. The use of shear-controlled rheological measurements gives important insight into not only the bulk fluid properties of gas hydrate slurries in oil systems but also the mechanisms behind gas hydrate agglomeration and blockages.