Measurements of Hydrate Formation Behavior in Shut-In and Restart Conditions

Transient operations in oil and gas production can result in conditions with a high potential for the formation of hydrate plugs. In restart operations, the shear flow and the increased pressure can induce rapid hydrate formation possibly leading to a plug or severe flow reduction. In order to study favorable and unfavorable restart conditions, experiments were performed in a high pressure cell coupled to a rheometer. Hydrate slurry behavior was investigated under transient conditions. Experiments were carried out in three-phase systems containing mineral oil or crude oil, water, and a model natural gas mixture of 92/8 mol % methane/propane. Two commercial antiagglomerants were added in the tests. Experiments were conducted at varying water volumetric fractions (10, 30, 50 vol %), subcooling (6 °C, 10 °C, 15 °C, 16 and 18 °C), pressure (42, 56, and 70 bar), and mixing rates (100, 200, and 300 rpm). The viscoelastic behavior was observed in most shut-in and restart tests. The experiments showed subcooling as an important parameter that affects hydrate morphology. Also, experiments varying the rotation speed showed that the apparent viscosity was unaffected by decreasing the rotation speed, suggesting that hydrate particle/aggregate size was unchanged. However, increasing the rotational speed resulted in a decrease of the apparent viscosity, in the case without an additive, or an increase in the apparent viscosity in the case with an antiagglomerant. Results using crude oil, antiagglomerant, and high water cut did not show viscoelastic behavior at shut-in and restart conditions. Both antiagglomerants formed hydrate dispersions, indicating that a flowable hydrate slurry had formed due to the antiagglomerant effect.