In this work, the effects of an anti-agglomerant (AA) and salt (sodium chloride, NaCl) on water-in-oil (w/o) emulsion stability with and without the presence of gas hydrates is presented. The characteristics of gas hydrate formation and the hydrate slurry transportability were determined using a high pressure autoclave cell, with continuous mixing. The stability of the emulsions was independently measured by performing bottle tests, where the stability of the emulsion was determined by observing any evidence phase coalescence for a period of one week after emulsification. In addition, the stability of the emulsion with hydrate formation and dissociation was tested using high pressure differential scanning calorimetry (DSC). From high pressure autoclave studies, it was observed that the formation of a stable emulsion was shown to with the subject oils led to transportable hydrate slurries. Note however, it was observed that for this specific oil at 75 vol.% water cut and the addition of both AA and salt, a highly viscous mousse with viscosity of ~100 000 cP formed when the emulsion was saturated with methane gas at 950 psia and 20 °C. Even without the presence of hydrate, the formation of a highly viscous mousse may not be desirable in the field production system since it could render the system inoperable (effectively plugged). Interestingly, it was observed that the highly viscous mousse produced dispersed slurry of hydrates upon hydrate formation. Based on the autoclave studies, the absolute motor current (which indicates the relative viscosity of the system) at maximum amount of hydrate is 2 times lower of its original viscosity.
Riser Base Gas Lift is one of several methods successfully deployed that can enhance production and suppress severe slugging. With few moving parts, it is a solution that can be accommodated into existing infrastructure with minimal upgrades on existing facilities while not being as maintenance intensive as other solutions. The flow assurance design is critical to successful implementation and this paper details this process through the means of a case study. The case study in question is a three well, deep water subsea tie-back. The subsea system consists of an 8" × 12" pipe-in-pipe round-trip piggable flowline loop which will experience severe terrain slugging. Riser base gas lift had been implemented with success on adjacent fields at this facility; therefore it was again selected for deployment. From a Flow Assurance perspective, design considerations discussed are: Optimum Injection Rate/Operating Range: Flowrate requirements that will effectively mitigate slugging and will not violate any operating constraints.Low Temperature Mitigation: Design against brittle material failure.Erosion Control: Assessment of droplet erosion.Production Chemistry: Design to prevent formation of hydrates/asphaltenes in the production system due to injection of gas.Testing/Benchmarking: Since system design is heavily dependent on the accuracy of simulation modelling, details of equipment flow tests to validate performance are discussed.Recommendations/Support to Operations: Development of standard operating practices associated with gas lift.Execution Issues: Detailing design adaptations to system changes during installation etc.Revalidation: Review of actual operational performance against design intent. For this field, a novel technological solution of a gas-lift umbilical riser was selected. This configuration provides the best balance of operability, reliability and cost. The operating philosophy uses a fixed subsea orifice /topside choke to control the gas flow (rather than the conventional variable subsea choke). The orifice will regulate the injection rate even as the system pressure varies due to slugging, allowing the umbilical to act as an accumulator that dampens the pressure variations and allows for a larger bandwidth and smoother operation. Ultimately this configuration is envisioned to be less dependent on operator interaction.
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