In this Case Study, AFS examines the impact of using live oil versus stock tank oil waxing potential laboratory measurements during flow assurance design. This overview summary is based on the paper “Flow-Assurance Aspects of Subsea Systems Design for Production of Waxy Crude Oils” originally presented by AFS staff at the SPE Annual Technical Conference and Exhibition (SPE-103242-PP).
Issue
During early project design, flow assurance laboratory screening measurements include stock tank oil measurements such as Wax Appearance Temperature (WAT) and pour point. If either a wax deposition or a gel formation (high pour point) risk is determined during screening, further stock tank tests are completed to assess wax deposition rates and/or gel restart pressures. The challenge is that using stock tank measurement results alone can result in the conservative over design of the system – particularly for gel forming fluids.
Process
As part of this case study, a range of waxing potential measurements were completed for a West African project with both stock tank and live oil samples. Subsequently, a flow assurance engineering study was completed which used either stock tank or live measurements as the basis of design. The impact of using live conditions on the measured waxing potential properties is shown in Figure 1.
Figure 1: Measured Stock Tank Oil vs. Live Waxing Potential Properties
As shown in this figure, the WAT decreases by 23ׄ°F and the pour point decreases by 19°F when the fluid pressure is increased from stock tank conditions to 700 psia. The WAT reduction obviously increases the amount of time the system can operate without having steady state wax deposition. Additionally, from a gel formation perspective, the gel (yield) strength of the oil at 700 psia is approximately 25% of the strength at stock tank conditions. The gel strength is directly proportional to the required pipe restart pressure. As shown in Figure 2, this gel strength difference between stock tank and live conditions can be the difference between a gel blockage and a system that can successfully be restarted.
Figure 2: Subsea Tieback Required Gel Restart Pressure vs. Fluid Pressure
Outcome
In summary, this study evaluated how the use of either stock tank oil or live oil waxing potential measurements impacted system design. To optimize the design of waxy systems, live waxing potential laboratory measurements are recommended. Additionally, the gel formation potential of the development should be evaluated via gel (yield) strength measurements. Even if gel formation is likely, if the required gel restart pressures are acceptable, gel formation can be managed without having to adopt extreme operating strategies.
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