Abstract
In recent times, studies on slugging have focused on the development of new and optimal slug mitigation techniques. This study focused on investigating a new proposed severe slug mitigation technique which involves combining self-lift and gas-lift. The self-lift slug mitigation technique is a unique approach that involves in-situ gas being tapped-off via a by-pass pipe along a pipeline upstream of a riser. The in-situ gas is then re-injected into the riser column via an injection point on the riser, to break liquid slugs within the riser column and mitigate severe slugging. This study adopted a methodology which involved validating the field data by comparing field pressure data with OLGA simulation based on input data from the field. The field case involved a deepwater oil field, consisting of well X1 and well X2 comingled along a 2712m pipeline section and a 1513m riser section. The pipeline-riser system in consideration in the case-study experienced hydrodynamic slugging, during the early life of the field; when the two wells comingled on the pipeline-riser system were operating at 3000 bbl/d. The 3000 bbl/d scenario (well X1 – 8.745 kg/s and well X2 – 25.13 kg/s) was initially modelled via OLGA. Furthermore, the 3000 bbl/d scenario was modified by moderating the flow rates at well X1 to 3.25 kg/s and well X2 to 12.13 kg/s until severe slugging scenario was observed, which was verified via pressure and holdup trend plots. Self-lift technique alone was able to moderate holdup within the riser column, however pressure trend at the riser column was observed to be over 20 bara (290.075 psi) which is the design pressure for the inlets of the separator. Further analysis of results obtained via the combination of self-lift and gas-lift at 2 inches by-pass diameter and 8kg/s gas-lift showed that pressure at the riser column was stabilized to approximately 20 bara (290.075 psi). This study showed that a by-pass internal diameter to pipeline-riser internal diameter ratio of (1 : 2) or less is crucial to the effectiveness of self-lift and gas-lift slug mitigation techniques combined. The new approach proposed in this study also has the potential of moderating the high compressive cost associated with gas-lift in deepwater scenario.
