Effect of dually combined under-inhibition and anti-agglomerant treatment on hydrate slurries

Abstract

Gas hydrates in deepsea pipelines can cause problems to flow assurance and also be a safety hazard. It can be expensive to prevent hydrate formation as this requires methanol injection in proportion to the water content of the system. As deepsea production facilities go to deeper depths of seawater and as the fields mature, the water content will increase, thereby reducing the economic viability of the production facility. This thesis explores the possibility of using an under-inhibited system to control hydrate formation along with an anti-agglomerant to prevent the hydrates from agglomerating together and plugging the pipeline. A high pressure autoclave cell was used as the primary apparatus for this work. The volume of hydrates formed and their morphology within the cell could be discerned by monitoring the motor current and pressure, temperature conditions within the cell. Different inhibition and anti-agglomerant conditions were used to explore the effects on the resulting hydrate morphology and the volume of hydrates formed. Experiments were also performed that investigated the effect of combining the inhibition and anti-agglomerant chemicals into the same system. Two experimental fluids were tested, mineral oil and a crude oil. The mineral oil was used in order to eliminate the uncertainty of using a fluid which has a lot of unknown chemistry present and to highlight the effects of the added chemicals. Results showed that there was no clear advantage to using a combined system of under-inhibition and anti-agglomerants. When the two chemicals were used, results did not show a significant improvement between cases that had just one chemical present and cases that had both chemicals present. Crude oil experiments showed that the usage of anti-agglomerants and salt at high water cuts would cause a highly viscous foam to form which would likely be unflowable in a pipeline. Hydrates that formed from this foam system, however, appeared to be very flowable. The foam did not form in the mineral oil experiments under the same conditions, showing that the natural chemistry within the crude oil causes the foam to appear. Results showed that both the mineral oil and crude oil system could be under-inhibited with 5 wt.% methanol and remain non-plugging. A slurry of hydrates would be formed from these experiments.