A study of the effect of pumps and desanding cyclones on oil droplets in produced water

Abstract

The production of petroleum resources includes co-producing water. This water contains oil products and needs to be treated before discharge to sea or re-injection into the reservoir. The amount of produced water increases with the age of a field, as does often the amount of solids. The pressure of the reservoirs will decrease with time. The primary equipment for treating produced water is the hydro cyclone. It’s operated with a lower limit with regards to oil droplet size of ~10 μm although smaller oil droplets have been experienced to separate. The hydro cyclone requires a certain differential pressure to perform at its maximum efficiency, and it is sensitive to solids. To protect the hydro cyclone a desanding cyclone could be installed upstream the hydro cyclone. A pump is often used to pressurize the water if necessary. The effect that this equipment has on the oil-water separation is a topic being discussed in the industry. The development in the industry, with increasing the life lengths of the fields and the need for development in areas of more stringent requirements, will benefit more knowledge regarding the effect of pumps and desanding cyclones on the oil- water separation. This thesis deals with these issues and presents the knowledge that exists with regards to pumps and desanding cyclones and their effect on oil droplet coalescence and break-up. This research has been done through contacts with suppliers, literature studies and evaluation of available test data related to this equipment. The literature survey and contacts with pump suppliers reveal that little work has been done to identify any effect of the pumps on the oil droplets. Both the literature survey and the suppliers agree that the eccentric screw pump generates the least droplet break-up, and that the centrifugal pump creates more shearing than other pump types. The offshore tests of centrifugal pumps and twin screw pumps indicate that the twin screw pump is gentler to the oil droplets than the centrifugal pump. While the tests of the centrifugal pumps indicate that increasing differential pressure increases droplet break-up, the twin screw pumps do not show such correlation. This indicates that the twin screw pump is more suitable, with regards to oil droplet sizes, for boosting the produced water to a hydro cyclone. The literature survey and the contacts with the desanding cyclone suppliers reveal that little work has been done to identify any effect of the desanding cyclone on the oil droplets. The experience of the suppliers is that the desanding cyclone gives coalescence, while the one revealed test show that the desanding cyclone isn’t damaging to the downstream separation. The offshore tests indicate that the desanding cyclone create coalescence or an insignificantly degree of break up and will not damage the oil removal performance of the downstream equipment.