Overview of the Produced Water System at the Prudhoe Bay Unit — Alaska, North Slope

Abstract

The produced water system at the Prudhoe Bay Unit (PBU) is one of the largest in the world, currently designed to handle in excess of one million barrels of produced water per day. Due to the large scale of the PBU production facilities, a major effort has gone into all the aspects of planning and operating the produced water system.

During the early stages of production from Prudhoe Bay the relatively low volumes of produced water were disposed to a shallow, permeable formation at low pressure. As the fields production matured sea water injection was initiated for pressure maintenance. Water- flooding also utilizes produced water which has continued to increase relative to oil production.

In some areas, as the produced water rates increased, injection water quality decreased due to separation equipment limitations and solids buildup in vessels. Expansion to the produced water system has included additional pumps for greater injection capacity as well as additional separation equipment to reduce oil and solids in injection water. The separation equipment expansion includes skim tanks, hydrocyclones and gas flotation cells. Additionally, sand jetting equipment has been installed in some upstream facilities.

Several other significant issues are currently being addressed relative to the PBU produced water handling system, including the impacts of increasing bacteria levels and the disposal capacity for water not reinjected to the producing formation. There is a significant effort underway to determine the impact of bacteria on injection water quality. Potential operating constraints and possible additional regulatory restrictions on produced water disposal are prompting an evaluation of options for handling the produced water not reinjected to the producing formation.