Design and Operation of Gas Flotation Equipment for the Treatment of Oilfield Produced Brines
Authors: Sport M.C. 
Published: October 2008

To eliminate the disposal of produced oilfield brine into the Gulf of Mexico and consequently any possibility of polluting State or Federal waters, Shell Oil Company's Delta Production Division has designed and installed the first phase of a $1.8 million produced water treating facility on the Southwest Pass of the Mississippi River. This facility, which removes oil from produced brine in a two-step process of gravity separation and dissolved gas flotation, has replaced and centralized water treating facilities heretofore located on sixteen production platforms within the Gulf of Mexico.

This paper presents an evaluation of the gas flotation process as a method of treating produced oilfield brine for the control of pollution. Contained in this paper is background information on principles of dissolved gas flotation, results obtained during a six month field pilot test of flotation equipment and details of flotation system design and operation.

INTRODUCTION

Shell Oil Company's South Pass Block 24 and 27 Fields currently produce some 90,000 barrels of oil per day and 200,000 barrels of water per day from approximately 700 Shell-operated wells. Due to water influx in the bottom water-drive reservoirs of these fields, total fluid production is expected to increase rapidly during the next three-year period. With this increase in total fluid production, water production will increase approximately 100 percent to an anticipated maximum rate of 265,000 barrels per day by 1971. To maintain current oil production rates and eliminate any possibility of polluting public waters, a field-wide facilities modernization program is currently nearing completion in South Pass Block 24 and 27 Fields. This program as it relates to the handling and treatment of produced brine provides:three new free water knockout platforms which will receive and separate from hydrocarbon production all free water produced from sixteen production platforms in the two fields, anda new field-wide central water treatment and pollution control system located onshore to receive, treat, and dispose all water transferred from the free water knockout platforms.

This central produced water treating facility, which will have an ultimate treating capacity of 290,000 barrels of water per day, removes residual oil from produced brine by gravity separation followed by dissolved gas flotation. Flotation systems provided for the final treatment of brine insure that the quantity of oil in the plant effluent is sufficiently low to satisfy existing pollution control regulations. Subsequent to treatment by flotation produced brine is disposed into the Southwest Pass of the Mississippi River, produced sand separated in the facility is disposed into surface pits for subsequent use as land fill and recovered oil is dehydrated and sold.

THEORY OF GAS FLOTATION

The flotation process is a unit operation utilized in the separation of dispersed solids and immiscible liquids from a continuous liquid phase. Separation is facilitated by the presence of finely dispersed bubbles, resulting from the addition of a gas phase to the system.