Separating Oil, Natural Gas and Water

Oil generally comes out of the well mixed with water and, often, small amounts of natural gas. Similarly, natural gas often comes out of the ground mixed with water vapor and other gases. These various components must be separated before "pipeline quality" oil and/or natural gas can be sent to market.

To remove water and natural gas from oil, the mixture is passed through a device that removes the gas and sends it into a separate line. The remaining oil, gas and water mixture goes into a heater/treater unit. Heating helps to break up the mixture so that oil separates from water, which is more dense. Any remaining natural gas, which is less dense than oil, rises to the top. The gas is removed for either processing or burning; water is removed and stored for further treatment.

Additional separation of oil from wastewater is accomplished using hydrocyclones. Hydrocyclones spin the oil/water mixture, and use acceleration to separate oil from water. Water is forced to the outside of the hydrocyclone, where it is removed. Because most wastewater is very salty, it cannot be used as a water resource. Instead, it is injected back deep into the subsurface, usually into the same formation where the oil and water came from, helping force more oil out of the reservoir.

In some areas of the western United States, the produced water extracted with natural gas from coal beds may be suitable for agricultural, livestock, and wildlife use. This water is usually much less saline than produced water from deeper gas-bearing formations.

One promising new separation technology is the freeze-thaw/evaporation (FTE) process. Using a freeze crystallization process in the winter and natural evaporation in the summer, wastewater is separated into fresh water, concentrated brine, and solids. The fresh water can be used for agriculture or livestock, and the volume of waste requiring disposal is greatly reduced. This approach is useful only in areas with hot summers and cold winters -- like the Rocky Mountains.

Offshore, the salty water is tested to make sure that it does not contain any oil or other impurities that could hurt sea life, and is then put into the ocean. This practice has been studied extensively and is closely monitored to ensure that it does not harm marine life. The fact that most offshore platforms are rich in marine life attests to the safety of this approach.