ExxonMobil’s Produced Water: What you need to know

With the review process for ExxonMobil’s Yellowtail project underway, and the environmental impact assessment (EIA) available for scrutiny, discourse has kicked up once again on the discharge of produced water into the ocean.

Here is what you need to know:

Produced water comes from a well where oil and gas is being produced. Sometimes called formation water, saltwater or oilfield brine, it tends to be hot and high in salinity, meaning highly concentrated with salt.  

Since oil production began at Liza Phase One in December 2019, produced water has come up with the crude. Two primary options are available for the disposal of produced water. It could be treated then discharged overboard, or it could be combined with seawater and re-injected into the well.

For all three approved developments (Liza Phases One and Two, and Payara), ExxonMobil had decided that it would be wiser to discharge the treated substance into the ocean. For Yellowtail, the decision is the same.

The EIA states that up to 225,000 barrels of produced water would be produced every day, to be treated and discharged.

“The overboard discharge of treated produced water at the FPSO location, is a common practice for deepwater oil and gas operations applied at many locations in the world,” ExxonMobil said.

What does treating produced water entail?

According to the Yellowtail EIA, oil and grease, and residual production and water treatment chemicals are potential contaminants in the produced water. It is also very hot.

Exxon has developed a quantitative standard for the treatment of the oil content in the produced water.

The EIA states that ExxonMobil plans to “treat to achieve an oil in water content of 29 mg/L (monthly average) and 42 mg/L (daily maximum)” using a produced water treatment system, prior to discharge. This, the company said, is in-keeping with the World Bank’s Environmental Health and Safety guidelines for Offshore Oil and Gas Development. 

ExxonMobil also said discharge will meet good international oilfield practice limiting increase in the temperature of the surrounding water. 

This is what the EIA has to say about the expected impact of the discharge into the ocean:

“Modeling indicates concentrations, the FPSO will discharge several primary effluent streams to the ocean. The FPSO systems associated with these discharges will be designed to ensure applicable discharge criteria are met, which may require treatment in some cases. Modeling indicates that concentrations of chemical constituents would be reduced to insignificant levels and temperature increases from cooling water and produced water discharges, will be less than 3 degrees Celsius (5.4 degrees Fahrenheit) within approximately 100 meters) approximately 328 feet) of the discharge point, resulting in a Negligible residual impact.”

In addition to meeting the World Bank’s standards, the produced water system ExxonMobil intends to employ meets or exceeds established standards for discharged water quality from the U.S. Environmental Protection Agency (USEPA), and the Convention for the Protection of the Marine Environment of the North-East Atlantic (also referred to as the OSPAR Convention).