Deepwater Horizon: Climate Change Impacts
Last Thursday marked the one-month anniversary of the Beyond Petroleum (BP) Deepwater Horizon oil rig disaster in the Gulf of Mexico just 40 miles off the Louisiana coast. A pipe rupture 5,000 feet below the ocean surface is spewing out crude oil and natural gas at an unknown rate. The scientific community has produced spill rate estimates that range from 5,000 to 100,000 barrels of oil per day. BP, after repeatedly denying the importance of obtaining an accurate rate estimate, has finally agreed to post a live video feed of the leak to help third parties create estimates.
The indirect environmental cost of a disaster like BP’s spill is hard to quantify. State governments are quick to estimate economic losses from affected fishing and tourism, but what about the disaster’s immediate effect on climate change? In this post we will offer a few calculations to demonstrate the magnitude of the spill’s environmental impact.
- Last Thursday BP reported oil capture (via an interception tube) at a rate of 5,000 barrels per day, but soon afterwards revised the amount to 2,200 barrels per day.
- BP estimates that half of the volume spilling out of the deepwater rupture is comprised of natural gas. BP’s interception tube was able to capture 15 million cubic feet of natural gas in a 24-hour period last week in addition to its reported capture of 2,200 barrels of oil.
- A federal task force dubbed the “ Flow Rate Technical Team” will try to produce a scientifically credible estimate of the spill rate this week once it has had time to model the spill.
- Steven Wereley, associate professor of mechanical engineering at Purdue University, modeled the leak based on a video from BP. Last Wednesday, Wereley told the House Energy Committee that about 70,000 barrels were spewing out of the rig rupture each day (± 20%).
Most experts agree that the Deepwater Horizon spill is more severe than the Exxon Valdez tanker spill of 1989. Exxon Valdez spilled 10.8 million gallons over the surface of the ocean off the Alaskan coast. At the lower end of Wereley’s estimate, the Deepwater Horizon rig could be spilling 56,000 barrels (by volume) per day into deep ocean currents.
After subtracting a conservative 5000 intercepted barrels per day (BP’s highest reported capture rate) and assuming 50% of the spill volume is methane gas, the spill could release about 25,500 barrels of crude to the environment per day. At this rate it would take just over 10 days for the rig to spill the same volume released by the Exxon Valdez. Thirty-six days have passed since the spill first erupted.
Natural gas, an odorless and invisible gas, is difficult to intercept once released to the environment. At 5000 feet deep, the ocean pressure is about 140-150 times the air pressure at sea level. If half of the leaked volume is indeed natural gas as BP claims it is, we can calculate that 25,500 barrels of gas is released per day. At deep ocean pressure and temperature (145 atmospheres and an assumed 0 °C), this amounts to about 280 tonnes of natural gas released to the environment each day.* Natural gas has about 20 times the global warming potential of CO2 over a 100-year span.
The amount of CO2 equivalent to the natural gas leak (280 tonnes * 20 = 5600 tonnes CO2 per day) is emitted daily by 390,000 average American cars (MPG = 20.4, traveling 11720 miles per year). It would take over 140,000 tree seedlings grown for 10 years to sequester the carbon in 5600 tonnes of CO2. All equivalencies were calculated with US EPA equivalency calculator.
Today, BP engineers are preparing a “top-kill” procedure to plug up the leak. If top-kill is successful, the leak could stop as early as this week. Such a procedure is commonly performed to stop sub-surface oil leaks, but never at depths as great as the Deepwater Horizon rupture. We can only guess when BP will successfully stop the leak. A worst-case scenario offered by BP chief operating officer Doug Suttles projects leak stoppage in August of this year.
*This is calculated using the ideal gas law and a molecular weight of 16 grams per mole (methane), assuming that all natural gas is in gaseous form. At high pressures and low temperatures the ideal gas law becomes less reliable, but the equation errs in the conservative direction. Real gas law calculations with van der Waal’s constants for methane yield a mass flow rate about 50% higher than the 280 tonnes per day calculated with the ideal gas law.
Note: The back-of-the-envelope calculations in this article are based on estimates provided by BP and Professor Wereley of Purdue University. These quick calculations help us appreciate the scale of the disaster, but they are not offered as facts. If you have better calculations or find errors in ours, we welcome you to comment!
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