WASHINGTON, D.C. – The U.S. scientific teams charged with determining the oil discharge rate of the Deepwater Horizon oil spill have completed a peer-reviewed report that assesses the methodologies to determine each technique's strengths and limitations.
The report does not change any of the numbers from the government estimate released in August 2010, which estimated the well's flow rate at 53,000 barrels of oil per day immediately preceding its closure and approximately 4.9 million barrels of oil over the course of the entire oil spill. The government estimate, based on pressure readings taken during the well integrity test and subsequent reservoir modeling, has an uncertainty of plus or minus approximately 10 percent.
The results are published today in the Flow Rate Technical Group's (FRTG) final report.
“To understand the flow of oil from the Macondo well, top scientists from government, universities, and national labs rapidly developed and deployed innovative techniques to measure flow rates in the deepwater environment,” said USGS Director Dr. Marcia McNutt. “The final, peer-reviewed FRTG report integrates the scientific knowledge acquired over the duration of the oil spill to better understand successful approaches to measuring deep-water flow rate. This report, and the excellent work of all the scientists who contributed to it, helps move the science forward for future applications in oil spill response.”
Given the lack of precedent and proven techniques for accurately estimating the release of hydrocarbons from a deepwater well blowout, the FRTG used all practical methodologies to generate an estimate, including observations at the ocean surface, video and acoustic observations from Remotely Operated Vehicles (ROVs), and reservoir and well modeling.
Using the August 2010 flow estimates as the primary benchmark for the assessment, the FRTG examined each methodology's timeliness, accuracy and ease of deployment as criteria for selecting the best approaches. The report concludes that the two best methods were acoustic and video, both of which involved making measurements from ROVs of the hydrocarbon plume as it exited the well in the deep sea.
The acoustic technique, which used an acoustic doppler current profiler to image the velocity of the moving fluid and a sonar to calibrate the size of the plume, yielded the most accurate estimate of flow rate. This method, however, required a more complex deployment of oceanographic equipment. The FRTG's video technique needed only high-resolution footage of the plume from a video camera, a standard tool for ROVs. Flow rate estimates based on video were available in a more timely manner than the acoustic technique; however, they were marked by larger uncertainty. The earliest video measurement attempts underestimated the flow rate as a result of lower quality video data and evolving understanding of the complex flow geometry. The report found that an on-site hydrocarbon sample reduced the uncertainty in the flow rate in both cases by resolving the proportions of gas and oil in the well fluid.
In comparison, the report concludes that any technique that relies only on measurements taken at the sea surface is likely to miss much of the oil that remains in the deep sea. Additional research is needed to understand the fate of oil in deep sea releases under a variety of oceanographic conditions.
In addition, the report found that attempts to model the flow rate from the well using industry proprietary data on reservoir properties and well flow paths resulted in a wide range of outcomes, some of which closely matched the August estimates. The report also recommends that blowout preventers in the future be better equipped with pressure gages, as reliable pressure readings would have provided additional evidence and greatly reduced the uncertainty in such models.
Dr. McNutt said she hopes that the scientific community will build on the results of the FRTG report and continue work on developing and advancing flow rate measurement techniques. She noted that a team of scientists within the National Oceanic and Atmospheric Administration is analyzing how airborne chemistry could be used to help measure flow rates from subsea spills. The results of that study are expected to be published soon.
National Incident Commander Thad Allen charged the FRTG to quickly generate a preliminary estimate of the flow rate, and, using multiple peer-reviewed methodologies, to produce a final estimate of the flow rate and volume of oil released. The Flow Rate Technical Group was led by United States Geological Survey (USGS) Director Marcia McNutt and worked in close cooperation with a team of Department of Energy scientists and engineers, led by Energy Secretary Steven Chu.
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