Officials nearly double minimum flow rate estimate for gulf spill

A federal team of scientists and engineers released an updated estimate of the deepwater Macondo well flow rate, putting it at 20,000-40,000 b/d before June 3 when BP PLC cut the riser pipe, temporarily increasing the flow.

Paula Dittrick
OGJ Senior Staff Writer

HOUSTON, June 11 -- A federal team of scientists and engineers released an updated estimate of the deepwater Macondo well flow rate, putting it at 20,000-40,000 b/d before June 3 when BP PLC cut the riser pipe, temporarily increasing the flow.

The Flow Rate Technical Group (FRTG), led by US Geological Survey Director Marcia McNutt, said it continues work to come up with how much oil has been flowing since June 3.

On June 10, McNutt released the 20,000-40,000 b/d flow rate estimate. Previously, the team estimated the flow rate at 12,000-25,000 b/d (OGJ Online, May 27, 2010). The USGS and BP initially agreed upon a 5,000 b/d estimate following the Apr. 20 fire and explosion on the Transocean Ltd.’s Deepwater Horizon semisubmersible. It was drilling the Macondo well for BP and its partners. BP operates the well.

The estimates vary depending upon several scientific methodologies being used, McNutt said.

Retired US Coast Guard Adm. Thad Allen, who is the national incident commander for the spill response, said, “Developing accurate and scientifically grounded oil flow rate information is vital, both in regards to the continued response and recovery, as well as the important role this information may play in the final investigation of the failure of the blowout preventer [BOP] and the resulting spill.”

The US departments of Energy and the Interior also directed BP to provide precise differential pressure measurements inside and outside a lower marine riser package (LMRP) cap to allow federal scientists to develop another independent estimate of how much oil is flowing from the runaway well.

“Each of the methodologies that the scientific teams is using has its advantages and shortcomings, which is why it is so important that we take several scientific approaches to solving this problem, that the teams continue working to refine their analyses and assessments, and that those many data points inform the updated best estimate that we are developing, ” McNutt said.

Methodologies used
A summary of the independent scientific methodologies that are being used to develop assessments of flow rates follows:

• An Analysis of Pressure Readings Team, led by Energy Secretary Steven Chu, is analyzing pressure readings from the BOP stack and the riser to assess flow rates and how flow rates may have changed as a result of the riser being cut.

• A Plume Modeling Team is observing video of the oil-gas mixture escaping from the damaged well, using particle image velocimetry analysis to estimate fluid velocity and flow volume.

• A Mass Balance Team is using remote sensing data from deployment of the Airborne Visible InfraRed Imaging Spectrometer and satellite imagery to calculate the amount of oil on the ocean surface on a certain day. The team is correcting the value for oil evaporated, skimmed, burned, and dispersed up to that day and divided by time to produce an average rate.

• A Reservoir Modeling Team will describe the geologic formations as well as composition and pressures of the oil, natural gas, and other compounds that are being released. Using open-hole logs; pressure, volume, and temperature data; core samples; and analog well or reservoir data; the team will populate computer models and determine flow rate from targeted sands in the well as a function of bottomhole pressure. Its work will be completed later this month.

• A Nodal Analysis Team will use input from reservoir modeling (including pressure, temperature, fluid composition, and properties over time) and pressure and temperature conditions at the leak points on the sea floor, along with details of the geometries of the well, BOP, and riser to calculate fluid compositions, properties, and fluxes from both before and after riser removal. The nodal analysis team is continuing to work on independent estimates that will be completed later this month.

• Woods Hole Oceanographic Institution experts are working with researchers from other universities to measure flow rates after the “top kill” attempt ended and before the riser was cut. Using a remotely operated vehicle, flow estimates have been derived from three different view angles above the riser pipe and three different view angles above the BOP.

Contact Paula Dittrick at paulad@ogjonline.com.

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