Among their findings, the Berkeley Lab team found that reservoir permeability had a strong influence on oil flow rate. This graphic tracks oil flow rate, in barrels per day, as a function of reservoir permeability and gas-oil ratio in a model in which the pressure at the blowout preventer is 4,400 pounds per square inch. |
The
first two weeks of June 2010 were a blur for six scientists from the
U.S. Department of Energy’s Lawrence Berkeley National Laboratory
(Berkeley Lab). As the world focused on the ongoing crisis in the Gulf
of Mexico after the blowout of BP’s Deepwater Horizon Macondo well, the
scientists dropped everything to estimate how much oil was flowing from
the mangled wellhead.
The
clock was ticking: Their work would help assess the environmental
impact of the disaster, as well as develop ways to cap the well, which
had been spewing unchecked since April 20.
They
used some of the world’s most sophisticated numerical modeling tools,
developed at Berkeley Lab over the past two decades for applications
ranging from geothermal energy production to environmental hydrology.
Working
quickly and amid abundant uncertainties, they estimated that between
60,000 and 100,000 barrels of oil were flowing into the Gulf each day.
Their calculations were in line with a final estimate derived two months
later based on much more information.
Their
research is recounted in an article published in an online
early edition of the Proceedings of the National Academy of Sciences.
“We
were able to harness Berkeley Lab’s expertise in multiphase flow and
computational tools to quickly take on this urgent problem,” says Curt
Oldenburg, a staff scientist in Berkeley Lab’s Earth Sciences Division
and lead author of the article. Also on the team were fellow Earth
Sciences Division scientists Barry Freifeld, Karsten Pruess, Lehua Pan,
Stefan Finsterle, and George Moridis.
The
scientists were part of a group established by the National Incident
Commander in May 2010 to estimate the oil flow rate from the wellhead.
One component of this effort comprised scientists from five Department
of Energy national laboratories, including Berkeley Lab.
The
Berkeley Lab team first developed a simplified conceptual model of the
system despite a lack of knowledge about the flow path from the
reservoir into the well, reservoir permeability, and pressure in the
blowout preventer. They then developed a coupled model of the reservoir
and wellbore using a numerical program, called TOUGH2, which simulates
fluid and heat flow in porous and fractured media.
Their
simulations painted a range of flow rates, from a low of 60,000 barrels
of oil per day to a high of 100,000 barrels of oil per day. Their
initial estimates are in line with a final estimate established in
August 2010 by the entire group and based on independent analyses and
observations. It pegged the rate at 62,200 barrels of oil per day upon
initial blowout in April, tapering to 52,700 barrels per day just before
the well was capped in mid-July.
The
Berkeley Lab team’s modeling approach also allowed them to determine
the role played by various uncertainties. For example, they found that
the rate of oil flow greatly increased as the length of the well in
contact with the reservoir increases.
Surprisingly,
they also determined that oil flow rate is relatively insensitive to
the pressure at the bottom of the blowout preventer. Common sense
dictates that as pressure drops at the bottom of the blowout preventer,
the oil flow rate increases. Instead, the scientists found that the
lower the pressure, the more natural gas exsolves from the oil. Natural
gas interferes with oil flow and counteracts the pressure that drives
oil upward in the well.
