November 23, 2011 / 5:31 PM / 8 years ago

Chevron oil spill fallout still being assessed-experts

* Spill illustrates risks in tapping offshore frontiers
    * One of two by U.S. companies in foreign ops this year
    By Kristen Hays
    Nov 23 (Reuters) - Chevron Corp's efforts to
contain an oil spill at its prospect offshore Brazil appear to
have worked, but it is too early to say whether the accident is
a major setback for offshore producers that push technological
limits to tap crude, experts said.
    "This is not something that's going to encourage people to
give kudos to the industry for safety," said Nansen Saleri, CEO
of Quantum Reservoir Management in Houston and former head of
reservoir management for Saudi Aramco.
    "It's definitely not in the positive column. How negative
it is depends on how Chevron and Petrobras manage
the situation," he said.
    Petrobras is Chevron's partner in the Frade project about
99 miles (160 km) off Brazil's shore, but Chevron is the
operator.
    Don Van Nieuwenhuise, the University of Houston's director
of petroleum geoscience programs, said Chevron's response shows
ability to tackle problems.
    "They did everything they could do and so far, from what I
understand, the amount of damage is minimal and they're working
very hard to fix it," he said.
    This week Brazil fined Chevron $28 million for causing the
spill, which the nation's National Petroleum Agency said
reached 200 to 330 barrels of oil per day at its height.
Chevron estimated a total of 2,400 barrels leaked.
    While that's much less than the more than 4 million barrels
of oil that spewed from BP Plc's  runaway
deepwater Macondo well last year, Chevron's accident
illustrates risks involved with tapping offshore crude at great
depths.
    WHAT HAPPENED?
    Chevron said the spill occurred because the company
underestimated pressure in the reservoir and overestimated
strength of the rock through which they drilled.
    The incident occurred on Nov. 7 after the drillbit hit an
unexpected pressure spike, causing oil and drilling mud to rush
up the wellbore, Chevron said.
    Chevron shut a blowout preventer atop the well to stop the
flow, but that force could have fractured brittle rock along
the wellbore, giving oil a sideways route out to natural upward
crevices, Van Nieuwenhuise said.
    Gulf of Mexico rock layers are generally more pliable,
prone to give in a similar scenario, he said.
    He said offshore producers study seismic and other data
about rock layer formations before drilling to avoid pressure
pockets.
    Sometimes engineers miss a pocket or it doesn't show up on
the data. Saleri said companies manage unexpected kicks by
circulating drilling mud, or a mixture of barite, clay and
water that is heavier than oil, and perhaps cement to hold back
the crude. Chevron said its mud was too light.
    "Particularly with gas pockets in shallow horizons, it
happens everywhere both in the states and overseas," he said.
"There are ways of responding to it. The blowout preventer is a
step, but it is a last resort."
    NOT THE ONLY ONE
    Similar incidents recently have affected other oil
companies elsewhere.
    A ConocoPhillips offshore operation in northern China's
Bohai Bay sprung two leaks in June.
    The first happened when the company injected water into a
well to boost oil production, overpressurized some reservoir
layers, and created cracks to a fault through which oil and
drilling seeped out. The company reduced injection operations
and the fault sealed itself, spokesman Rich Johnson said.
    The second leak, like Chevron's, occurred after
ConocoPhillips unexpectedly drilled into a pressure zone. The
company shut the well, increasing pressure in some higher
layers that opened cracks for oil and mud to seep out.
ConocoPhillips plugged and sealed the well.
    Overall, the company said about 700 barrels of oil and
2,500 barrels of drilling mud leaked into the bay over more
than two months.
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