2 Challenges of Deepwater Drilling |
4. Deepwater drilling depths are sometimes defined
as greater than around 400m, while water depths of greater than
1500m are defined as "ultra-deepwater". Mr Malcolm Webb,
Chief Executive of the industry association Oil and Gas UK, told
I don't think there is an agreed industry definition
of what constitutes deepwater [...] When we started in the North
Sea over 40 years ago, depths of 100 or 200 feet [30-60m] would
have been regarded as deepwater, and as our abilities and technologies
have moved forward so the definition of what is "deep"
has moved with it.
5. Compared to conventional offshore drilling methods,
deepwater presents unique technical challenges related to greater
water depths, higher pressures, manipulating the extra long riser
pipe connecting the wellhead to the rig (over 1,500m in the case
of the Deepwater Horizon), extreme temperature gradients and added
costs. We found it interesting to note Mr Webb's observation that
the intervention in the well at the seafloor switches from divers
to Remotely Operated Vehicles (ROVs) at "about 500 feet [150m]",
as this seems to be an obvious threshold for deepwater operations.
6. The pressure in the well is controlled by ensuring
that the pressure of the drilling fluid (known as mud) in the
well boreknown as the bottomhole pressureis sufficient
to oppose the pressure from the oil, gas and water in the reservoir
(known as the formation pressure or the pore pressure). This prevents
fluids from the reservoir entering the well. Dr Tony Hayward,
BP's former Group Chief Executive, informed us "the pressure
on the drill pipe and the volume of [drilling] mud [...] are the
two most important parameters that are monitored and measured
on a continuous basis".
If the formation pressure is greater than the bottomhole pressure
oil and gas would enter the wellbore, and would lead to a blowout
if uncontrolled. The drilling fluid engineer monitors the formation
pressure and increases the density of drilling mud to balance
the pressure and keep the well bore stable. These active pressure
control systems are the first line of defence against losing control
of the well.
7. Mr Webb told us: "deep water brings some
particular risks with it".
Deepwater is characterised by young rock formations that differ
from shallow-water or onshore exploration. This is exemplified
by the narrow gap between the pressure of the oil and gas in the
reservoir and the typically small changes in pressure required
to fracture the rock around it (known as a low fracture gradient,
this is typically low under deepwater). Small increases in formation
pressure can therefore cause rock fractures to occur, destabilising
the borehole and potentially leading to an influx of gas and oil
(known as a kick) which if uncontrolled could lead to a blowout.
This inclination for fractures to occur is caused by an increased
weight pressing down on the oil and gas bearing rock formation
(known as overburden). This can necessitate using lighter drilling
fluid, which could potentially make it more difficult to control
the well, and a lighter cement mixture (used when fixing the pipe
casing into the borehole), potentially making the well more vulnerable
to the formation of channels around the casing up which gas could
8. Deepwater environments also present the combination
of low temperatures, high seabed pressures, gas and water that
cause "gas hydrates" to form. Gas hydrates are cages
of frozen water molecules with gas trapped inside and have a tendency
to bond with metal, resulting in blockages (as occurred during
BP's "top hat" operation to kill the Macondo well).
9. Even though the incident in the Gulf of Mexico
took place in deepwater, Mr Webb told us: "The depth of water
is not the critical element here".
Mr Roland Festor, Managing Director of Total Exploration and Production
UK, argued that: "Macondo has fundamentally nothing to do
This is because, once the blowout had occurredwhile the
depth of the water made the response to the incident more difficultit
was the fact that it was a high-pressure, high-temperature (HPHT)
well that made it more challenging to control.
UK Deepwater Drilling Activity
10. The majority of wells drilled on the UK Continental
Shelf (UKCS) are in water-depths of less than 100m, but oil and
gas exploration companies have increasingly been drilling in deeper
waters as reserves in more accessible areas run dry. Mr Webb told
us: "the deepest well so far drilled in the UK Continental
Shelf was at 6,000 feet [over 1,800m] of water".
According to DECC statistics on existing production installations
in the West of Shetland (WoS) Basin, BP has a platform on the
Clair field, and two processing ships in the Foinhaven and Schiehallion
Fields. The Clair began production in 2005, and has a water-depth
of around 140m. The Foinhaven has a water depth of between 400-600m
and the Schiehallion 350-450m. We heard from Paul King, Managing
Director of Transocean's North Sea Division, that: "The Paul
B Lloyd [a Transocean rig] is working for BP west of Shetlands
at the moment in up to 3,000 feet [over 900m] of water".
The Tormore and Laggan fields are being explored by Total and
lie in 630m of water.
Total is also searching 1,600m underwater in the Tobermory field,
north of the Clair field. Chevron is exploring the Rosebank-Lochnagar
fields in 1,115m of water.
In comparison, the Deepwater Horizon was drilling in ultra-deepwater
at a depth of 1,544m.
Operations on the UK Continental
11. The Macondo Well in the Gulf of Mexico was being
drilled into a high-pressure, high-temperature (HPHT) "over-pressurised"
oil and gas reservoir. Over-pressurised wells are hazardous as
the fluid in the reservoir can escape rapidly. Total pointed out
that while they have operational experience of such fields in
the Central North Sea, the geological conditions encountered WoS
are very different and "no significant overpressure"
has been encountered in that area to date. 
Dr Hayward told us: "there is nowhere where we are drilling
in deepwater [in the UKCS] and the reservoirs have high pressures
12. DECC figures estimate that the deepwater oil
and gas resource (which includes estimates for future discoveries)
of the West of Shetland and the less well explored West of Scotland
account for 15-17.5% of UK total resources. However, the resource
estimates for the West of Scotland (located north of the Outer
Hebrides) area are highly uncertain.
13. As part of Total's development of the
Laggan-Tormore area (600m in depth) in the WoS, a new gas pipeline
system is being built that will connect these discoveries to the
UK mainland. The new system has been "oversized" with
the expectation that further exploration and development in the
coming decades (including prospects that in isolation could not
justify the cost of this infrastructure) will take advantage of
the excess capacity. This development has started with the construction
of a new gas plant in Sullom Voe, the Shetland Island's oil and
gas terminal. Total hope that discoveries such as Tobermory, located
eight blocks north of the Clair field and in 1,600m of water,
will create opportunities for new fields and infrastructure that
"would further protect the UK's security of supply".
14. DONG Energy is one of the largest acreage
holders in the WoS region and a partner in Total's recently sanctioned
Laggan-Tormore gas development. DONG Energy is not currently drilling
as operator in UK territorial waters, but drilled the WoS Glenlivet
gas field in 2009 and has interests in a further six discoveries.
15. On 1 October 2010 the Government gave the go-ahead
for the first deepwater drilling off Britain since the Gulf of
Mexico incident. This consent was given to Chevron to drill
in the Lagavulin Prospect located in the West of Shetland area
at a depth of just over 1,500m (comparable to the Macondo Well).
Chevron and its partners plan to drill an exploration well in
the prospect with an expected duration of six months. In accordance
with UK regulation, an Environmental Statement for the well has
been in the public domain since March 2010.
Chevron has drilled 18 deepwater wells in the WoS since
1987 without serious incident, and is also exploring the Rosebank-Lochnagar
fields in 1,115m of water.
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