continue the quest for discoveries in the Gulf of Mexico -- that
sturdy backbone of domestic hydrocarbon production where the deep
water area now represents the only homeland "frontier" accessible
to the industry.
The Minerals Management Service (MMS) announced recently
that the number of rigs drilling in deep water (greater than 1,000
feet) had reached a record high of 45. Ten of these were deployed
in the ultra-deep water of more than 5,000 feet, where Unocal set
a new world record for drilling in 9,743 feet of water at Alaminos
This is an environment where drilling costs can soar
into the stratosphere and operational challenges are rife -- but
the potential payoff is huge.
"The deep water Gulf is a key exploration focus area
for BP," said David Rainey, exploration manager deepwater Gulf at
BP. "All the global analog work we've done suggests the evolution
of the deep water to look something like the shelf.
"We would expect that ultimately it could deliver
40 billion barrels of oil equivalent," he said, "but it's going
to be a hard slog, because it's incredibly complex."
Indeed, complexities are rife. Besides the complicated
trapping geometries and seismic imaging problems related to the
salt bodies, there's the problem of dealing with the high pressures
and low formation strengths indigenous to the deep water GOM.
In this environment of rapidly deposited, unconsolidated
younger sediments, the fracture pressure and pore pressure gradients
are exceedingly close. A small increase in pore pressure from one
section of the hole to the next can cause a blowout, whereas a drop
in fracture gradient might result in lost circulation.
The solution when drilling through such problematic
sections has been to set extra casing strings, which is a pricey,
time-intensive process. And it creates a telescoping effect that
can result in a smaller wellbore with a production string too small
to produce the hydrocarbons in sufficient volumes to pay for the
A whole new solution is being offered to operators
via innovative technology called Dual Gradient Drilling that has
the potential to revolutionize deep-water drilling, according to
some of the folks leading the effort.
David Adams, general manager at SubSea MudLift Drilling
Co., puts it in perspective:
"I'm reminded of a comment by the CEO of Global Marine,
Bob Rose, a few years ago, when he said 'this is the biggest change
our industry has seen since we put the blowout preventer (BOP) on
Dual Gradient Drilling relies on two fluid gradients
to provide the same bottomhole pressure (BHP) ordinarily achieved
with a single fluid gradient. In fact, the technology is premised
on changing the longtime methods for controlling wellbore pressures.
It's designed to allow operators to penetrate ultra-deep exploration
targets while achieving the desired casing size across problem zones
at shallower depths.
"Much of the cost of drilling deep-water wells is
fighting your way through the first 10,000 feet of sediment," Rainey
said, "where there's a very little window between pore pressure
and fracture gradient.
"Dual gradient drilling lets you thread casing strings
through that narrow window in a much more efficient manner," he
said. "You can start with a smaller wellbore, use less casing strings
and have less 'flat' spots in the drilling program.
"It all becomes much more efficient." Rainey said.
Here's the blueprint:
In single gradient drilling, a single mud column
extends from the rig floor to the bottom of the hole, resulting
in a hydrostatic BHP. Pressure gradients are referenced to the rig
Using dual gradient technology, BHP is maintained
via the combination of two fluid gradients:
- A slightly heavier mud column than with single gradient that
reaches from the mudline to total depth.
- Seawater that fills the annulus from the mudline to the rig
- A subsea pumping system installed on the seafloor provides
the energy to lift the mud from the annulus back to the surface
in the riser return lines.
All gradients are referenced to the seafloor, and
the margins between fracture gradient and pore pressure are much
greater while drilling the well.
The water, in effect, is moved out of the way, and
the well is "tricked" into drilling as though the rig were on the
Sometimes erroneously referred to as "riserless"
drilling because of the initial interest in the early 1960s on eliminating
the riser, dual gradient drilling took on a sense urgency in the
1990s with the advent of several significant deep-water discoveries
in the GOM. Deepwater drilling rigs were in short supply, and there
was a need to extend the capabilities of shallow water rigs.
The dual gradient concept appeared to offer a way
for smaller rigs to move into deeper water by reducing riser weight
and station-keeping requirements as well as mud volumes.
The main driver, however, came to be the narrow margin
between pore pressure and fracture pressure gradients, prompting
three separate industry groups to begin developing dual gradient
At the forefront of the effort to make the technology
a reality is the SubSea MudLift Drilling (SMD) Joint Industry Project
(JIP), which recently completed the world's first subsea field test
of a full-scale dual gradient drilling system. It took five years
of effort and $50 million dollars to accomplish this feat.
The SMD JIP was formed in 1996 with Conoco as project
administrator and Hydril as project designer. The 22 participating
operators, contractors and service companies set a goal to provide
a total solution for dual gradient drilling -- both hardware and
the methodology to safely and efficiently use the hardware.
Participation eventually contracted to eight companies,
primarily because of the focus on the GOM that came out of the program's
The recent field test was a partnership between Texaco,
Diamond Offshore and the SMD JIP. The test was implemented in an
intermediate water depth of 910 feet with a known pore pressure
environment at a ChevronTexaco-operated well in Green Canyon Block
The test rig was Diamond Offshore's Ocean New
Era, a second-generation semi-submersible with 1,500-foot water
depth capability. The rig was modified to accommodate the added
weight, power requirements and ancillary equipment that the SMD
system installation needed.
The dual gradient SMD system is ultimately intended
for use in 4,000 to 10,000-foot water depths. The test venue allowed
evaluation of the operation without the high cost of drilling from
a larger rig with greater water depth capacity.
"We now have proof of concept in 900 feet of water
by drilling the world's first dual gradient well," said Ken Smith,
JIP project manager at Conoco, "and we also have proof of mechanical
integrity and design in 9,000 feet of water equivalent.
"Any changes to the design as it's actually deployed
in deeper water will not be significant and will be primarily at
the sub-component or detail level," he added. "For instance, if
this were a car, it's like 'do I want velour or leather seats,'
instead of 'do I want seats.'"
Besides participation in the JIP, ChevronTexaco and
BP are also helping to fund the DeepVision dual gradient drilling
project, which is a joint venture between Baker Hughes INTEQ and
Transocean Sedco Forex. The program currently is in the development
mode, according to Pete Fontana, director of business development
He said the final lab testing phase was expected
to begin in November and include hyperbaric testing of all system
Given its lengthy track record in the deep water,
it should come as no surprise that Shell International E&P is
heavily involved in the dual gradient drilling technology push.
"We started work on a dual gradient system in 1997,"
said Rome Gonzalez, project manager subsea pump project, "and we'll
test it as a whole system next year. We're building a prototype
for our own use."
While all three systems appear viable, there are
some significant differences, including the type of pumps and how
they're powered. Also, the Shell system removes large pieces of
"gumbo" and larger cuttings and discharges them subsea, whereas
the other systems carry all cuttings and fluids to the surface.
Gonzalez emphasized that only environmentally friendly
drill fluids are used, and the subsea discharge is in total compliance
with stringent MMS and EPA regulations.
A perusal of the list of benefits of dual gradient
drilling outlined by the SMD JIP leaves no doubt why these groups
are expending so much effort and money to develop the technology:
- Fifteen-to-twenty percent savings per well by using fewer casing
strings and drilling fluids.
- Ability to complete a well with a larger diameter production
string for high-performance reservoirs.
- Fewer lost circulation and well control problems for enhanced
margin of safety.
- Ultra-deep objectives can be reached in virtually any water
- Riser filled with water instead of mud, improving riser tensioning
- Water depth capability of smaller rigs may be extended.
- Friendlier to the environment because emergency disconnect
will not result in significant release of mud to the environment.
Finding the Comfort Zone
Although the focus thus far has been on the application
of dual gradient drilling in the GOM, the technology is applicable
in other environments where the mud weights are quite high and the
formation fracture pressures are relatively low. These include West
Africa, the Caspian Sea and Brazil, among others.
Still, this is an industry that is loath to embrace
change for the most part. Convincing operators to actually apply
dual gradient drilling technology is no slam-dunk.
"There's a tendency to try to extend conventional
technologies into the deep water instead of designing something
specific," said Adams at SubSea MudLift Drilling whose parent, Hydril,
owns the patents associated with the core components and the mud
lift pump the SMD JIP developed. "But this is not giving as much
flexibility as operators want in their developments, and that's
why a lot of deep-water projects in the Gulf may be uneconomical.
"There's a bridge that's needed to get the industry
comfortable with these technologies," he said. "Just putting the
mud pump on the seafloor concerns a lot of people.
"The perception now is that it's complicated, but
if you look at the concept of dual gradient, it's not that complex,"
Adams said. "In fact, I see similarities to the uptake for the now-commonplace
extended-reach and horizontal drilling, which the industry was slow
to adopt because of perceptions it was too complicated."
He cautioned that dual gradient drilling must be
a collaborative effort that is project-based, and emphasized it's
not a cookie-cutter approach where you can build a test kit and
snap it on every rig. Actual deployment of a system likely will
impact an AFE to the tune of $35,000 to $40,000 per day, he said.
However, the SMD JIP is based on the premise that utilization of
the technology can save perhaps as much as $5 million to $12 million
"Ultimately the people who get the most economic
benefit from dual gradient drilling technology are the people who
own the reserves," Adams noted.
"The industry recognizes the need," Smith said. "It's
difficult enough and sometimes impossible to drill the wells in
the deep Gulf of Mexico, and dual gradient technology is the solution
to drilling in ultra-deep water.
"To me, it's only a matter of time before this becomes
the way to drill in deep water."
His enthusiasm is matched by BP.
"Bring it on," Rainey said.