Dual Gradient Providing a Solution

High Pressures, Weak Formations Cause Problems

Operators 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 Canyon.

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 well.

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 the seabed.'"

Dual Action

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.

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Operators 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 Canyon.

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 well.

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 the seabed.'"

Dual Action

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 floor.

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 floor.
  • 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 ocean floor.

First Steps

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 solutions.

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 initial phase.

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 136.

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.'"

Cooperation

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 DeepVision.

He said the final lab testing phase was expected to begin in November and include hyperbaric testing of all system components.

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 depth.
  • Riser filled with water instead of mud, improving riser tensioning requirements.
  • 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 per well.

"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.

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