The deep geologic horizons of the shallow-water Gulf of Mexico Outer Continental Shelf offer the best near-term opportunity for increasing US natural gas production.
While the shallow-water gulf OCS has been heavily explored, sediments at depths of greater than 15,000 ft in water depths less than 200 m are relatively unexplored. Only about 5% of all wells drilled on the OCS have penetrated below 15,000 ft subsea.
To be economic, deep gas finds on the shelf require larger structures and high flow rates.
But deep gas potential on the gulf shelf is sizable. The US Minerals Management Service has estimated the gulf’s deep gas resource potential at as much as 20 tcf. Recent deep gas discoveries on the shelf have provided completions producing as much as 20-80 MMcfd. The average deep gas discovery is about 20 bcf, with finds in the Norphlet trend averaging 105 bcf.
And the abundance of platforms and pipelines already on the shelf ensures that any economic deep shelf gas finds will flow to market quickly.
Technology challenges
In tackling the deep shelf gas opportunity, industry must overcome the major technology challenges associated with drilling wells encountering high temperatures, high pressures, and corrosive environments.
In deep shelf areas with salt intrusion, there are concerns with lost circulation of drilling mud systems when drilling through salt.
Bill Coates, president, Schlumberger North America, contends that, in many ways, drilling for deep shelf gas is more difficult than drilling in the deepwater Gulf of Mexico.
Accordingly, operators must weigh prospect size against drilling risk: “Deep shelf gas involves a complicated blend of well planning, pressure management, and completion techniques.”
Such complications can mean a costly dry hole. “These [extreme] pressures resulted in a premature P&A of the Blackbeard well,” notes David Mullen, vice-president, North and South America, Transocean Inc.
Mullen also cites as crucial technology hurdles the lack of knowledge about deep shelf gas reservoir characteristics, concerns over the drilling window on frac and pore pressure gradients, and the fact that depleted zones “further complicate the well-construction process, especialy with respect to the number of casing strings.”
Elliott Pew, Newfield Exploration Co. executive vice-president, exploration, contends, however, that technology is not the major hurdle in finding, developing, and producing deep shelf gas.
“Rather, costs need to decline to economically develop smaller fields on the shelf. We know pretty well how to find these prospects and risk them appropriately.”
Seismic concerns
With greater target depth comes the need for better seismic imaging, points out Steve Hadden, senior vice-president, exploration and production, Devon Energy Corp.
“For example, pore pressure affects our ability to develop deep shelf resources. With only a few scattered deep wells and poor seismic velocities, at these depths it can be difficult to properly design casing programs.”
He also notes that “legacy seismic data are not adequate to image the deeper section. Increasingly, however, new data are being acquired on the shelf with depth migration as a final product.
“There also is no risk reducer in the deep shelf. Traditionally, shallow shelf prospects carried low risk due to having amplitude-supported targets. With increasing depth, amplitudes diminish because the sand and shale seismic velocities become very similar.
Hadden also contends that the lease term for shelf blocks (5 years) is too short in order to better image deep prospects and let them mature to a drillable stage.
“Although the MMS has allowed term extensions under certain conditions, these conditions-and the [deep gas production] royalty suspension program-should be significantly upgraded, given current risks…and results of deep wells drilled to date.”
Drilling issues
Drilling high-pressure/high-temperature (HP/HT) wells in tandem with drilling through salt layers pose especially daunting technology challenges, according to Chris Oynes, MMS regional director, Gulf of Mexico.
“Ultradeep shelf operators and contractors are evaluating high-cost metal alloys such as nickel-chromium and titanium as options for downhole production equipment to withstand corrosion and HP/HT,” he says.
“Additionally, drilling through layers of sediments and salt combined with variations in HP/HT may require drillers to use as many as 10 different runs of casing to line the walls of a single well drilled to 30,000 ft subsea on the shelf. These wells may take more than a year to drill and cost in excess of $100 million.”
Dominion’s Rick Fowler contends that industry’s challenge on the deep shelf is to drill deep, high-pressure wells at a reasonable cost.
“The main technology hurdle involves developing the ability to predict formation pressures accurately before drilling,” he says. “With existing technology, we often must design for contingency casing strings that requier larger hole sizes throughout the well at significantly increased cost. We need enhanced drilling procedures and equipment that will allow higher penetration rates in the deeper rock.
“We’ve had some success with underreaming while drilling in this area. Managed pressure drilling has proven successful in other parts of the world and may be helpful in the Gulf of Mexico in the future.”
Downhole conditions also can present added risks, Hadden notes: “In addition to this play being in a high-cost environment, an increased level of operational risk is present. This additional risk component must be factored into the overall risk profile of the play.
“Finally, issues such as rig and service ability can impede production of deep shelf resources. The current rig fleet capable of drilling these types of wells is limited.”
