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Providing coverage of Alaska and northern Canada's oil and gas industry
May 2008

Vol. 13, No. 20 Week of May 18, 2008

Shooting for more pay

Marathon Excape well technology evolves from Cook Inlet into wider application

Alan Bailey

Petroleum News

A number of years have passed since Marathon invented its Excape technology for completing wells in the challenging multi-sand Beluga reservoir of the Cook Inlet Kenai gas field. But since that time the technology has evolved and has found application in places outside of Alaska, Phil Snider, a Marathon operations engineer, told the Alaska Unconventional Gas Resource Forum on April 7.

Marathon developed the Excape technology to make the gas reservoirs of the Cook Inlet more viable by increasing gas production above what can be achieved using conventional completion techniques, Snider said.

The thin, somewhat discontinuous sands of the gas-bearing Tertiary section result in gas reservoirs in the form of stacked, multiple compartments.

In addition, the extreme variability of Cook Inlet geology makes it difficult to extrapolate the geology found within a well far out from the well. What’s at the pin point of a well penetration may bear no resemblance to what’s 20 feet away — the sand may become better or it may become worse, Snider said. And depending on the quality of an individual reservoir zone, the zone may require fracturing (or “fracing”) using pressurized fluid to break the rock and thus stimulate the flow of gas into the well.

In the 1990s Marathon would use conventional perforating and fracing techniques to cherry pick the better zones, Snider said (perforation involves punching holes in the well casing to allow gas to flow from the field reservoir into the well). “That really wasn’t very effective,” Snider said. “We went past too much pay.”

Exterior guns

A search for greater efficiency led to the Excape technology, in which the placement of multiple perforating guns on the outside of the well casing enables rapid perforation for multiple sand horizons — hydraulic control lines external to the casing enable operators on the surface to control the firing of the guns. The lines can later be used for operations such as reservoir pressure monitoring.

The guns shoot inwards into the casing, rather than perforating the casing by shooting outwards from inside the well. Placement of the guns outside the casing avoids the time and cost of going in and out of the hole with coiled tubing, wireline and other equipment during multiple perforating and fracing operations, Snider said.

At the same time, valves between guns at different levels in the well enable the isolation of each reservoir sand horizon during perforation and fracing. After completion of the perforation operations, the valves can be broken out using the coiled tubing that is used for certain types of directional drilling.

In a typical operation, an operator will place the well casing and then fracture each interval in turn, to prepare the well for production. As many as 16 separate intervals would be fractured within 24 hours, with the well going into production within 30 hours, Snider said.

Refinements

Following initial development of the Excape technology, refinements have included downhole technology to monitor bottomhole pressures.

“Because we’re running all these external devices anyway, it’s no big deal to, say, run a control line to get real time bottomhole pressures,” Snider said. “Those real time diagnostics really help us with the frac job.”

Also, by dealing with each reservoir zone individually during perforation and fracturing, it is possible to understand the production characteristics of each zone.

“In Alaska we frac every single zone … by itself, and truly if you look at the production logs they’re not all productive,” Snider said. “That really hurts the economics.”

Further work is needed to understand how to pick out the better zones, but part of that issue comes back to understanding the reservoir quality away from the well bore, he said.

Refinement of the technology over time has improved efficiency and safety, while also reducing costs. In fact, the efficiency of the basic Excape technique minimizes gas venting during well completion and that factor alone earned Marathon a U.S. Environmental Protection Agency award, Snider said.

Lower 48

After initial success in Alaska, Marathon adapted the technique for use in unconventional gas wells in the Lower 48. New applications included multiple perforations in horizontal wells — in one south Texas horizontal well the drillers used the technology to fracture eight zones in one day, Snider said. The technology has been used in horizontal gas wells in the Barnett shale, he said.

And the technology has proven ultra reliable with Marathon achieving a near-perfect firing success rate of 99.7 percent, Snider said.

“For new technology I guess I’d say that’s not bad,” Snider said. “In our horizontal wells we’ve had 100 percent success.”

Enhancements to the technology over the years have included the automatic removal of isolation valves during the perforation process. That has eliminated the significant cost of coiled tubing for breaking the valves manually.

Another modification allows the fracing to be spread across several perforated zones along the well, a technique that can prove important in the Barnett shale. In Alaska, an adaption of this technique allows two zones to be fractured simultaneously, thus improving the economics of the operation, Snider said.

Gas lift

In tight gas wells, where a reservoir zone at the bottom of the well may not be producing, Marathon sometimes runs a line down the outside of the casing to provide a form of gas lift to that lower zone.

“We’re trying to design these artificial lift systems to keep the well unloaded from the bottom,” Snider said.

And because the artificial lift system is outside the casing, it is possible to do remedial work in the well without having to remove the system.

Another system enables a zone that is producing water to be isolated. And now Marathon is in the process of building a perforated sliding sleeve system that enables the perforating, fracing, testing or isolating of zones in any order.

“We’re trying to make it where we can do all of the things remotely,” Snider said. “… We’re trying to build up technologies where we can do anything in any order.”

But the technology has already proved itself, he said.

“This technology works,” Snider said. “It has opportunities in unconventional (gas), particularly in the high-cost environments. … And we continue to try to improve it as we go.”






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