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Well yields Tertiary core Cross-agency team recovered 400 feet of core from Franklin Bluffs well, found surprisingly low thermal maturity in coals of central North Slope By Alan Bailey Petroleum News Staff Writer
The drilling of a shallow well at Franklin Bluffs in the central North Slope in the summer of 2005 is leading to some interesting new findings, USGS senior research geologist Charly Barker has told Petroleum News. The findings came despite a break in the drill stem prior to reaching the planned maximum well depth of more than 2,000 feet.
The project formed part of a multi-year cooperative program involving the Bureau of Land Management, the U.S. Geological Survey and the Alaska Department of Natural Resources, to investigate the potential for coalbed methane as an alternative to diesel fuel as an energy source in rural Alaska. The Franklin Bluffs well enabled an initial test of Arctic drilling at a relatively accessible location on the slope, prior to drilling at a more remote site at the village of Wainwright on the Chukchi Sea coast in the next couple of years. The Franklin Bluffs project focused on the use of a lightweight rig for drilling through permafrost; looking for subsurface stratigraphic information; and testing for coalbed methane.
The Franklin Bluffs drilling site is on an old construction pad just off the Haul Road, 40 miles south of Deadhorse. The team used the CS 1000 drilling rig that it bought in 2004 to successfully drill a 2,278-foot well in Fort Yukon in Interior Alaska. Coring at intervals At Franklin Bluffs the team used an interval coring technique, involving coring for 200 feet and then reaming out the hole before rotary drilling for perhaps another 200 feet, USGS drilling supervisor Art Clark told Petroleum News.
“We were coring intervals and then we would open hole intervals,” Clark said. “What we were trying to do was catch core at various places in the permafrost.”
And that approach resulted in rapid progress.
“We were able to drill quite rapidly through the conglomerates of the Gubik formation and then on down through the permafrost in a few days,” Barker said.
Although the drillers were achieving really good core recovery, they weren’t finding much gas. So, at a depth of about 1,600 feet the team decided to stop coring and instead rotary drill down to about 2,000 feet before recommencing coring, Clark said. Because the lightweight rig can only handle heavy-duty steel drill stem down to about 1,500 feet the team decided to use lightweight core rod to continue the rotary drilling — the team had used this same technique to great success when drilling the Fort Yukon well. Broken stem Unfortunately one of the lightweight rods snapped at a depth of about 420 feet in the well. The exact reason for the break remains unclear but Clark thinks that washout (cavities where the side of a well has disintegrated) allowed excessive flexing of the drill stem. Then, the flexing may have exceeded the capabilities of the lightweight rod.
“They’re really not made to rotary drill — it’s a lightweight tough rod but still it can only take so much torque,” Clark said. “… I think once the drill steel started to torque up a little bit there was no sidewall stability”
It’s also possible that previous use of the lightweight rods at Fort Yukon had weakened the rods.
Although the team could detect a point in a steel rod at a depth of about 480/490 feet, the top of the stem appeared to have lodged itself at the side of a washed out portion of the hole. And, with the 6-inch casing at the top of the well limiting the size of the hooks that the team could use, numerous attempts to retrieve the detached string proved unsuccessful.
“We spent three approximately 24-hour days trying a variety of hooks and spears … and we never could get it,” Clark said.
Attempts at redrilling the lower portion of the well, both deflecting off the detached stem and deflecting off a cement plug, also failed.
“We could not deflect off and at that time we were pretty much out of time and funding,” Clark said.
Unfortunately, because the drilling rods remaining lodged in the well the team was unable to do any well logging. Good core Before the drill stem broke the team did manage to obtain about 400 feet of core from the well. That core, which is still being studied, should provide invaluable insights into some of the stratigraphy of the area.
“We can still do physical sampling, things like porosity, permeability studies, provenance studies and sedimentology studies,” Barker said. “We can get at reservoir quality and such for these shallower rocks on the North Slope.”
Barker thinks that information gleaned about the relatively young rocks from the Franklin Bluffs well may help develop offshore oil and gas plays.
“These Eocene rocks are more important as we go offshore towards the northeast … because the section’s quite thick there and it’s not been well tested,” Barker said. Low thermal maturity And the well encountered numerous coal seams, amounting to about 50 feet of coal and carbonaceous shale. However, vitrinite reflectance measurements showed that the coals are of surprisingly low rank, varying from lignite to material almost like peat.
“We hit a much less mature section than we expected — the bottom vitrinite reflectance was 0.29 which is in the lignite range,” Barker said.
That result conflicts with previous higher vitrinite measurements from coal in nearby wells, especially from the Pipeline State No. 1 well. Barker thinks that the higher vitrinite values may have resulted from making the measurements from well cuttings rather than from cores, as at Franklin Bluffs. Uphole cavings and other factors can cause people to overestimate coal ranks measured from cuttings, whereas with cores you have to report the lowest rank material, Barker said.
So, the team’s findings indicate that the thermal maturity of the area is a bit less than what people had previously thought, he said. Little gas A lack of gas in the permafrost at Franklin Bluffs did match previous findings in the area.
“We basically confirmed the result from two other wells that there’s very little gas in the shallow coals,” Barker said.
The coals that the team found contain less than 10 standard cubic feet of methane per ton. And, although the coals are still desorbing gas, the gas content is unlikely to exceed 10 scf per ton, Barker said.
This lack of gas in coal in the permafrost has become the subject of much speculation. Cores from the Hot Ice well south of Prudhoe Bay have demonstrated ice fractures and one popular hypothesis is that gas escaped through these fractures at some time in the past.
Barker thinks that the gas moved downwards to incorporate with gas hydrates at the bottom of the permafrost, rather than bubbling upwards as might at first be thought. The isotopic composition of the subsurface water indicates that the water has come from the surface, probably as a result of ice forming downwards from the surface at the onset of glaciation. The ice would have sealed the surface and then expanded, forcing water downwards and flushing the water through the coal, rather like waterflood in an enhanced recovery system.
But the low rank of the coal at Franklin Bluffs leaves major uncertainty about whether much gas ever formed in that area, Barker said.
The team would like to test its hypothesis about the downward movement of gas, using samples from wells to be drilled on the slope.
“We’re working with industry to get on some of the wells that are being drilled this winter in the drilling season and are in the hydrate zone and have coals in permafrost and below,” Barker said. Wainwright next The team also hopes to obtain more information about gas in the permafrost when it drills at Wainwright. Gas has shown up in conventional wells near Wainwright but the gas there also appears to be absent in the permafrost.
“What we want to do then is go and check this hypothesis and drill over in the Wainwright area,” Barker said. “We still want to go there because we know it’s a higher rank coal there.”
Also a prime purpose of drilling at Wainwright is to test the feasibility of producing coalbed methane as an energy source for the village.
But how will the team deal with the issue of the broken drill stem when it comes to drilling at Wainwright?
“It certainly isn’t going to have any long-term ramifications … as far as whether we do or do not do Wainwright,” Clark said.
Clark said that in future the team will probably not attempt to use lightweight rods to rotary drill new sections of the well. Below about 1,500, where the CS 1000 rig cannot handle heavy-duty rod, the drillers will advance the well by coring with lightweight rod. The drillers will then ream out the cored well using a rotary bit, also on lightweight rod — reaming does not impose the same stress on the rod as rotary drilling new well.
And the team is moving ahead to obtain all of the funding needed for the drilling project at Wainwright.
“Right now the drill rig is stacked at Deadhorse,” Barker said.
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