A multi-year U.S. Department of the Interior Alaska Rural Energy Project which has been searching for coalbed methane near rural Alaska villages hit pay dirt in June and July when a test well at the Chukchi coast village of Wainwright found a significant gas resource in coal seams under the village.
Wainwright with a population of 500 to 600 depends on expensive diesel fuel for electricity generation — an ability to switch to natural gas as an energy source for the electricity power plant could represent a substantial economic benefit for the community. However, the results of the summer drilling are still preliminary in nature and it remains to be seen whether the production of coalbed methane at Wainwright is technically and economically viable, Art Clark, co-project leader of the DOI project, told the Arctic Energy Summit technical conference on Oct. 16.
“But we are able to say that there is enough coalbed natural gas contained in sub-permafrost coal seams underlying Wainwright and vicinity to serve as an alternative energy source,” Clark said.
Fort Yukon firstThe project, led by the U.S. Bureau of Land Management and the U.S. Geological Survey, and involving at various times Alaska’s Division of Geological and Geophysical Surveys and the U.S. Department of Energy, started out by drilling a coalbed methane test well in 2004 at Fort Yukon in Alaska’s Interior.
That well encountered coal seams but did not find viable quantities of gas. In 2005 a well at Franklin Bluffs, just off the Haul Road in the central North Slope, tested the potential to drill for coalbed gas in an area of deep permafrost, prior to drilling in the less accessible Wainwright area.
The project team used a lightweight Atlas Copco CS 1000 rig for both the Fort Yukon and Franklin Bluffs drilling. And in August 2006 the team barged the rig and other drilling equipment from Prudhoe Bay to Wainwright, in preparation for drilling in the Chukchi Sea village during the following summer.
Arctic Slope Regional Corp., the North Slope Borough and Olgoonik Corp. (the Wainwright village corporation) were partners in the Wainwright drilling.
Locate coalThe primary objective of the drilling was to determine the number, thickness and depth of the coals directly under Wainwright — it was already clear from surface rock exposures, seismic data and a couple of oil exploration wells drilled 25 to 30 miles away that coal existed under the village, Clark said.
But it was particularly important to discover how much of that coal lay below the thick regional permafrost zone. Production of gas from the permafrost is unlikely to be feasible.
“Even if there was a large amount of gas-bearing coal in the permafrost, more than likely that would not really provide a viable resource,” Clark said.
Drilling started in early June and reached a total depth of 1,613 feet. Continuous rock coring resulted in 95 percent core recovery below the surface casing, Clark said. That resulted in about 1,500 feet of core in about 250 boxes for geologic analysis of the subsurface.
And during the drilling, the well penetrated many coal seams.
“We penetrated 70 feet of net coal in 24 beds greater than or equal to 1 foot in thickness. There were numerous other thin coals,” Clark said.
Took coal samplesThe drilling team took from one to four 1-foot samples from each of the coal seams that were at least 1 foot thick, and then rapidly transferred the samples into sealed plastic canisters for measurement of the amount of gas desorbed from the coal.
“The quicker you can get it into your canister and start taking measurements, the less gas you actually lose, although once you make your calculations you do have ways to determine what that lost gas is,” Clark said.
The team stored the canisters at a steady temperature in a desorption trailer and periodically measured the amount of gas released from each coal sample. Sampling intervals started at five to 10 minutes, but increased to once every few hours or more once the desorption rate slowed. At the end of the drilling project the samples were transported back to the USGS facility in Denver, for further monitoring and testing.
“Most of these samples are still desorbing back in Denver. It will take a long time for all of this gas to come off,” Clark said. “… I’m taking readings approximately every 10 days now.”
The team has also taken samples of the desorbed gas and gas from the drilling mud for chemical and isotopic analysis.
“We also (now) know that the gas contained in the coal is almost pure methane,” Clark said.
Although the testing of the coal is still incomplete, preliminary results indicate a natural gas content of 50 standard cubic feet per ton from samples taken from a depth of 200 feet, increasing to 180 standard cubic feet per ton from the deepest coal sampled at 1,470 feet. There is a very direct relationship between the depth of the sample and the amount of gas it contains, Clark said.
After completing the drilling and coring, the drilling team ran some geophysical logging tools down the well, using a portable winch system. The logs enabled identification of the exact depths and thicknesses of the coals, and of the other rock strata encountered.
Pressure recovery testThe next step involved isolating a single coal seam to test how quickly the pressure in the seam recovered from a pressure draw down — time and the project budget limited this type of testing to just one seam.
“Our coalbed of primary interest, our thickest coal, was at 1,250 feet,” Clark said. “We took four samples from that coal and they’re averaging out at a little over 150 standard cubic feet of gas per ton of coal.”
The team isolated this 7.5-foot seam by placing nitrogen-filled inflatable packers in the well above and below the seam, with a slotted pipe placed between the packers adjacent to the coal. A pipe through the upper packer connected the well hole within the seam to the surface. By using high-pressure air to drop the pressure in the pipe, the drill team could draw water from the coal seam to the surface. By then measuring the pressure recovery in the seam over a period of 12 hours, it was possible to calculate the permeability of the coal.
The permeability, a measure of how easily fluids can flow through the coal, turned out to be approximately 100 millidarcies, a value that Clark described as “a pretty nice, mid-range number” that ought to enable gas production without the need for artificial stimulation.
After completion of the draw down and recovery test, the team used a Teflon baler to recover a water sample from the coal seam for testing. A total dissolved solids reading of 10,000 to 12,000 milligrams per liter indicated that the water is highly brackish. That’s a mid-range dissolved solid content for a coalbed methane field but certainly not drinkable — water produced during gas production would likely have to be injected into a subsurface rock layer, Clark said.
From a technical perspective, one of the more intriguing discoveries from the well was the existence of numerous gas-bearing coal seams inside the permafrost zone (there have been questions in the past about the existence of coalbed methane in permafrost).
“They did contain gas, somewhat surprisingly — they contained more gas than we thought they would,” Clark said. “But that gas more than likely is not going to be producible.”
Having completed the sampling and testing at the well, the team set an anti-freeze filled PVC well tube in the completed well for long-term monitoring of subsurface temperatures.
Estimated resourceSo, what does all this mean in terms of a natural gas resource for Wainwright?
Just taking the single coal seam used for the draw down and recovery test, multiplying up the numbers for the coal volume and gas content indicates that there would be approximately 1.24 billion cubic feet of gas in one square mile of the seam. Assuming that 50 percent of that gas could in practice be recovered that amounts to 0.62 bcf of recoverable resource, Clark said.
“That is … a very realistic and probably conservative recovery ratio,” Clark said.
Currently Wainwright uses about 450,000 gallons of diesel fuel per year for electricity generation. The energy content of that volume of diesel equates to about the energy content of 62,500 thousand cubic feet of natural gas, thus indicating that one square mile of the single coal seam could fuel Wainwright electricity generation for 10 years, Clark said.
Taking the estimated gas content from all of the sampled sub-permafrost coal seams results in an estimate of a 26-year power supply from a total of 21 feet of coal. And extending the area of extraction to four square miles could provide power for the village for 40 to 100 years, depending on how many coal seams are tapped, Clark said.
Much work remainsBut much work remains to be done before it will be possible to say whether coalbed methane production at Wainwright will be possible.
The project team has yet to determine the methane saturation of the coals, a critical piece of data in determining how much water would have to be pumped out before taking gas out of the ground. But an initial assessment looks promising.
“Our initial feeling is that these coals are highly saturated, maybe even fully saturated, with respect to gas, which from a production standpoint is a very good thing,” Clark said. However, actual saturation levels cannot be determined until isotherm analyses are conducted on fully desorbed coal samples. Because the coal cores are still desorbing, this won’t be done for at least several more months.
The next step at Wainwright would be a multi-well production test, to test the engineering challenge of producing gas and water through the permafrost and to measure the characteristics of the coal seams as gas reservoirs. It would also be advisable to drill to a depth of 2,500-3,000 feet to look for possible deeper coal seams and to locate a potential zone for produced water disposal, Clark explained.
The multi-well test would require a larger truck-mounted drilling rig than the CS100 rig used for the test well.
“We need to transport a larger rig up to Wainwright,” Clark said. “We have that rig in the Lower 48, which we can barge up and then fly into Wainwright in a C-130. … We’re already working with DOE trying to put together a multi-well test plan.”
But the project team will need to work with the project partners to secure funding for all of that — current funding does not extend to production testing, Clark said.
However, the preliminary results from the Wainwright test drilling seem very promising and show that a lightweight rig can successfully test for shallow subsurface resources in a remote rural location.
“The 2007 Wainwright project shows that shallow to mid-depth test drilling to assess local-use resources can be successfully and economically conducted in remote Arctic locations using this type of portable equipment,” Clark said. “… Similar equipment and techniques could be used for comparable studies in other frontier locations where little subsurface information is available.”