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Team investigates Barrow gas hydrates East Barrow and Walakpa fields may have gas hydrate resources that are maintaining reservoir pressures after years of production Alan Bailey Petroleum News
For most Alaska rural communities escalating fuel costs have been triggering a crisis that threatens the continued economic viability of rural life. But not so Barrow, located at the northwest end of Alaska’s North Slope.
For many years Barrow has enjoyed the benefit of plentiful natural gas from three gas fields just a few miles from the city perimeter: the East Barrow, South Barrow and Walakpa fields. And it is possible that methane hydrate deposits in those fields could extend gas supplies for Barrow for many more years than people have previously expected.
Methane hydrate (often referred to as “gas hydrate”) is a crystalline substance that traps methane molecules inside a lattice of water molecules within a certain range of temperatures and pressures known as the methane hydrate stability zone. When decomposed the crystals yield about 164 times their volume of methane gas.
Because of the low temperatures in and below the deep permafrost of the North Slope, widespread areas within the methane hydrate stability zone are known to exist under the North Slope region. Gas hydrate deposits have formed under the central North Slope and scientists have speculated that more as yet undiscovered deposits exist elsewhere in the region.
Some of those deposits may exist in juxtaposition with free gas in the Barrow gas fields, Tom Walsh, principal partner and manager of Petrotechnical Resources of Alaska, told the Arctic Energy Summit Technical Conference on Oct. 16. The North Slope Borough and the U.S. Department of Energy National Energy Technology Laboratory are funding a research project investigating the possible existence of hydrates in the fields. PRA and the University of Alaska Fairbanks are doing the research, with assistance from an advisory committee of experts in gas hydrates and North Slope geology, Walsh said.
First discovery 1949 The U.S. Navy discovered the South Barrow field in 1949 during exploration of the Naval Petroleum Reserve No. 4, later to become the National Petroleum Reserve-Alaska. The East Barrow and Walakpa fields were discovered during U.S. Geological Survey exploration programs in the 1970s and 1980s.
The South and East Barrow fields, both east of the City of Barrow, are reservoired in the Barrow gas sands in a similar stratigraphic setting to the Alpine oil field, 135 miles to the east. The reservoir for the Walakpa field, southwest of Barrow, lies in the Pebble Shale unit, a major petroleum source rock for the North Slope.
All of the fields have been supplying gas to Barrow for heating and power generation.
By 2004 the South Barrow field had produced about 22 billion cubic feet of the originally estimated 32 bcf of gas in the field, Walsh said. The current estimate for original total gas in place at Walakpa is about 250 bcf, of which about 14 bcf have been produced, he said.
Puzzling But the production history for the East Barrow field is puzzling.
Although the original estimate of gas in place for that field was just 6.2 bcf, 8 bcf have been produced from the field, Walsh said. And, perhaps more significantly, during the years of production the reservoir pressure has not declined — normally the gas pressure within a field reservoir should drop as the field depletes.
People had thought that subterranean water was maintaining the pressure in the field but no breakthrough of water into the reservoir has ever occurred. The lack of water subsequently led to speculation about the presence of methane hydrates in contact with gas in the reservoir — as gas is withdrawn from the reservoir, the reduction in pressure might cause some hydrate to disassociate, thus releasing more gas into the reservoir and maintaining reservoir pressure.
Hence the current research program.
“We haven’t really seen that (water breakthrough) and that’s one of the reasons that this hydrate study was commenced,” Walsh said. “…The lack of a decline in the East (Barrow) field and some peculiar characteristics of the decline curve in the Walakpa field led us recently to want to reinvestigate this issue and see if hydrates are contributing now as an energy source to the free gas fields.”
Do they exist? The first step in the research, started in November 2006, has involved using existing information to determine whether it is feasible that gas hydrates exist in any of the three fields. In addition to assembling reservoir, fluid and gas data from previous studies and from well files, the team has been analyzing seismic data from the gas field area, Walsh said.
By piecing together knowledge about subsurface geologic structure and about the subsurface temperatures and pressures, it is possible to estimate the depth of the hydrate stability zone and evaluate how that zone might interact with field reservoirs — if the stability zone does not intercept any part of the gas reservoir, methane hydrate cannot exist in the field, Walsh explained.
It is also critical to assess the mechanics and timing of the petroleum system that gave rise to the Barrow gas accumulations and that may have resulted in the formation of methane hydrates, he said.
Promising results The research results look promising for the East Barrow field.
“Based on the salinities of the water and the gas composition that we’re seeing we do have a hydrate stability zone that intercepts the top of the reservoir and penetrates into the reservoir in the middle Barrow sands at the East Barrow field,” Walsh said.
In fact a structure map of the field indicates that there is about one square mile of the East Barrow reservoir inside the stability zone, he said.
Although the research team found several sets of temperature gradient data for the Walakpa field, the most recent data set leads to a conclusion that the base of the hydrate stability zone is at the crest of the currently mapped Walakpa structure. But there also appears to be plenty of potential reservoir acreage inside the stability zone, up dip from the producing reservoir — hydrates in that acreage could hugely increase the resource potential of the field.
“It would take about 4,000 acres of reservoir within the hydrate stability zone to double the reserves at Walakpa to say 500 bcf of gas,” Walsh said. “So that could be a huge impact. Even if it was 10 percent of that it would be a huge impact.”
On the other hand, the South Barrow field is unlikely to contain any methane hydrate.
“We didn’t see these kinds of results on the South (Barrow) field,” Walsh said. “The methane hydrate stability zone did not intercept the top of the reservoir in the south field.”
Next phase Having established the likelihood of methane hydrate associated with the East Barrow and Walakpa fields, the research team has now embarked on detailed reservoir characterizations of the two fields, using seismic data, well data and other information. For the Walakpa field the team is looking closely at the seismic data to determine whether there is enough reservoir sand updip of the producing field to reservoir significant volumes of hydrates.
The team will then need to use reservoir modeling to assess whether hydrate disassociation appears a feasible gas production mechanism in the fields.
Depending on the results of the reservoir characterization and modeling, the project may eventually drill a test well to core methane hydrates at one of the fields. But drilling is not currently funded and remains a long way off in the future, Walsh said.
However, establishment of a methane hydrate resource near Barrow could have a major economic impact in the region, as well as providing a significant contribution to scientific research, Walsh said.
“We’re on the very practical end of hydrate research,” Walsh said.
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