BP Works to Unlock Mystery of Hydrates
rigzone.com
by Tim Bradner
Alaska Journal of Commerce 11/8/2005
BP Exploration (Alaska) Inc. may drill a test well on the North Slope to confirm the presence of hydrates around producing oil fields, the company says. No decision on the well has been made, but planning is underway and it is the logical next step in a joint BP-U.S. Department of Energy research program on hydrates, officials with the company and the DOE said in interviews.
The test well would be what is known as a stratigraphic well that is drilled to gather geologic information to plan future drilling rather than to actually discover oil and gas.
Hydrates are underground structures of frozen water and methane that exist under certain temperature and pressure conditions. They mainly occur offshore in the seabed adjacent to coastal areas in many parts of the world, but they occur onshore in Arctic regions, within and just below shallow permafrost.
If gas can be produced from hydrates, it would be a major new source of energy. Huge volumes of methane, the main component of natural gas, can exist in a hydrate, and the U.S. Geological Survey estimates that as much as 500 trillion cubic feet of gas could be physically present in hydrates across onshore areas of the North Slope. This is many times the confirmed 35 tcf of proven gas reserves now known on the Slope.
The key question is whether gas from the hydrates can be commercially produced, and the North Slope oil fields could provide an ideal test lab for finding out, according to Bill Lawson, director of the Strategic Center for Natural Gas and Oil at DOE's National Energy Technology Laboratory.
There is existing infrastructure in the area, hydrates are known to be present at shallow depths and operating companies already know a lot about them, Lawson said. All the pieces are in place on the North Slope for a test of hydrate potential, he said.
"We're within the hydrate stability zone where these occur, they're in sandstone reservoirs with which industry is familiar, and industry infrastructure is there so we can get to the hydrates," Lawson said.
Mark Myers, the recently resigned director of the state Division of Oil and Gas, said the highly faulted nature of the local geology also creates effective traps in which gas could accumulate.
Myers said the resource modeling shows potential for as much as 100 tcf of in-place gas resources trapped in hydrates around the Milne Point, Prudhoe Bay and Kuparuk River fields, where existing infrastructure can be used.
Only part of the in-place resource might be commercially produced, however.
If the gas can be produced the additional reserves could help underpin the resource base for a planned $20 billion natural gas pipeline, Myers said.
The scenario the state foresees is production of free gas that has accumulated in traps below the hydrates, which exist within and just below the shallow permafrost layer on the Slope, Myers said.
What makes the Prudhoe Bay area unique is that local rock formations are highly faulted, creating trapping mechanisms to hold the free gas that has accumulated below the hydrate, Myers said. Gas from these traps could be produced using fairly conventional well technology, he said.
The production concept is that as gas is produced, pressure in the free-gas trap is drawn down, which would cause additional gas to come out of the hydrate into the gas trap. Hydrates exist in particular temperature and pressure regimes, and if one of these changes -- in this case the pressure -- the hydrate will begin to dissolve, releasing methane.
This mechanism may be at work in the North Slope Borough's small, shallow South Barrow gas field in Barrow, 180 miles west of Prudhoe Bay. Geologists have long been puzzled at why the field shows relatively little reserve decline despite years of production. Some believe the conventional gas reservoir is being replenished from hydrates occurring above the field.
BP itself is very cautious in its assessment of hydrate potential, however.
Gas could be produced from the shallow gas traps, but what isn't known is how quickly hydrates might replace the produced gas, says Scott Digert, BP's western North Slope subsurface team leader.
Digert says BP is pleased with the results of "desktop" work done so far on hydrates, which has focused on assembling known seismic and well data and developing better seismic interpretation techniques to identify hydrate locations with more precision.
BP and the DOE are considering a shallow stratigraphic test well that would confirm that hydrates are where its new interpretations predict they should be, Digert says. No decision to drill the well has been made or just where in a known hydrates area it would be drilled, but a "strat" well is the logical next step to work done so far, he said.
The work is also useful in planning conventional wells because hydrates can create high-pressure gas zones that are drilling hazards, he said.
An actual production test from hydrates might come later. This could involve drilling to produce directly from the hydrate rather than producing gas from a free gas trap below.
What's critical is learning how much gas can actually flow from the hydrate over time, he said.
If gas can be produced from traps below the hydrates it is important to know how fast the produced gas can be replaced from the hydrate, Digert says.
DOE's Lawson said the horizontal production wells now being drilled to tap viscous oil deposits on the Slope might be the type of well technology that can be used to produce gas from hydrates, particularly if the gas flows slowly.
A concern with hydrates is that if the methane is produced, the hydrate could dissolve and cause a sinking of the surface, which could pose risks to nearby oil and gas facilities. One idea being investigated by University of Alaska Fairbanks scientists is injecting carbon dioxide into the hydrate that, when frozen, would replace the methane in the ice structure, preserving it. This could be also a way of storing carbon dioxide produced in field operations underground rather than having it released to the atmosphere.
Lawson said hydrates could be widespread across the Arctic Slope. The continuous permafrost conditions that create hydrates at Prudhoe Bay occur across northern Alaska, which means that hydrates could be present from the Arctic National Wildlife Refuge in the eastern Arctic Slope all the way through the National Petroleum Reserve-Alaska to the west, he said.
Hydrates are also known to exist in Canada's Mackenzie River Delta, where a test program was previously carried out, as well as in Siberia. Hydrates also occur offshore, but Arctic onshore regions could be reached more easily and could hold huge volumes of gas, Lawson said.
The federal energy bill passed last August has two provisions to encourage hydrate development. The first is an extension of a five-year federal hydrate research program that will provide federal funds to support work by the DOE and BP on the North Slope. A second section authorizes relief from royalty for any gas produced from hydrates on federal lands.
While this wouldn't benefit hydrate gas produced on state-owned lands, it could be an important incentive for hydrate exploration on federal lands, such as those within the National Petroleum Reserve-Alaska and the Arctic National Wildlife Refuge. |
