(Editor's Note: This article was originally published in Norwegian on geoforskning.no and again on the Norwegian Petroleum Directorate homepage, as part of the "Seven Mysteries in Norwegian Geology" series. PennEnergy staff has broken up the article into two parts for ease of reading. Re-published with permission by NPD. Part 1 can be found here.)
Underpressure in Adventdalen
The purpose of the Longyearbyen CO2-lab project, headed by UNIS, was to examine the possibility of injecting and storing CO2 on Svalbard.
Several wells down to a depth of 1000 metres were drilled, logged, tested and monitored.
The goal for CO2 injection is Jurassic and Upper Triassic sandstone.
Snorre Olaussen, who followed up the drilling, witnessed extreme pressure conditions. Well Dh-4 went through approx. 100 metres of permafrost and into sandstone from the Helvetiafjellet Formation that had groundwater with some overpressure.
The deeper sandstone layers in the Jurassic and Triassic are sealed from the Helvetiafjellet Formation by approx. 400 metres of shale.
Injection testing in 2010 showed that the pressure in the low-permeable Triassic sandstones was extremely low, up to 50 bar below hydrostatic pressure.
Alvar Braathen and his colleagues have described the results from the project in the Norwegian Journal of Geology. They also write about pressure conditions, and state that they have not arrived at a final explanation for the extreme underpressure.
When I talked to Snorre, he was very interested in the Fingerdjupet sub-basin. Could these areas of underpressure have something in common?
|Schematic profiles through the drillings at Longyearbyen (top) and the Fingerdjup sub-basin with the three wells 7321/7-1, 8-1 and 9-1. The blue dotted line shows the bottom of the permafrost. The underpressure is sealed by permafrost where the layers come up to the surface towards the Sassen Valley toward the ENE. If thawing or other processes at the bottom of the permafrost in this area create a deficit of water in the pores, it will affect the pressure conditions in the well. The profile in the Fingerdjupet sub-basin (cf. map of depth to chalk), shows schematic Jurassic and Upper Triassic reservoir rocks (yellow), depth to the bottom of the gas hydrate area (red line) and maximum calculated depth to gas hydrate during the last ice age (blue line). The black lines are faults.|
Permafrost and gas hydrate – important geological factors in the Arctic
When water freezes to ice, the volume increases by about eight percent. Ice that freezes in the pores of a sedimentary rock consequently takes up more space than the water and will therefore create overpressure and frost heaves.
Pingos are a type of gigantic frost heave, large mounds with ice in the core. They are formed where the overpressure bleeds off and ice builds up. If the pore water that freezes contains salt, the ice will be fresher than the initial pore water, which leads to concentration of salt in the remaining pore water.
|There are a number of pingo structures in the bottom of the valley in Reindalen on Svalbard, some of them have crater-like structures with melt water on the top. The pingos are typically a few hundred metres wide and approx. 30 metres high.
Aerial view, source Norwegian Polar Institute, http://toposvalbard.npolar.no/
If permafrost thaws, the resulting water volume will be less than the ice volume, and in theory underpressure could occur. Transitions between water and ice in freezing and thawing create much larger volume changes in the pore water than the other processes discussed in this article.
The pressure conditions in the drillings at Longyearbyen are so extreme that it is reasonable to suggest that processes related to permafrost contribute to creating the underpressure, while the sealing properties of the Jurassic shale contribute to preserving it.
The schematic profile above shows a proposal for how underpressure could occur.
If one were to verify this hypothesis, it would be interesting to compare the conditions in Adventdalen with data on pressure conditions under permafrost in Siberia and other areas in the Arctic region.
The Fingerdjupet sub-basin is located at such significant water depths that it could not have had permafrost during the last ice ages, but gas hydrate has been widely distributed.
The figure shows similarities between the two geological profiles with underpressure.
The ice on fire
A lump of gas hydrate looks like ice. It has a crystalline structure where gas molecules are trapped in a cage of water molecules. In nature, the hydrate usually contains methane, with a small percentage of heavier components such as ethane and propane. Other gases, e.g. CO2, can also form hydrates. If one were to thaw one cubic metre of methane-hydrate, about 160 cubic metres of methane would be released at atmospheric pressure. Methane hydrate is only stable at high pressure and low temperature. At atmospheric pressure, the gas will quite quickly fizzle out of the ice structure.
|Gas hydrate is not stable under atmospheric pressure. The methane gas that is trapped in the hydrate flows out and could ignite.
Naturally occurring gas hydrates have been extensively studied the last 20 years. They are regarded as a potential gas resource, and gas released from decomposing hydrates may be a factor contributing to greenhouse effects. In the early 1990’ies I was introduced to gas hydrates as a geological factor by Anders Solheim who was then a researcher in the Norwegian Polar Research Institute. He had discovered a field with crater structures in the Bjørnøya trough, using bathymetry and shallow seismic data. The km-size craters looked like giant pock-marks but were formed in solid Triassic sedimentary rocks. Anders published his results together with his coworkers in 1993, suggesting that the craters were formed by blow-out of gas from decomposing gas hydrates. This hypothesis has been supported by later investigations, and much larger crater areas have been discovered. This is described by Karin Andreassen and a number of co-authors in an article in Science from June 2017.
At locations with considerable water depths where hydrate is stable on the seabed, pingo structures will form, analogous to permafrost pingos. If pressure and temperature change, the gas hydrate core of the pingos will decompose and the result is a crater at the sea floor.
In the Bjørnøya trench, with a water depth of 400 metres and a temperature at the seabed that is nearly 0 degrees, methane hydrate is stable in the sediment at a depth of 600-700 metres. At maximum glaciation during the last major ice ages, the glaciers calved out at the edge of the shelf on the entire Norwegian shelf. The water pressure under the ice was great enough for gas hydrate to remain stable at the sediment surface.
Consequently, the crater fields suggest that large volumes of gas hydrate existed in the Bjørnøya trough, possibly limited to areas with significant methane seepage.
Gas hydrates and volume change
One of my colleagues, Oddbjørn Nevestveit, made a calculation of what would take place in a shallow gas field if temperature and pressure change so that gas hydrate becomes stable. Methane and water in hydrate form take up less volume than methane and water separately. When the gas converts into hydrate, the pressure will drop dramatically if the gas field is located in a reservoir volume with poor communication to the seabed or major groundwater systems.
|The curve shows the volume occupied by water and methane when 1 m3 methane is dissociated, at different burial depths. Vertical axis shows depth below sea level in meters. Methane hydrate is stable in deep, cold water settings, in sediments down to a few hundred meters below the sea floor. Assumptions: Pure methane hydrate, gas expansion factor equal to depth divided by 10, no methane dissociation in water.|
Such calculations indicate that large losses of fluid volume, and consequently- underpressure could be caused by gas hydrate formation. In the Fingerdjupet sub-basin, gas hydrate has existed in larger volumes, but it decomposed once the ice age ended. If the reservoir volume were completely sealed the entire time, the volume of gas and water would have returned to the starting point when all of the hydrate had thawed. If the pore volume in the sediment were then also preserved, the pressure would return to normal. But such complete sealing and preservation of pore volume is hardly realistic. The crater formation further east shows that large volumes of fluids escaped from the system, and it seems likely that underpressure in the aquifer could be preserved. More mapping and more complex models are required in order to understand more about these processes.
There is little data on the effects that formation and thawing of gas hydrate have on pressure and liquid flow in the subsurface. The processes are of significance for those examining the possibilities for recovery of gas from gas hydrate and for exploration and production of hydrocarbons in areas that have been exposed to glaciation. And – most likely – for those looking for explanations to the underpressures in the north.