|The drilling site, Adventdalen - Svalbard|
(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.)
Early in the 1980s, large oil and gas discoveries were made in the newly opened areas north of the 62nd parallel.
The Smørbukk discoveries on Åsgard in the Norwegian Sea only had small deviations from hydrostatic pore pressure in the reservoir, while wells drilled on the same type of prospect nearby encountered high overpressure. The overpressured wells were dry.
For many years after, it was therefore a commonly held belief that the overpressured areas on the Halten Terrace were not prospective.
Being able to predict pore pressure so as to plan and carry out safe drilling operations has always been important in the oil industry; however, in the 1980s, some geologists started using their understanding of pore pressure for prospect evaluation. In high-pressure areas, it is probable that hydrocarbons could accumulate in deeper traps even though the seal of the shallower structures was fractured.
The people that cracked the high-pressure code in the Norwegian Sea were eventually given the opportunity to drill deeper prospects, which e.g. led to Saga Petroleum’s Kristin discovery in 1997.
In the Barents Sea, there was hydrostatic pressure in the discoveries on Snøhvit, and virtually all other exploration wells, and hardly anyone imagined that the pressure conditions would offer up surprises when the areas further to the north in the Barents Sea were being explored in 1988.
However, unexpected problems occurred in the first well in the Fingerdjupet sub-basin, 7321/7-1, with large mud loss to the reservoir formations.
There was a powerful underpressure in the reservoir in all three wells drilled in this basin, 8 to 20 bar below hydrostatic pressure.
To my knowledge, naturally occurring underpressure of this magnitude had never previously been reported in offshore basins. What could these underpressures tell us about the geological development?
This also had consequences for exploration: All three wells contained traces of gas, but were dry. Did this have something to do with the underpressure? How is the discovery probability affected by such low pore pressures?
These events were what caused me to realise how important it is for a petroleum geologist to analyse and evaluate the pore pressure data.
|Depth to base of Cretaceous and water depth in the southwestern Barents Sea. The area without colour is the Loppa High, where the Jurassic and younger beds are eroded. The profile northwest of the Loppa High runs through the three wells in the Fingerdjupet basin. These boreholes are located in the deep part of the Bjørnøya trough where occurrences of gas hydrate are interpreted based on seismic data. Black dots: Exploration wells.|
Subsidence, hydrostatic pressure and overpressure
If you have a well at your holiday cabin, you can look down at the water surface. It is at the same level as the top of the groundwater.
The downward pressure in the water column increases according to the formula P= ςgz, where ς is the density of the water, g is the gravitational acceleration (9.81 m/s2) and z is the depth difference from the water surface to where the pressure is measured.
When the pressure going down in the subsurface increases according to this formula and is in equilibrium with the water table (or sea surface), we say that it is hydrostatic.
The water table in a well on land will be deeper during a dry summer than a wet autumn, and the groundwater table varies with topography.
The sea surface does not vary much, but the hydrostatic pressure in offshore geological formations is affected by the tide and will adjust according to sea level changes.
Overpressure builds up in a water volume when it is confined inside the pores of a sealed reservoir under subsidence as it is buried under more and more sediment load.
The available pore volume will decline when buried due to compaction of the reservoir. The compaction is caused both by the sediment grains being physically squeezed together and chemical processes that lead to dissolution and precipitation of minerals in the pore cavities.
The water volume in the subsiding pore volume will increase with thermal expansion, and the total volume of liquid could also increase with the addition of hydrocarbons from mature source rocks.
The principles are simple: As a result of the burial, the pore volume will decline while the volume of liquid will have a tendency to increase. If water can escape and there is communication to the seabed (open system), the pressure continues to be hydrostatic. If the excess water cannot bleed off quickly enough, overpressure will build up. The compressibility of water is so low that it only takes small volume changes to create major changes in pressure. If the reservoir is sealed and the pore volume shrinks, the pressure will continue to rise until it reaches the resistance to fracture (fracturing pressure) of the sealed layers.
As a rule of thumb, if there is a 0.5 to 1 per cent surplus of water in a closed groundwater system, one can expect the pressure to increase by 100 bar. This calculation takes into account compressibility of the rock. The pressure increase in the water will in fact be counteracted by some compression of the network of mineral grains in the reservoir.
Can erosion cause underpressure?
In a subsiding basin that is being buried by more and more sediment, it is thus natural for overpressure to build up. In central parts of the North Sea and in the Norwegian Sea, it is common to encounter overpressure with burial of 3000-3500 m or more.
But the geological beds underneath the seabed in the Barents Sea do not subside. Most of the Barents Sea shelf has a history of net erosion through the ice ages, and vast areas had their maximum burial 30 – 40 million years ago.
When subsidence leads to high pressure, is it then reasonable to expect that a removal of sediments will lead to low pressure? This would e.g. happen if the compacted pore network started to expand when it was unloaded. But one can see on both Svalbard and in the Barents Sea that a sedimentary rock that has been compacted will remain compacted even if the load is removed. Many of the processes that lead to compaction cannot be reversed.
In 2009, I performed a comparison of pressure data from the Barents Sea in the Glacipet research project to see whether there could still be correlations between unloading and low pore pressure.
One of the points of departure was that the Fingerdjupet sub-basin has been exposed to 2000 – 3000 metres of erosion since Eocene. This area has deeper erosion than the other areas drilled in the Barents Sea at that time.
Underpressure and tendencies for underpressure in a smaller scale had also been measured in certain other exploration drilling wells. The data I had was assessed in relation to the physical processes that occur during unloading.
- If water can move into or escape from the reservoir to the seabed, the pressure will be hydrostatic.
- Stress reduction and cooling will take place underneath a surface that is eroding, since the temperature increases with depth, and the unloaded sediment comes closer to the surface. The water volume in the pores will shrink due to the cooling. The reservoir will also contract due to cooling. The net effect of this on a sealed reservoir is not known, but was assumed to be minor, since certain wells in the erosion area have overpressure.
- The pore volume can expand somewhat because the stress caused by the weight of the overburden declines. Such an expansion effect is probably most relevant in unconsolidated sediment. In consolidated rocks, changes in stress can lead to cracking and make it easier for gas and liquids to move out of the reservoir.
- In a closed volume that originally contained both water and hydrocarbons, the volume of liquid in the pores will decline if gas can escape from the system. This will contribute to underpressure. There are many observations of residual hydrocarbons in the wells in the Barents Sea, which indicates that there have been extensive leaks.
One interesting result of the comparison for Glacipet was that observed underpressure of 1 – 2 bar occurred in reservoir volumes that were either small or had very low permeability and that there also was or had been a gas leak.
The explanation for these small underpressures could be that gas seeps out faster than the water flows back into the pores. A deficit of gas and water in the pores leads to pressure reduction.
There were many gas leaks in the Barents Sea during the ice ages, but in reservoir systems with high permeability and good communication to the seabed, the water pressure remains hydrostatic if the inflow of water is in equilibrium with seepage of gas. The figure shows that the major Jurassic aquifer in the Hammerfest basin has hydrostatic pressure. The pressure is seemingly somewhat higher than hydrostatic, but this could be explained with the pore water having a high salt content and therefore higher density than seawater.
|Plot of pore pressure against depth in relinquished exploration wells in the Barents Sea. Blue lines: Hydrostatic pressure with water with normal and high salt content. Red rings: Underpressure wells in the Fingerdjupet sub-basin (Fing) and Longyearbyen CO2 lab. Blue arrows: Pressure points from various local systems with weak underpressure.|
The figure shows that around the Loppa High and the Finnmark coastline, both the Jurassic and Upper Triassic stratigraphic sequences are in communication with the seabed over large areas.
These considerations could explain observations of minor underpressures, but did not fully explain the significant underpressures in the Fingerdjupet sub-basin.
A few years later, I received a phone call from Snorre Olaussen, currently professor of Arctic geology at UNIS, a call which opened for approaching this mystery from a new angle.
Continue to Part 2: Underpressure in Adventdalen to read more about additional geological factors in the north.