Gulf of Mexico as compared with the Barents Sea and Lofoten
15/09/2010 It is highly unlikely that a potential blowout off Lofoten could have a comparable blowout rate as the blowout on the Macondo prospect in the Gulf of Mexico.
This is the conclusion of NPD´s comparison of petroleum-related conditions in the spill area in the Gulf of Mexico and in The Norwegian Sea, the waters off Lofoten and the Baretns Sea.
A total of about 70 discoveries have been made in similar or deeper waters as the Macondo prospect which is located in waters that are 1520 metres deep. This is is a common depth for drilling activity in the Gulf of Mexico today.
On the Norwegian shelf, the only wells drilled in such deep waters are in the Norwegian Sea, where three exploration wells have been drilled in waters from 1 495 to 1 721 metres deep. All of them were dry.
The Barents Sea is, for the most part, shallower than 300 metres, and there is little likelihood of discovering oil in the unopened deepwater areas outside the Barents Sea. The continental shelf off Lofoten and Vesterålen is narrow, The 50 prospects mapped by the NPD on the continental shelf off Lofoten and Vesterålen are mainly located at depths of up to 200 metres. Three prospects have been mapped in waters that are from 1 000 to 1 200 metres deep.
Depth of reservoirs
The reservoir rocks in the Macondo prospect in the Gulf of Mexico are located at depths of about 5 500 metres (water depth 1 500 metres and reservoir depth 4 000 metres). Reservoirs at this depth in the Gulf of Mexico are often under very high pressure. The pressure in the Macondo prospect is estimated at 825 bar and the temperature is 128 oC.
On the Norwegian shelf, we have experienced pressures up to around 1 000 bar in the Norwegian Sea (the Kristin area) and in the southern part of the North Sea (2/12-1 Mjølner in Sentralgraben). The Norwegian authorities designate areas as HPHT areas (High Pressure High Temperature) when the pressure exceeds 690 bar and the temperature exceeds 150 oC. Very strict requirements apply in these areas for the qualification of new companies, the companies’ organisation, competence and experience with this type of drilling.
The deepest exploration wells in deep water in the Norwegian Sea reach to a little more than 5 000 metres below sea level. The maximum reservoir depth here is 3 800 metres below the seabed. No reservoirs have been encountered in deep water in the Norwegian Sea with pressure as high as 690 bar. The temperature in the rocks in the Norwegian Sea rises more rapidly as depth increases compared with the Gulf of Mexico, and temperatures up to 140-150 degrees C have been recorded.
Because of the geological history in the area, we do not expect to find very high pressure on the shelf and the shelf slope in Lofoten-Vesterålen. Uplift of the shelf area and erosion have led to a significant reduction in pressure and temperature. This is also the case in the unexplored areas of the Barents Sea, cf. the report ”Geotechnical assessment of the petroleum resources in the waters off Lofoten, Vesterålen and Senja” (NPD 2010).
Preliminary information on the composition of the petroleum from the Macondo project indicate a gas-oil ratio of 500 Sm3/Sm3, i.e. a light oil. Light oil with high gas content will have higher flow rates than oil with little gas content.
Both oil and gas cases have been considered for the prospects in the management plan area outside Lofoten and Vesterålen. In the oil cases, we have estimated a gas/oil ratio of 50 to 150 Sm3/Sm3, which yields an oil with less gas than in the Macondo prospect. This is because the majority of the prospects off Lofoten are much shallower than the Macondo prospect. Shallower reservoirs have a lower gas content in the oil.
On the Norwegian shelf, the deepwater areas in the Norwegian Sea have the greatest potential for containing oil. Generally speaking, however, we expect to find mostly gas in the deepwater areas of the Norwegian shelf.
However, the sandstones in the Norwegian Sea have considerably poorer reservoir properties than sandstones in the Gulf of Mexico at the same subsurface depths. This may be linked to the fact that the reservoir rocks in the Norwegian Sea are much older, and the temperature is higher than in the Gulf of Mexico.
The combination of good reservoir properties, high pore pressure and crude oil containing gas entails high flow rates in the Gulf of Mexico. In the geographical area defined by the Management Plan for Lofoten and the Barents Sea, however, it is very unlikely that we will find the same combination of good reservoir properties, very high reservoir pressure, oil with high gas content and deep waters as on the Macondo prospect in the GoM. Therefore, a similar discharge rate in the event of a potential blowout off Lofoten is also extremely unlikely.