A good layer of oil

10/04/2007 It was lost when the gas was produced at the Frigg field, it has become a goldmine at Troll and a decision is eagerly awaited at Snøhvit. Thin oil zones have become a challenge.

Text: Janka Rom, Eric Mathiesen and Leif Erik Abrahamsen. Photo: Emile Ashley

Photo: Emile Ashley

The oil on the Norwegian continental shelf mostly lies in layers of sand. The layers can be anywhere from a few centimetres to several hundred metres thick. How thick an oil zone must be to be profitable to produce depends on its natural characteristics and the available technology. Oil zones as thin as seven-eight metres are presently produced using horizontal wells with several thousand metres of pipes suctioning oil from the saturated sands.

Hopes for Snøhvit
Lately, the most frequently mentioned field has been the Snøhvit field in the Photo: Emile AshleyBarents Sea. Snøhvit has a 14-16 metre thick oil zone with more than 80 million standard cubic metres (scm) of oil present in the reservoir. How much of this it is possible to recover depends on the production characteristics in the reservoir, the available technology and the oil price assumptions used as a basis. Preliminary estimates indicate approx. 100 million barrels of recoverable oil.

The oil in Snøhvit is time-critical. This means that it will gradually be lost as the reservoir pressure drops when the gas outtake starts in the autumn of 2007.

When the development plan for the Snøhvit gas was presented in 2001-2002, production of the oil zone was not included. The oil project was then calculated to have a break even price of USD 18-20 per barrel, while the price expectations were just USD 12-15 per barrel. This made the licensees decide to give priority to the gas and leave the oil where it was. However, the NPD pointed out that there was a need to collect more data from the oil zone. The discovery of the Goliat field could change the situation, and the NPD demanded new assessments.

New assessments, increased knowledge of the reservoir, improved well technology and higher oil prices have lead to a new review of the oil project. The Snøhvit oil alone can yield an income of NOK 5-15 billion. Coordination with the large resources present in the Goliat discovery could yield even more.

The present plan is to drill an appraisal well in the western part of the Snøhvit field in mid-2007 to collect more reservoir data. The operator, Statoil, will at the same time carry out a number of technical studies. The licensees in the Snøhvit field and the Goliat discovery have together started studies to look into potential synergy effects from coordinating the development and operation of the oil in Goliat and Snøhvit.

The Troll dilemma
There are several fields with thin oil zones on the Norwegian shelf. Sleipner Vest in the North Sea has two zones with oil which is difficult to get to. One of the wells on Ormen Lange, Norway's second largest gas field, proved an oil zone of at least two metres.

The two best known, however, are the oil zones in the Frigg and Troll fields in the North Sea.

The Troll field is the by far largest gas field Norway has, and one of the world's largest offshore gas fields, but it also contains large volumes of oil.

The field has two main structures, Troll Øst, mainly filled with gas, and Troll Vest, with a gas zone of up to 200 metres. The entire Troll field has a thin oil layer under the gas, but it is only thick enough to be commercially viable to produce in Troll Vest.

Under the Troll Vest gas lies a 14 metre thick oil zone. Under the oil lie large voumes of water. When the development plans for Troll were assessed in the first half of the 1980s, the option of recovering this oil was far from given. The well technology was mostly limited to vertical wells or wells with a maximum vertical deviation of 60 degrees.

The authorities wanted to produce the oil and pushed for getting an oil operator on Troll Vest. In 1986, Hydro was awarded this task, while Shell became operator for the gas development of Troll Øst. Later, Statoil took over as operator of the gas production at Troll.

Hydro faced several challenges related to this oil. Horizontal wells had to be drilled, something which had not been previously tested on the Norwegian shelf. Horizontal wells had been drilled in Australia and Russia, but the reservoirs could not be compared with Troll.

The reservoir sand in Troll is loose, like the sand you find at the finest beaches. This could cause problems with the boreholes collapsing. The sea depth in the area is 300 metres, and the oil therefore had to be transported from templates on the seabed to a production platform. On the basis of tests, Hydro, Sintef, Franlab and the NPD believed that these challenges could be resolved.

The NPD insisted that a horizontal test well be drilled, preferably on the Troll field. However, the first horizontal well on the Norwegian shelf was drilled at Oseberg Vest by Hydro. It was successful. A horizontal well was then drilled at Troll Vest. The well was supposed to do test production for one year. This well was also a success. The well made twice as much as it cost and contributed to covering the costs of the preliminary work before the test production was completed. Thus, it was proven that the Troll oil could be produced with a profit.

Of course it can be done
Drilling further horizontally would make it possible to recover even more of the oil, and by increasing the number of branches from each well, even larger areas could be drained of oil.

At present, more than a third of the Troll wells have several branches, some as many as six. The longest horizontal sections are up to 4000 metres long. More than one billion barrels of oil have been produced since production started in 1995. The remaining reserves have been estimated at 450 million barrels.

Over the last ten years, Troll has produced more oil than Ekofisk, Norway's largest field measured in resources in place.

The well paths at Troll are directed with a high degree of precision just above the contact between oil and water in the rock. As the oil is produced, water production increases. Treating produced water is very energy-intensive. To save energy and reduce environmental discharges, attempts are underway to separate the produced water on the seabed and inject it back into the subsurface.

Just 20 years ago, recovering the oil in Troll was considered not commercially viable. With new well targets and technology, it is presently likely that the operator will reach the goal of producing 1.9 billion barrels of oil or more. The technology and competence development has created enormous value, but it was also decisive to the increase in value creation that both Hydro and the NPD had enthusiasts onboard.

Troll oil is a success story because the authorities stuck firmly with their demand that the oil should be produced and the operator challenged the supplier industry to deliver good technology products. In 1998, Norsk Hydro was awarded the Norwegian Petroleum Directorate's IOR Award (Improved Oil Recovery) for its work to continuously find methods to increase the value creation from Troll oil. Baker Hughes INTEQ and Halliburton were given the IOR Award in 2006 for progressive development and application of advanced drilling and well solutions at Troll.

The history of Frigg
Frigg was the first field on the Norwegian shelf where a thin oil zone was proven, in the third largest gas field in Norway. The reservoir had a thin oil zone in addition to the gas. It was only five to nine metres thick and contained 125 million scm of oil, almost 80 million barrels. Frigg was proven in 1971. In the development plan, the operator Elf stated that producing the oil in the field was not profitable. In its annual report in 1973, the NPD wrote that more than 300 wells would have to be drilled to recover the oil with the technology then available. The next year, the development plan was approved, without oil production. From start-up of production in 1977 until the field shut down in 2004, Frigg sold more than 116 billion scm of gas. The oil was left in the reservoir and is presently considered lost. This is due to the fact that the pressure in the reservoir is reduced when the gas is produced, causing the drive mechanism needed to recover the oil to disappear. After 27 years of gas production, the oil is smeared all over the gas zone in the entire field as the gas pressure has been greatly reduced.

Several fields near Frigg have thin oil zones: Heimdal, Nordøst Frigg, Odin and Øst Frigg. All of them, with the exception of Heimdal, have been shut down without it being possible to produce the oil.

 

Updated: 14/12/2009