Big pay-off from 4D

06.01.2015
NOK 6 billion in extra revenue was the return from just over a decade of four-dimensional seismic surveys on Gullfaks. Operator Statoil believes that further advances with this technology could yield at least as much again over the next 15 years.

| Astri Sivertsen

Photo: Harald Pettersen/Statoil

Suitable field.
A complex reservoir makes Gullfaks very suitable for 4D seismic.
(Photo: Harald Pettersen/Statoil)

 

Half of all the oil produced from the North Sea field between adopting 4D in the late 1990s and 2008 can be attributed to the 4D survey technique, reports Tor Vegar Mårdalen in Statoil.

These big additional earnings derive specifically from 19 wells which are unlikely to have been placed where they actually were without the contribution of 4D data.

Although such surveys are primarily used to plan where new wells should be drilled, they also make it possible to avoid bad well positioning and to save on drilling costs.

“Being able to cancel wells on the basis of new 4D information is very valuable,” affirms Mårdalen, who is responsible for geophysical work on Gullfaks.

Given that a well on the NCS can easily cost several hundred million kroner to drill, it is easy to understand that such savings quickly add up.

The first three-dimensional seismic survey on Gullfaks was shot in 1985, before the discovery even had a name or production had begun.

Following a successful 4D pilot in 1995, survey ships have crossed the field at regular intervals after 1996. Since 2001, too, hydrophones and geophones have been installed on the seabed to collect other types of seismic data than can be picked up by streamers on the surface.

All this has generated huge volumes of information – Mårdalen estimates 2 000-2 500 seismic volumes each covering the whole main field – which are in constant use.

Right now, for example, the geophysicists are engaged in reprocessing the 1985 data. That was acquired before the arrival of GPS, and using other criteria for placing and registering shot points, vessel location and streamer position.

“But technological progress is making data quality ever better in terms of both navigation and seismic shooting,” Mårdalen explains. “New noise removal methods and algorithms have been developed, revitalising old data.”

 

"Being able to cancel wells on the basis of new 4D information is very valuable."

 

Behind

Norway was not the first to adopt 4D technology. The country was 10-15 years behind the USA, where the method had been used on land for many years.

Martin Landrø, winner of the NPD’s IOR prize for 2014 (see separate article), worked at Statoil’s research centre in Trondheim from 1996 to 1998 and headed the team involved in the early 4D phase on Gullfaks.

Three of the other scientists in the group moreover came from BP, which had experience of using this technology on Britain’s Magnus field 160 kilometres north-east of Shetland.

A few 4D surveys had been conducted on the NCS for well 2/14-14, which leaked 20 000 barrels of oil per day for almost year before operator Saga Petroleum managed to halt the blowout just before Christmas 1989.

Norsk Hydro had also carried out some tests on the Oseberg field in the North Sea, but Landrø says that Gullfaks was the first on the NCS where the method was adopted on a large scale.

One reason for choosing this field is that its sub-surface geology is complex. “If the reservoir were a single big tank, 4D surveys would have had less point,” explains Landrø.

“We knew that the potential was substantial. Extensive faulting on Gullfaks provides many opportunities for concealing reserves.” By comparison, adopting 4D technology at a later date on Statfjord added much less value than on Gullfaks precisely because this field is a big “tank”.

Nevertheless, Landrø observes that Statfjord’s eastern flank is a little more complex. “The more complicated a reservoir, the more use there is for such technologies as 4D.”

Another key reason for adopting the method was that it gives a strong seismic reflection (echo) from the top of a reservoir where oil exists, and a much weaker one for water.

In other words, a strong indication existed that the seismic surveys could detect the difference between oil- and waterfilled reservoirs.

This was precisely what the geoscientists working on Gullfaks were looking for – the ability to see where oil had been replaced in the formations by water.

Mårdalen adds that subsurface conditions on Gullfaks provide very good 4D images – including a reservoir temperature of 78°C, ideal for maximum contrast between oil and water.

“When you replace oil with water, you get very clear images – a strong 4D signal,” he explains. That was a crucial factor in launching the project in the 1990s.

“The hydrocarbon zones lit up on our first-amplitude map – which shows the acoustic contrast between two rock strata in the sub-surface – for the Brent formation in Gullfaks,” he says. “We saw that the potential was massive.”

 

Photo: Harald Pettersen/Statoil

Gullfaks A.
The drilling crew at work on the platform.
Improved well positioning has yielded substantial revenues.
(Photo: Harald Pettersen/Statoil)

 


Standard tool

Collaboration between the operations team and the Trondheim research centre is highlighted by Per Digranes as a key reason for the great success of the Gullfaks development.

Now head of one of Statoil’s geophysical units, he had experience of 4D surveys from Norsk Hydro before becoming involved with the Statoil project in 1997.

Three of the scientists from the Trondheim centre were at the Gullfaks office in Bergen twothree days a week during 1997-99, for instance.

And cooperation between the various disciplines involved was very good. Geologists, geophysicists, reservoir engineers and production engineers worked closely together throughout.

“This type of seismic survey was in its infancy at the time, and we had to come up with reasons why it should be adopted,” Digranes notes.

The project team made some simple value calculations which showed the return from positioning wells more accurately and with greater confidence by using 4D data.

“That was important in arguing why the method should be utilised,” he says. “When you can show the impact on the bottom line, it’s much easier to get a go-ahead.”

Reservoir management on roughly 20 Statoil-operated fields currently uses 4D seismic data. Digranes says this figure can vary, because some fields are close to depletion while new ones get added.

Assessing the use of 4D seismic surveys is now mandatory for all new developments. “What we achieved on Gullfaks has converted 4D from a research project to a standard tool,” he observes.

“Before, you had to make a case for acquiring this type of data. Now it’s almost the opposite – you have to justify not doing this.”

 

"Before, you had to make a case for acquiring this type of data. Now it’s almost the opposite – you have to justify not doing this."