A sinking feeling

Ekofisk is an iconic field, but its uniqueness gives operator ConocoPhillips some specific challenges around subsidence. Elaine Maslin reports.

The Ekofisk Complex. Photo from ConocoPhillips Norge.

Ekofisk is the iconic field on the Norwegian Continental Shelf (NCS) – the first field to be discovered and the largest field on the shelf in terms of oil in place.

Its uniqueness is far from limited to its scale (some 6.5 billion STOIP), place in history or the fact it is now expected to still be producing for 40 years hence.

One of its most unique attributes, however, is its challenging subsurface characteristics.

A chalky discovery

Ekofisk was discovered in 1969, about 280km southwest of Stavanger. Two years later, on 9 June 1971, test production was started with full production in 1974 from the Ekofisk 2/4 A, B and C central processing facilities. Oil is transported via pipeline to Teesside in the UK and the gas to Emden, Germany.

The reservoir is in chalk, but a unique type of chalk similar to that in the Valhall field, also on the NCS. The field is dome-shaped with two major formations, Ekofisk and Tor, roughly 400ft thick each, some 2900-3250m below sea level. The Greater Ekofisk Area also contains the Eldfisk and Embla fields, and a further six fields produced from the Ekofisk complex, which have since ceased production. At its peak, the Greater Ekofisk Area had around 30 platforms in 70-80m water depth, including the first test production installation, the Gulftide jackup.

Subsidence

But, while the firm has been able to increase recovery rates from the field as its understanding in it has deepened – increasing recovery rates from about 17% to 50% today – it has also had a large issue to contend with; subsidence.

“With any large oil field particularly water flood, there are lots of typical challenges – logistics, capacity, dealing with water production, surveillance, dealing with growth, aging infrastructure,” says John Leslie Hand, speaking at the SPE Annual Technology Conference and Exhibition, Amsterdam, late 2014. “These are typical challenges. What’s unique for Ekofisk is the subsidence and compaction. These [Greater Ekofisk Area fields] are very unique fields. It’s a consistent chalk, although highly stratified and highly fractured with lots of faulting and break-up of the rock. You have activation and reactivation of faults in the reservoir and through the over burden.”

The Ekofisk 2/4 K platform, from which water injection started in 1987. Photo from ConocoPhillips Norge.

“Over 40 years, the sea floor has subsided about 9m, or about 30ft and that’s huge,” continues Hand. “That’s quite a challenge,” he adds, not least in the reservoir, some 9000ft below where the result is well buckling and in some cases well collapse, which is why there are some 100 active wells drilled to date and 250 wells bores in the field. “We also have more complex well paths, because of number of wells already there but also because of the subsidence and stress and strain,” Hand says.

The problem is that the rock has not got a lot of strength and the hydrocarbons that were in place were giving support to the overburden. Once they started to be depleted, the support was also depleted.

In 1987, as production declined, Conoco started water injection at Ekofisk, which, along with the Ekofisk II redevelopment in 1996-8, helped bring production back to its peak in the 2000s, re-orientating the field center. It was thought that once water injection started they would replace the fluids and stop the subsidence. But that was not the case.

“We had been injecting water since 1987, and we still continued to have subsidence [after that],” says Hand. “The reason is the water fundamentally changes the chalk. What we see is a phenomena we call water-weakening, the water changes the integrity of the rock and changes the permeability by an order of magnitude. 10 millidarcy becomes 1 millidarcy, 1 millidarcy becomes 0.1 millidarcy.”

In 1987, due to the subsidence, ConocoPhillips had to jack up all the steel jacket-based platforms in the field by 6m – “a huge engineering accomplishment at the time.” Protective walls were also installed to protect the Ekofisk tank against the ocean.

Drilling difficulties

Drilling is also a big challenge in these conditions. “As we drill through the reservoir we also see huge pressure variations,” says Hand. “[While] drilling a horizontal well, we maybe drilling through a water swept region and we will hit the water front and see a 1500-2000 psi pressure drop from one side of the water front to the other side over the course of a couple of 100ft. Trying to manage that through drilling is very difficult.”

The changing reservoir structure, with collapsing and compaction, also results in bucking in some wells, which results in new wells with complex wells paths, avoiding existing well paths, having to be drilled. “We have incredibly thick well bores but they’re still undergo buckling,” Hand says. “Over time, you run wireline and you get stuck higher and higher up the well bore, making reservoir management very difficult. So reservoir management and being able to see water is important, particularly if you’re replacing six wells a year.”

To help improve the understanding of the water flood and to help manage and understand what was going on in the overburden and in the reservoir, ConocoPhillips installed a fiber optic cable permanent reservoir monitoring system over the entire field. It is hooked up directly to shore via a cable as well as via satellite.

“There is enough movement in this field that we can shoot a survey every six months and see something different,” says Hand. “This asset has been producing for 40 years and there has been a lot of change and evolution going on. The repeat surveys helped us see better in the reservoir, under some areas that were obscured areas. But also it is very important to understand where the saturation changes and trying to map where that water front is going, not only from a reservoir management stand point but also from a containment standpoint and also when drilling new wells.”

The Greater Ekofisk Area. Image from ConocoPhillips Norge.

Renewal

Ekofisk has not just been evolving, the Greater Ekofisk area has been going through various periods of renewal. In 2005, the Ekofisk area saw a further extension, under the Ekofisk Area Growth plan and in 2014, ConocoPhillips started up the Ekofisk South platform. The last half a dozen years has also seen some 120,000-tonne of jackets, wells and platforms removed from the field.

More recently, the area has seen further new infrastructure. “We are adding new infrastructure, including a new hotel platform,” says Hand. “We have the new Ekofisk South platform, a 35-well platform, also subsea injection templates, which means you have to worry about wave height and can place the template closer where you need to drill the well and avoid subsidence areas. At Eldfisk, nearby, we’re also adding some new infrastructure to extend the life of that.

About 40% of the field’s resources have been produced to date. But, after more than 40 years in production, ConocoPhillips has aspirations not to wind Ekofisk down, but to continue the Ekofisk complex, which takes in the Eldfisk, Embla and Tor fields, for another 40 years.

Hand says: “It’s known as the pioneer field on the Norwegian continental Shelf, the first discovery, and it really set up the Norwegian oil industry. It has had an about 43-year production history and, aspirationally, we would like to see it produce for another 40 years.” But, he adds, “an 80-year life requires a lot of nurturing.”