One of the North Sea’s truly original offshore pioneers, BP, had enormous success developing its huge Forties Field discovery. The project almost single-handedly established the U.K. operator’s status as a world-class player for decades to come.

But by 2002, more than 30 years after that company-defining find was made, it was time to move on. The field, which came online in 1975, had been in steady decline for years.

A bold redevelopment scheme that would have kept the field producing well into the 2030s was put on hold because of various factors, including the fact that with an oil price at that time of around $12/bbl the project’s economics were considered marginal. “If we’d done it, it would have been an enormous success,” said Mark Richardson, North Sea projects group manager for Apache, who was working for BP at the time. “But at the time it didn’t meet the metrics.”

The field itself was discovered in 1970 when BP’s 21/10-1 well was drilled into Paleocene sands to a total depth of about 2,135 m (7,000 ft). At the time the area was little explored, with only several smaller discoveries having been made, but the 1969 Ekofisk discovery offshore Norway renewed interest in the region.

The possibility that oil could be present was recognized as early as 1965, although the subsequent success exceeded all expectations.

A seismic grid shot in 1967 defined a structural nose extending southeast into Block 21/10 and indicated 40 sq km (16 sq miles) of closure in the block. The discovery well was drilled on this feature, with the probe producing 4,730 bbl/d on a ⁄-in. choke. New seismic data were then acquired.

This spectral decomposition image shows the morphology of the channel systems. The Charlie Channel axis is clearly seen along with the thicker wing sands to the west. The thinner wings do not show up as well. Lineation within the channel belts also is visible, reflecting the stacking of individual channels. (Images courtesy of Apache Corp.)

Overlain on the spectral decomposition is the 1988-2000 sweep pattern. In the lower right-hand corner the sweep along the original oil-water contact is evident. This is in the area of the Echo platform that was brought online just before the 1998 baseline survey was acquired.

500,000 bbl/d
The appraisal well, 21/10-2, was spudded in 1971 a few miles northwest of the discovery well. It found an oil column of 33.5 m
(110 ft), and a later appraisal well hit an oil column of 126 m (413 ft). At that point BP was developing plans for four platforms to develop the field.

Once onstream, Forties was eventually averaging 500,000 bbl/d of oil through the early 1980s. In the late 1980s BP installed the Echo platform to do infill drilling, and later the operator also installed artificial lift systems.

But by the early 2000s it was clear that the field no longer fitted comfortably into the BP portfolio.

With almost immaculate timing, in 2003 Apache Corp. came searching for North Sea prospects. The company was originally interested in BP’s Montrose Field asset but decided it was too small. Forties was offered up as an alternative option, and within
two weeks the transaction was complete. “It was a very fast turnaround,” Richardson said.

That was just the beginning of the story because Apache’s success in revitalizing Forties has been nothing short of phenomenal.

Since taking control, it has drilled more than 160 targets at a 76% success rate. Apache purchased the field with 144 MMbbl of oil reserves on the books but has so far produced more than 235 MMbbl of oil from Forties, with 120 MMbbl coming from Apache-drilled wells.

Starting with an intensive drilling campaign on the Echo platform, Apache was able to boost the field rate from 45,000 bbl/d of oil in 2003 to 80,000 bbl/d of oil in 2005. The Forties platforms now produce more than 50,000 bbl/d on average. The three satellite fields Maule, Tonto and Bacchus currently contribute about 7,500 bbl/d of oil to the total. Infill drilling and satellite development have resulted in the establishment of a remarkable late-life production plateau for a field nearing the start of a fifth decade of production.

This success can’t be attributed to any one technology, but it all starts with geophysics.

Urge to infill
Phil Rose, team lead for the Central North Sea at Apache, said the company bought the field because there were solid base reserves with a well-defined upside target portfolio. BP already had amassed a high-end geophysical database that included 4-D seismic and prestack lithology inversion, he said. Initially there was a strong target portfolio of 38 targets.

“I joined just after the purchase and remember my slightly skeptical interest when the chief geophysicist at the time told me how we were going to drill some 20 targets in the first year and that with time geophysical resolution would improve to reduce target size and greatly increase the number of opportunities,” he said. “I could see six or seven but thought getting to 20 would
be a struggle.

“However, as we started drilling in 2004, the experience of success soon made more opportunities apparent, and as they say, the rest is history.”

Apache relies on an integrated multidisciplinary approach to continue to find new targets. Datasets include well log-derived attributes, seismic amplitudes, seismic geobodies and production information. These are integrated into the Petrel platform, which Rose said has been “invaluable” in combining these datasets. He added that it was clear from the start that continuing to acquire detailed reservoir geophysics would be the key to future opportunities. “Not only are 4-D data key to defining prospects, but every seismic inversion and imaging enhancement defines new places to drill,” he said.

BP had acquired a baseline survey over the field in 1988 and followed up with two monitor surveys in 1996 and 2000. Apache continued the trend, acquiring three more monitor surveys in 2005, 2010 and 2013.

World-class
The main Forties Field is a Paleocene-aged turbidite channel sand complex trapped within a large four-way structural closure. Its stratigraphic complexity results in localized accumulations of bypassed pay. Its favorable rock properties make it a world-class example for the application of reservoir geophysics.

“It’s not surprising that 4-D works, but it probably works beyond what you would ever imagine,” Rose said.

Initial goals were to monitor the sweep patterns of the reservoir, which Rose said are never how he imagines them to be when the wells are drilled. “Therein lies the power of the technique,” he said. These eye-opening datasets have led to numerous infill opportunities within the field. Eight years in, Apache had identified more than 100 targets with an overall success rate of 74%. This led to a return to surface drilling on the five platforms when sidetrack donor wells became scarce.

These seismic sections indicate small discontinuities present in the field derived from seismic coherency and spectral decomposition. They have been shown to be associated with sandbody edges as well as small faults. Often these discontinuities line up with the edge of a swept zone.

Lithology prediction also has evolved over the years. Apache’s lithology prediction methodology uses amplitude vs. offset gradient associated with shale-to-sand interface reflections. The gradient response doesn’t reflect the fluid content or acoustic impedance (AI) of the overlying shale. Gradient impedance (GI) and AI inversion volumes are created from the offset stacks and are cross-plotted. A lithology project angle is determined in this volume and is then used to generate a lithology prediction volume. According to Rose, the GI aspect is required to separate out the Forties shales from the sands.

Overall, new infill targets are defined by direct hydrocarbon indicators, 4-D seismic lack-of-sweep targets, development geoscience targets and simulation-derived targets.

However, Rose said that Apache doesn’t rely on simulation runs as much as other companies. “If you look around the North Sea, a lot of people are into reservoir development, which is very much engineering-led,” he said. “It’s difficult to get things justified without a reservoir simulation model.

“We’ve been drilling on seismic and actual observations. That’s the great thing 4-D gives you—a real measure as to how things are draining. We do our best to push the data as far as we can, get the best data we can, and then allow ourselves to be guided by those data in terms of drilling. I think that’s how we’ve managed to keep finding new opportunities.”

New data, new fields
Infill targets are not the only things being discovered at Forties—several satellite fields and near-field opportunities also have been discovered in the region. Satellite fields include Bacchus, discovered in 2005, Maule in 2010 and Tonto in 2013.

The Bacchus Field, in which Apache has a 50% working interest, currently produces 5,500 bbl/d of oil (gross). Gross cumulative production to date is 9.8 MMbbl of oil, and EUR is 18 MMbbl of oil. At a finding and development cost of $33/boe, the field is economic even at low commodity prices. This subsea development is tied back to the Forties Alpha platform.

Maule was drilled from the Forties Alpha platform and discovered a 14-m (46-ft) net oil column in Eocene-aged Brimmond sands. The company was able to develop the field under the U.K.’s small field allowance scheme, and it was brought onstream in less than nine months.

The field was identified on farstack seismic data, which clearly showed the lateral extent and thickness of the reservoir. The field overlies the Forties, but earlier wells lacked the logs to indicate the presence of hydrocarbons. The field has produced 2.6 MMbbl of oil to date.

Tonto began production the same year it was discovered and currently produces from two wells drilled from the Forties Bravo platform. It too is under the small field allowance scheme. It has produced 1.1 MMbbl of oil to date.

Another discovery, the Aviat Field, is one that has been used to great effect by Apache. The gas field is being tied back to Forties Alpha not to produce its commercial gas but to provide a fuel source for the Forties complex. The development plan is a two-well subsea development.

Pictured above, sweep can be seen with associated sweep down by the contact due to high-rate wells drilled in the core of the Charlie Channel. The influence of the stratigraphy is evident in the Bravo/Alpha Channel. During the next period there was a focus on drilling wells in the Charlie Channel and infill drilling throughout the field.

Pictured above, sweep response is filling in the holes with a strong response in the Charlie Channel. Apache is also seeing fluid movements on the western flank of Charlie. It has two successful wells in the area with plans to drill additional wells.

A wild ride
For Rose, his time working on Forties has been a great deal of fun. “I can’t believe it,” he said. “The time has gone by very quickly, and it’s been an amazing journey. I really didn’t dream that I’d still be drilling Forties wells 12 years after I joined Apache. That was not my expectation at all.”

Being able to continually acquire data has made a huge difference, he said. “I think there is a really clear connection in our experience,” he said. “Every time we’ve improved our inversion techniques and gotten a high-definition lithology image, every time we shot a new 4-D survey, particularly as we’ve increased the definition of that survey and done things like removing time shifts in the data to clean things up or produced normal root mean square error pictures, these things have provided another way to evaluate whether a location is going to be a good or bad place to drill.”

He added that Apache doesn’t really have any “silver bullets” that other operators lack, but the geophysicists push the contractors to get the best data possible. During the last monitor survey that was shot, for instance, Apache personnel were closely involved with the contractor to make sure the survey had very high levels of repeatability, critical for a successful 4-D survey. “It’s not so much that we’re devising new algorithms,” he said. “We’re making sure we get the maximum value out of what’s out there.”

Both Richardson and Rose are quick to point out that the company’s culture plays a key role in its success at Forties and elsewhere. “We don’t have any special technical skills,” Richardson said. “We don’t have any special equipment or fields; we’ve got some of the oldest fields in the North Sea. Yet we still produce some of the highest standards of operational efficiency.

“The idea of not [taking risks] is total anathema to Apache, where it’s all about delivery and turning things into action rather than discussing them.”

Added Rose, “When asked the question, ‘Are we going to drill the well,’ it’s ‘yes’ unless you can give me a really good reason why not to.”

This story is part of a special report. Read each story:

Forties At 40

Technological Innovation Key to Enhancing Forties Field Drilling

Editor’s note: Part 2 of the Forties Field special report will appear in January’s E&P issue.

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