Enhanced recovery promotes record flow from Danish chalk reservoirs

Siri, Arne South to boost production by 60%

Water injected into Danish fields. [4,254 bytes]
The load-out of the Dan FF topsides for Maersk Olie og Gas AS, which was built by Grootint B.V. in the Netherlands. (Source: Ben Seelt) [31,231]

Oil and condensate production in the Danish sector could surge 60% to 20 million cu meters by the year 2000. The Danish Energy Agency's forecast is based primarily on supplies from new developments such as Siri and Arne South.

However, the figure also reflects the success of several enhanced recovery schemes on existing fields employing a combination of horizontal wells and injection of water into tight chalk formations. This has led to the unusual situation where several fields are now generating more oil than during their initial production stages in the early 1980s.

The main beneficiaries of EOR techniques are the Gorm and Skjold Fields, plus Dan, which is undergoing a phased US$930 million facilities expansion program. Limited water injection began on Skjold in 1986, spreading to Dan and Gorm in 1989. The scale of injection rose sharply from 1993 onwards, reaching a total of 22 million cu meters of water for the three fields in 1996.

A further 60% hike to 35 million cu meters is expected in the next few years with operator Maersk planning to virtually flood the three reservoirs in order to displace oil from the reservoir rock. If successful, combined recovery should climb by 20%.

Water production

Until 1991, only 1.5 million cu meters of water was produced annually from Danish fields. However, that figure more than quintupled to 9.9 million cu meters by 1996, when water also accounted for around 45% of all liquids produced. This scenario was not unexpected from Denmark's predominantly chalk reservoirs, where oil tends to dominate the initial production yield, giving way to water as the years pass.

Skjold today actually produces more water than oil (around 60% of total output), while in two minor Danish fields, Regnar and Dagmar, the percentage is 88% and 96% respectively. Ultimately, water breakthrough will reach almost 100% in all five of the fields mentioned. For Maersk, the challenge is to delay the inevitable as long as possible. Produced water is throroughly purified before discharge into the Danish North Sea.

The 20% rise in Skjold's water-oil ratio last year only led to 2% higher oil output. But water injection's impact is still considered favorable. New wells drilled on the field in recent years have penetrated some short zones with low oil saturation levels, but also some long zones with high oil saturation - suggesting that large reserves remain to be tapped, particularly in the field's western sector.

Skjold comprises Danian, Upper Cretaceous, and Zechstein carbonates, some highly fractured. Where the reservoir has been flooded, low residual oil saturation has been reported, suggesting that displacement of the oil was effective. The goal now is to ensure better distribution of water throughout the reservoir.

Gorm, 10 km to the northwest, is characterized by Danian and Upper Cretaceous chalk. Oil production of nearly 3 million cu meters last year was higher than the figure recorded in 1986. That rise is due not just to water injection, but also to drilling of productive horizontal wells. Gas lift will also come into play more following the recent addition of new wellhead compression facilities.

Last year 18 horizontal or deviated producer/injector wells were drilled on Danish sector developments. Nine of these were re-drills of existing wells. This swelled the number of horizontal wells in operation off Denmark to 106, of which 83 were producers and 23 water injectors.

The main increase came from Dan, where the re-drills occurred. Dan, a Danian and Upper Cretaceous chalk reservoir, is Denmark's biggest oil field. A fifth stage of the development is in progress, involving a new platform, FF, featuring a 150,000 b/d water injection package. According to London-based field analysts Scanboss, this should increase oil output from the reservoir 50% to 90,000 b/d.

Last year's re-drills included three horizontal wells in the central part of the A Block, drilled to boost oil production under Dan's gas cap. One horizontal producer and four horizontal injector wells were drilled in the B Block to raise reservoir pressure quickly, thereby lowering the gas/oil ratio and in turn improving recovery conditions.

The result of this program was slightly higher oil output, with produced water levels similar to those of 1995. Maersk concluded that large-scale, high-rate water injection ensured rapid distribution of water within the reservoir through the pressure-induced fractures, leading in turn to efficient displacement of the oil through the tight chalk. Dan's well pattern is optimized to aid recovery further.

A new delineation well in the western flank also detected high oil saturations, suggesting that the Dan reservoir extends further west than assumed previously.