Seismic images of the Danish North Sea’s mature fields help Maersk Oil to target its well interventions, which lead to increases in production rate.
A time lapse seismic map with red and blue anomalies is a key tool for the Danish Business Unit’s pursuit of oil from the mature reservoirs. The different colours hold valuable information about where oil is hidden.
“The images give us a better understanding of how the subsurface has developed over time,” said Monica Calvert, Lead Geophysicist in Maersk Oil’s Danish Business Unit and an expert in interpreting seismic images.
The images show where the reservoir has been changed by oil production and water injection. If it is blue, it can indicate water replacing oil in the reservoir. If it is red, it can indicate an increase of gas in the reservoir.
“What the image doesn’t tell us is the exact cause of the anomaly because the reservoir pressures and fluid saturations are constantly changing. The resulting time lapse (4D) seismic image is a combination of these changes through time. In an integrated team with multi-discipline professionals, we can combine the 4D seismic data with all the other information we have, such as well geologic and production data,” says Calvert.
One of her colleagues is Reservoir Engineer Luke Vagg. His job is to transfer the knowledge into concrete actions that can help Maersk Oil extract a larger portion of the oil from its fields.
“In the teams we identify opportunities that can unlock oil from the reservoirs. We analyse what a specific intervention would cost and what our gain would be. This is all pooled into a portfolio of opportunities where we select the best ones. The 4D seismic images are a key tool for us when we are planning future work,” said Vagg.
A hidden pocket of oil
When the 4D seismic images from the latest acquisition were analysed, it showed an anomalous area around an existing oil producing well. The anomaly was in fact where a well from the nearby Skjold field crossed beneath Halfdan production wells. “The Skjold well crosses beneath Halfdan, and in places comes within approximately 10 metres of the Halfdan oil producers. The concern was that water could have travelled from the deeper Skjold well into the Halfdan oil producers. However, the 4D seismic data showed that there was limited risk of that happening, and that we could open production zones in this area without harming the other wells,” said Calvert.
When the long horizontal well, HDA-14, was drilled, Maersk Oil decided to keep one of the 21 production zones closed because there was a potential risk of a connection between the underlying Skjold well and Halfdan wells. This would mean that water could ‘shortcut’ via the Skjold well and thereby result in more produced water and poor oil recovery.
Equipment called coiled tubing was used to open zone 5, several kilometres down in the well. The procedures are performed kilometres below the sea surface without being able to actually see the operation so you don’t know if it is a success, until you see the production flow.
“Within hours of the operation we saw an increase in the oil production, which indicated that our predictions were correct and that we have a significant area of oil that we can reach with existing wells,” says Vagg.
This specific intervention on the HDA-14 well increased the daily oil production rate by 500 barrels per day (bpd) to around 2,000 bpd. This was achieved with only three to four days of work and at a fraction of the cost of a new well.
“I was impressed to see such immediate results from this operation. We see this as a concrete example of where 4D seismic can help us understand a mature field better. This encourages us to do more similar work,” said Vagg.
Adding low cost barrels
“We can see that these targeted well interventions unlock access to low-cost barrels. By getting a better understanding of the subsurface we can recover more oil from the reservoir,” said Brian Pagaard Nielsen, Asset Manager of Halfdan.
“In the Danish Business Unit we have, in recent years, increased our focus on producing better from our existing wells. Targeted intervention can add and protect a significant number of barrels,” he said.
4D Seismic data is a key part of Maersk Oil’s mature field development and an integrated tool in the Well, Reservoir and Facility Management programme where all wells are reviewed to check for the optimal performance.
Today the Halfdan field produces around 40% of Denmark’s daily oil production, and it still holds potential. Despite its size, the field was discovered in an unusual way.
In early 1999 a long horizontal well was drilled from the Dan platform called MFF-19C.
“No matter how far we drilled, higher than expected oil saturations were encountered. This led us to think there was more to gain,” said Brian Pagaard Nielsen, Asset Manager of Halfdan.
“After the successful Dan well, it was decided to drill to the same area from the west, the Skjold field area. The new Skjold well confirmed our suspicion. We had discovered a new oil reservoir, later known as Halfdan,” said Pagaard.
Halfdan came on stream in 2000 and has produced around 400 million barrels per 1 January 2014. The Dan well that first reached the Halfdan field was the longest well drilled in Denmark at that time. Today, The HBB-09 oil producer in Halfdan is the longest well in Denmark at just over 9.5 kilometres long.