Two of DTU Offshore’s research programmes on Improved Recovery, “Advanced Waterflooding” and “Tight Reservoir Development”, are in the process of being completed. After years of scientific work at the highest level, the results are clearly showing the value created for industry and society. For example, the results from the Advanced Waterflooding programme show a clear link between the composition of injection water and how much oil can be extracted from the reservoirs.
Waterflooding, to pump water into oil reservoirs to increase production, is one of the most widespread drive mechanisms for production of oil globally, and has been deployed in Danish offshore fields with great economic success. Despite this, circa half of the original oil-in-place remains unrecovered in Danish chalk fields due to physical limits to sweep efficiency when seawater is being used as a displacement fluid.
Experimental tests on chalk samples from the North Sea
The Advanced Waterflooding programme was started in 2015 to investigate whether new displacement techniques could be developed to improve oil recovery efficiency, for instance by changing the composition of injected water. Initially the focus was on desalination of seawater as this was proven to be of benefit in sandstone reservoirs elsewhere in the world. Experimental tests on chalk samples retrieved from offshore reservoirs, restored and tested under actual reservoir conditions in the lab, showed that oil recovery can indeed be increased by changing the salinity of injection water.
Surprisingly, this effect was noticed at lower as well as higher salinities, and further investigation demonstrated that improved recovery mainly depends on the sulfate content of the injected water rather than its total salinity. Lowering the sulfate concentration of injection water helps change the wettability of the studied chalk pore space towards waterwet conditions and thus lowers the amount of residual oil left behind. This increases oil recovery and reduces water production.
Sulfate content is important
The finding that the sulfate content of injection water is the main key for improved recovery has implications on many levels:
- Economics: Removing sulfate from seawater requires less energy than removing all salts, which makes implementation more cost effective. Also, the produced water will be cleaner, requiring less physical and chemical treatment and causing fewer flow assurance issues, which helps reduce operational costs and energy consumption.
- Environment: Removal of sulfate from injection water will reduce reservoir souring and scaling/corrosion issues caused by sulfate and H2S in produced fluids. This can help reduce the usage of chemicals that scavenge H2S or inhibit scaling and corrosion, which improves the environmental impact.
- Recycling: Produced water from most Danish fields is already low on sulfate content and may be re-injected. This was done in the Gorm field, which has led to a high recovery efficiency but at the cost of increased scaling and corrosion issues due to mixing of formation water and seawater. Re-injection of produced water without mixing with seawater is beneficial to the environment and may help improve oil recovery.
Part of national energy strategy
The IOR programmes were initiated as part of Denmark’s long-term national energy strategy. They were the cornerstone of the centre’s activities in the years after the formation in 2014. Recent evaluations show that the value created by the centre’s programmes is equivalent to 1.7 billion DKK of which circa 300 million DKK is thanks to the Advanced Waterflooding programme.
Even if improving oil recovery is no longer a priority, modification of injected seawater would still help reduce the amount of water required to support a given oil production rate. This will benefit the environment thanks to reduced use of chemicals and a reduced water handling energy footprint.
Publications for chalk studies are listed here.
