Process water is a heterogeneous complex mix
For each barrel of oil extracted, it is estimated that between four to eight barrels of water are required as part of the process.
This water is injected into the well to improve or enhance oil recovery and emerges as so-called produced water.
"A heterogeneous and complex mix, this process water represents a major environmental challenge."
A heterogeneous and complex mix, this process water represents a significant environmental challenge.
Not only as a result of both the large volumes produced – millions of cubic metres every year – but also as a result of the complex mix of molecules and materials it contains.
“It’s not an easy to treat mixture,” explains Jan Post, deputy program director at Wetsus, the European Centre of Excellence for Sustainable Water Technology.
“For instance, the problem with adding salt to the mixture is that the polymers fold up and will lose their functionality of increasing the viscosity of the process water.”
A cost saving on the chemicals
This injected water has added chemicals, for example, polymer surfactants such as partially hydrolysed polyacrylamide (HPAM). These chemicals increase the viscosity of the water to a level that is similar to the oil itself and support the better movement of the oil.
However, produced water also includes oil and salts leached from the surrounding rocks.
While the cost of water and its subsequent discharge generally represents a marginal overhead for the oil industry, more efficient treatment and recovery are attractive.
It potentially represents a significant cost saving on the chemicals that are added to the produced water to increase oil recovery.
“About 50 per cent of the water that comes out of the oils fields is reused, the other 50 per cent is either discharged or deep well injected,” adds Post. “We are looking into reuse options, but these come with challenges."
Developing better water treatment tech
Part of the challenge is that the current established commercially available technologies cannot remove all the components from the waste stream of produced water.
Oil, polymers or surfactants, salts and minerals cannot easily be removed with standard physical or chemical separation treatments such as cyclone separation.
"With a cyclone, you can only remove the big droplets of oil,” says Ettore Virga, theme coordinator concentrates at Wetsus. “With sedimentation, it is only possible to remove the layers of oil that are not stabilised by the chemicals. A lot of oil remains in the water because those chemicals stabilise it. We are working on a number of technologies to address these issues.”
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Wetsus is exploring nanofiltration membranes, where the separation of oil and water is possible even in the presence of all the other materials found in produced water.
Over the last few years, polyelectrolyte multilayers (PEMs) have been established as a potentially powerful method to support the functioning of hollow fibre-based nanofiltration membranes.
However, surfactants in produced water can affect the stability of the polyelectrolyte multilayers clogging the membrane, reducing efficiency and adding costs.
Research underway at Wetsus, (pictured above) is exploring the treatment of membranes with antifouling layers to prevent or reduce the impact of contamination over time.
"This is a technology that already existed, but we have synthesised novel types of polymers to prevent fouling of the surface,” says Virga.
While membrane technology is one of the few techniques that can successfully remove the smallest (< 10 µm) and most stable oil droplets, to address the fouling issue Wetsus is looking at novel surface chemistries. These are optimised towards low fouling using layer-by-layer deposition on ultrafiltration supports.
The potential of desalination
Another promising area of research is desalination.
“Nanofiltration membrane is one route that we are taking, but we are also working on desalination with the recovery of everything in the stream, so we recover the water and also the polymers by removing the salts where we use electrodialysis,” adds Post.
Ion exchange membranes are used to desalinate produced water and restore the expanded state of the polymer to increase the viscosity of the solution and lead to a reduction in the consumption of both freshwater and polymer chemicals.
According to Post, desalination treatment may result in savings of a factor of 5-10 on the cost of polymers, suggesting a payback time of less than two years.
"These treatments are complementary but may also be considered as alternatives to each other," says Post.
Indeed, multiple and layered approaches are expected to emerge using different technologies that can extract the maximum value from the produced water.
"We are not betting on one solution. We are thinking about how we can combine these solutions."
“We are not betting on one solution. We are thinking about several alternatives but are also thinking about how we can combine these solutions to include a nanofiltration membrane layer and ion exchange elements,” he adds.
Collaborating on water treatment
Wetsus’ research programme includes 20 teams that are consortia of academic institutions and commercial enterprises and which share laboratory space and instrumentation.
Within these consortia are not only oil and gas companies but also the membrane companies and technology providers covering the whole supply chain. By engaging with industry, the opportunities for commercially viable technologies to emerge are enhanced.
"Working with Shell and Aramco has allowed us to bring this knowledge further."
“Working with companies like Shell and Saudi Aramco has allowed us to bring this knowledge further,” explains Post.
It is almost unique to find two nominally competitive enterprises working together on a single R&D project in which they will share the outcomes. Nonetheless, both Shell and Saudi Aramco are working with Wetsus on nanofiltration membrane technology, for example.
Where commercial enterprises are investing in this primary R&D to develop these processes, any intellectual property that may emerge is retained within the specific consortium.
Wetsus is also publicly financed, so it is required to publish its findings and research work, but its approach does make it much easier to collaborate.
Collaborating on water treatment
With the industry investing in the R&D phase in the development of pioneering technology, the potential environmental benefits are evident, but there are commercial advantages too.
"The water saving is an environmental benefit, but if you look to the economics of this, especially when you add chemicals, then you can save a lot when you reuse the chemicals that are already present in the produced water,” says Post.
"Although we are not heading to a fossil-free future yet, the water use issue will become bigger as we look to exploit the reservoirs that we already have."
He adds: “Although we are not heading to a fossil-free future yet, there will be fewer new fields and the water use issue will become bigger as we look to exploit the reservoirs that we already have.”
As more emphasis is placed on sustainability, the expectation is that it will drive additional investment into water treatment technology; not just for oil and gas but for many other types of complex water treatment challenge.
“The impact of water treatment on sustainability will certainly increase,” concludes Post.