Scaling Up Smart Water

For nearly a decade, Saudi Aramco has been studying how altering the chemical makeup of seawater injected into its reservoirs can increase production. The result is "smart water" that can boost the sweep effectiveness of a waterflood.

August 20, 2016

Journal of Petroleum Technology

For nearly a decade, Saudi Aramco has been studying how altering the chemical makeup of seawater injected into its reservoirs can increase production. The result is an increasingly complex view of the interactions caused by the makeup of seawater that explains why seawater does more than just add pressure and help sweep the remaining oil out of a reservoir.

The goal is to cost-effectively maximize production by altering the chemistry of seawater. Hundreds of technical papers have been written on the potential benefits of reducing the salinity in seawater injected into formations.

More recently there has been a growing body of work on how other ingredients found in seawater—particularly sulfate, calcium, and magnesium—can add to oil output by freeing oil from reservoir rock.

Saudi Aramco has focused its research, dating back to 2008, on how seawater is able to increase production from carbonate reservoirs. Two recent technical papers from Saudi Aramco show that it has begun considering how it might modify its water treatment system to turn seawater into “smart water” (SPE 179564), and also offers an update on laboratory studies investigating how the active ingredients in seawater affect oil production (SPE 179590).

What is clear from those papers and other sources is that seawater, which Saudi Aramco turned to as a cheap option to scarce fresh water, can enhance the amount of oil ultimately recovered from the ground. The research also suggests that it can make seawater more effective by altering its chemical makeup, turning it into smart water. But this option is neither simple nor cheap.

Reducing the salinity of the extremely salty seawater used by Saudi Aramco would require desalination on a massive scale. Further changes to make it smart water add processing steps and may require new technology to reduce the energy required and to overcome the fact that available water treatment methods were not designed to selectively remove ingredients from water.

Turning seawater into smart water is a logical next step for the company, which has long been processing seawater on a huge scale to maintain production from its fields. Its Al-Qurayyah Sea Water Plant processes millions of barrels a day of seawater, removing small particles, microorganisms, and oxygen. Now the company has begun considering whether to add further processing steps to apply what it has learned about seawater chemistry by fine-tuning the makeup of the water.

In 2011, Saudi Aramco reported (SPE 143550) significant gains when it flooded core samples from a reservoir with seawater diluted by various amounts of water. The most effective was 10 parts of fresh water for every part of seawater. “Varying the salinity of seawater provided a substantial increase in oil recovery of up to about 18% beyond conventional water flooding,” the paper said.

Paper SPE 179564 on smart water processing said that a single-well test using chemical tracers concluded that “chemistry optimized water” could reduce the oil saturation level in the reservoir by 6–7%. That paper, delivered at an SPE conference in May, said that a multiwell test was being designed.

Those results will be valuable because there is no good substitute for reservoir testing. While there have been more than 300 papers on low salinity and smart water injection, Norman Morrow, a retired professor from the University of Wyoming, said that he has found it difficult to reproduce studies of water injection in a lab.

Determining which brine will work better is hard to do because performance varies based on multiple variables, reservoir conditions are tough to reproduce, and it has been difficult to scale up the results to predict performance on a reservoir level. “In spite of all the problems of poorly defined and unknown variables, I still believe identification of the factors that control waterflood recoveries is highly worthwhile,” Morrow said.

Saudi Aramco is considering a range of smart water treatment options, beginning with reducing the salinity of seawater by diluting it with desalinated seawater. SPE 179564 offered a proven water processing technology—reverse osmosis (RO)—and two emerging technology options: forward osmosis (FO) and membrane distillation (MD), which might be more efficient.

Using dilution to significantly lower the salinity of the seawater will also reduce the concentrations of sulfate, calcium, and magnesium ions, said to free oil from reservoir rock. If the levels of the ions resulting from the dilution are considered insufficient, other processes were described (below) to increase their levels in the mix of the diluted seawater.

A proposed process to increase the levels of sulfate, calcium, and magnesium ions is to first remove them from the seawater using nanofiltration (NF), which does not remove salt. The resulting ion-rich brine could be added back to the low-salinity seawater to increase the ion levels.

The processing problems posed by seawater look simple compared with those associated with recycling water from Saudi fields, where the salinity levels are beyond the limits of membranes, which are also easily damaged by even trace amounts of oil or other organics.

The paper suggests reducing the salinity in produced water by using two emerging technologies, carrier gas extraction (CGE) and dynamic vapor decompression (DyVaR), which are said to be able to process such extremely saline water. The water would be mixed with desalinated seawater and spiked with the three ions to reach sufficient levels.

Mixed Signals

Smart water research is an outgrowth of work on low-salinity waterflooding by companies such as BP, which will use the method to enhance the output of its Clair Field in the North Sea.

The term “smart water” was coined by researchers who realized the gains were not just a product of reduced salinity. As they have learned more, though, it is also clear that the ideal recipe for the smart water cocktail depends on many variables, including the temperature of the reservoir and the minerals found in the reservoir rock.

The Saudi Aramco paper on smart water processing said that the formula needs to retain only “sufficient quantities” of three ions: sulfate, calcium, and magnesium.

The sulfate ion (SO₄²⁻) is a prime example of why the amount considered sufficient may vary widely. On the plus side, the negative ion plays a key role in freeing oil. Sulfate ions have a stronger affinity to the positively charged carbonate rock surface than the acidic components in oil, allowing them to displace the oil by neutralizing the charge that had attracted the acidic components.

Calcium and magnesium contribute to the transition from mostly oil wet—attracting oil—to mostly water wet—attracting water.

Reducing the level of sodium chloride (NaCl) in the water helps those ions gain access to the rock surface where they can do good, according to an article by Puntervold et al. (2015) that recommended removing more than 90% of the NaCl.

While sulfate plays a key role in freeing the oil droplets, there are some downsides to consider when deciding how much sulfate to use.

The recent Saudi Aramco research update pointed out that it may limit the ability of oil drops to coalesce into an oil bank. When oil droplets were exposed to a sulfate-rich solution, there were changes in their surface properties that made them less likely to combine and ultimately coalesce into a bank of oil. But the paper added that other ions present in the reservoir, such as calcium and magnesium, are expected to “cause favorable interactions” that could overcome the “negative effect of the sulfates.”

Injecting seawater into a reservoir causes changes on the surface of the reservoir rock, freeing oil droplets to form an oil bank, which is swept out by the waterflood. Saudi Aramco is working to adjust that formula to make it more effective.

Reservoir conditions also change what constitutes a sufficient level of sulfate. In reservoirs with lower temperatures (100°C or less), spiking the injection water with sulfate should significantly increase the field’s output, reported Austad et al. (2015).

That does not apply in Saudi Arabia reservoirs, where the higher temperatures (greater than 130°C) increase the effect of the sulfate. Also the rock there contains a natural source of the ion—anhydrite (CaSO₄)—which when dissolved produces sulfate as well as calcium, according to Austad et al.

Adding more sulfate to the injection water can also cause costly operational problems by causing scale to build up in pipes. It is often removed before injection into offshore fields where bacteria in the reservoir may convert it into hydrogen sulfide, souring the reservoir.

Manufacturing Challenges

In the laboratory, water blends can be created from off-the-shelf ingredients like a recipe for soup. Changing the makeup of millions of barrels of seawater a day is far harder because the available water treatment methods are aimed at creating drinking water, where the goal is to remove everything but the water.

As the paper reviewing the available water chemistry alteration methods concluded: “There is no commercial technology yet available to selectively remove specific ions from the seawater in one step and meet requirements of SmartWater flooding.”

The mostly widely used method for desalination—reverse osmosis—pushes water through a fine membrane that removes nearly all the dissolved solids.

It is a proven technology that can reliably be used to desalinate the large volumes of Saudi seawater, which has a salinity level of 57,000 ppm, down to the 5,000-ppm level used in all the examples in the paper on smart water processing options. This process also lowers the concentration of useful ions, such as sulfate and calcium, which may or may not be a problem. The paper offered options to fine-tune the water mix, process it more efficiently, and consider how to begin recycling produced water. The goal is to “capitalize on existing huge produced water resources in Saudi reservoirs to generate SmartWater and minimize waste water disposal during field-wide implementation.”