Comparing aeration systems in terms of cost and efficiency
In Part 1, we discussed the basics of aeration and how it’s applied in the oil field. We discussed what type of bacteria (anerobic) aeration is most effective on and how it can provide iron and sulfide control. We covered the different types of aeration systems and the how bubble size affects oxygen mass transfer efficiency, the role of computational flow modeling and the emergence of nanobubble systems. With that, let’s continue with our discussion on nanobubbles.
In Part 1, we referred to the emergence of nanobubble systems and the potential for significant increases in mass transfer efficiency.
The goal of aeration is to introduce oxygen into water. Nanobubble systems allow you to accomplish this with only a fraction of the air required. Ordinary bubbles rise to the surface and then burst at the surface loosing oxygen.
Nanobubbles do not rise, allowing you to use most of the oxygen introduced. Systems generating ordinary bubbles can have efficiencies as low as 10 to 20 percent, while nanobubble systems report efficiencies as high as 90 percent. This could let you reduce the amount of air introduced by over a factor of 5. In theory, this looks like a potential new way to significantly reduce the cost of aeration.
Field testing is confirming significant increases in mass transfer efficiency, but not the 90 percent that is being advertised. Nonetheless, there are significant increases in mass transfer efficiency.
As promising as these results are, there are two limiting factors. Oxygen solubility in produced water and cost.
The goal of aeration is to introduce oxygen into water. Nanobubble systems allow you to accomplish this with only a fraction of the air required.
Oxygen solubility in water limits how much oxygen is solubilized or absorbed into the water. Salinity reduces the solubility of oxygen in water. Seawater, for example, has about 20 percent less oxygen solubility than freshwater, and in produced water, it is even significantly greater.
Temperature also has a dramatic effect on oxygen solubility. Produced water is typically hot from the wellhead, but will cool over time. As a result, oxygen solubility in produced water is much lower than most other applications. This means that the rate oxygen can be absorbed is significantly limited in produced water.
Consider this solubility limitation like a funnel. You may have a lot of oxygen in your water, but solubility only allows some of it to participate. In order to get all your oxygen to participate, you need much longer retention times to take advantage of the reservoir of oxygen created by nanobubble systems.
Retention times can be significant in larger pits, but still may not be long enough. These limitations do not exist in fresh water or even seawater applications.
Air is free. Maybe you’ve heard this before. The cost of air works against most nanobubble systems. The increase in mass transfer means you can use much less air, but air is free. But you can also decrease the size of your delivery system, smaller pumps, small blowers, but generating small bubbles needs energy.
When you combine the energy and the cost of the nanobubble delivery system, this cost is significantly greater than the reduced cost of the lower air flow required, partly because air is free.
We are going to consider two different scenarios to review and evaluate from a cost standpoint. The first is an aboveground storage tank (AST) scenario where aeration is used for pre-treatment. The second scenario is large pit used for storing produced water after treatment. These two scenarios are the most common applications in the oil field.
Table 1 is an example of a cost comparison of two nanobubble systems compared to a standard microbubbletype venturi. In this example, this is a pre-treatment system taking untreated produced water with high iron and bacteria, requiring a significant amount of oxygen.
The mass transfer efficiency of the nanobubble system is significantly better, so the air required is much lower, but the cost of these systems is significantly higher.
When looking at total cost, you see how the standard venturi provides the lowest cost even though it has a lower efficiency. To determine annualized cost, we took our initial capital investment and spread it over five years. The useful life should be much greater than this, but we are doing a conservative evaluation. You’ll see the real difference in these options is less about initial capital cost and more about the energy required.
In this next comparison, we evaluated standard diffuser-type systems to a venturi system and nanobubble systems. In this example, we are taking treated water with no iron and low bacteria and only trying to maintain the treatment and disinfection. As a result, oxygen demand is low.
The diffuser type in this example is a relocatable system allowing it to be moved around. You can also place the diffusers exactly where you need to have the oxygen distributed. In large pits, venturi systems must be designed to have enough flow to distribute oxygen everywhere you need to.
Pumps are used to create the vacuum and distribute the oxygen, and as we mentioned earlier, pumps performance can degrade over time, especially if they are not properly maintained. This degradation reduces flow and increases bubble size, greatly affecting the oxygen uptake while losing efficiency. As a result, we prefer diffuser-type aeration for large pits. There are also other considerations in this decision we will discuss further.
Your overall cost is lower for the portable diffuser system, and there are other considerations. The concern over pump performance over time gives the advantage to the diffuser-type system in larger pits. The relocatable feature of the diffusers allows the expansion or relocation of these types of systems easy, giving them additional value in larger pits.
STANDARD VENTURI VERSUS DIFFUSER
The nanobubble systems, even with their efficiency gain, still can compensate for their energy cost and cost of the delivery system. The decision here comes down to the standard venturi versus the diffuser type. You may want to consider the nanobubble 2 system, but the lack of experience in the oil field is a major drawback here.
With time, the nanobubble system can be validated and may become an option down the line. Other than the diffuser type, all the other systems are pump driven requiring significant energy, and we’ve already discussed pump performance issues over time.
In most produced-water storage scenarios, the purpose is to aggregate a large enough quantity of produced water to reuse in a well-completion program as a completion fluid. Because well completions/fracs schedules can change, it is likely you may not use your storage system year-round. A relocatable system gives you an advantage in that you can move them around as you move your completion program and use other pits or ASTs.
There are other critical considerations here, as well. Some of these systems have your air injection taking place underwater. This means your delivery system is also underwater, and due to the high scaling potential of produced water, these components will require a cleaning schedule.
When looking at total cost, you see how the standard venturi provides the lowest cost even though it has a lower efficiency.
We’ve been using venturi eductors for many years, and we must regularly clean them. Although many suppliers say cleaning isn’t an issue, most of these systems have either never been applied to produced water or have limited experience with produced water. There will be scale.
This drawback has made us a proponent of diffusertype systems in larger pits. Your delivery system does not require water flow, so scaling isn’t a concern. We have already seen some of these systems start to show performance degradation, which we believe is sign that scaling is beginning to develop inside them.
Cleaning would entail completely draining the pit and disassembling the delivery system to clean it, taking the entire system out of service for a while. The risks of pump performance, scaling of the delivery system outweigh the price advantage for us, and we continue to promote relocatable diffusers for larger pits.
If the relocatable diffusers have this advantage, you may ask why consider venturis for ASTs. In an AST, you don’t have the large area to distribute oxygen, taking away most of the pump-performance risk. The scaling is still a concern, but Hydrozonix has developed a proprietary design that keeps our delivery system out of water, allowing us to remove and clean in a few hours without emptying a tank. These design features take advantage of the lower cost of the venturi systems while minimizing the risk.
We use what we refer to as a candy-cane design so the delivery system can be placed over the edge of the AST without requiring entry into the water. Keeping the delivery system not only makes it easier to clean, you also aren’t moving air to overcome the hydraulic head of the full tank, cutting down on the energy cost.
In summary, aeration in the oil field requires careful consideration. You must size the system properly, use modeling to ensure you have a good design and are distributing oxygen everywhere you need it. You must not just consider capital cost, but operating cost and longerterm maintenance issues.
Taking all these things into consideration will leave you with a lower overall cost, but also a good working system. And good aeration is a good thing.
Authored by Mark Patton, President of Hyrdozonix