Produced-water management is an ever-growing and changing subject. Water quality has been a moving target as frack formulas get more tolerant of salts, and worse water quality becomes acceptable. As a result, produced-water reuse has been relatively uncomplicated and an extremely low-cost option. In simple terms, produced-water reuse comes down to:
• Iron Control
• Bacteria Control
• Solids Control
Sure, sulfides and sulfates come up from time to time, but it’s not an everyday occurrence for most operators. These tend to be more isolated issues to specific areas or non-reoccurring. What about oil/water separation? Yes, this can be an issue when you have upsets, but we see this problem slowing going away as operators optimize their gathering systems and separators. Nevertheless, we will discuss all these topics in future columns, but let’s get back to basics.
Most operators will tell you they have an iron goal of somewhere between 5-25ppm. But most aren’t sure if it’s ferric or ferrous iron, which can make achieving their goal difficult. So, let’s cut to the chase. Ferric iron is generally solid or part of your total suspended solids (TSS), while ferrous iron is soluble and part of your total dissolved solids (TDS). When you oxidize ferrous iron, you typically get ferric iron. So, aeration and oxidation will get you there and form ferric iron, which is a solid and can be filtered. Bad news is you need about a 5-micron absolute filter to catch small iron particles or use a coagulant with a larger filter or a solids removal system. So, what does that mean? I can oxidize iron with aeration or oxidation and achieve a less than 5ppm goal if I am measuring ferrous or soluble iron. If I am measuring total iron, then I won’t meet the goal and need a removal strategy, which is basically a solids control approach.
When you oxidize ferrous iron, you typically get ferric iron. Aeration and oxidation will get you there and form ferric iron, which is a solid and can be filtered.
Here’s where things can get confusing. My iron goal is 5ppm, and I use total iron, but my solids control strategy is a 10-micron filter. Believe it or not, this is not unusual. Time for a meeting to discuss the difference between ferrous and ferric iron or change your solids control strategy. Depending on whether your iron problem is ferrous or ferric, the first step is to identify the difference. Then the next is to convert ferrous to ferric and apply a solids control strategy that will capture my iron.
Another approach we are seeing is using a chelating agent to bind up the iron to reduce the overall oxidant demand of water. This may reduce the overall cost, but gives you an orangeor rust-colored fluid and keeps your iron in suspension. Most people don’t like the idea of pumping orange/rust water and are concerned about chelate stability in the formation. Let’s not consider this risky option and consider a more practical removal approach.
Why remove iron? There are plenty of reasons from degradation of friction reducers and scale inhibitors to formation damage, proppant fouling and friction increases. Iron can act as a coagulant and reduce core flow, cause bridging in proppant while affecting your frack fluid. There is also iron’s role in scaling and as a food source to iron-reducing bacteria (IRB). Yes, iron is a problem and needs to be addressed in your reuse program.
Iron can act as a coagulant and reduce core flow, cause bridging in proppant while affecting your frack fluid.
Bacteria control really comes down to two main strategies. Oxidizing or non-oxidizing biocides. The common belief is oxidizers can degrade friction reducers and non-oxidizing biocides don’t. Well, that’s kind of true, but not really. Oxidizers do oxidize and will oxidize friction reducers, so you must have good dose control. Non-oxidizing biocides have compatibility problems with friction reducers, so test your biocide. A friction loop test is probably the best method. Oxidizers are fast acting and allow for a real-time method to evaluate performance, while non-oxidizing biocides are generally slow acting and don’t allow for a real-time method to evaluate performance.
What does this mean? You can use dose control to optimize your oxidizer, and you have to take a wild guess with non-oxidizers. Bacteria tends to fluctuate wildly, so guesswork is risky. Bacteria can also form a resistance to non-oxidizing biocides, leaving them ineffective, but not to oxidizers. Lastly, there are some studies that conclude that high-salinity water like produced water can affect non-oxidizing biocides efficacy. More importantly, oxidizers provide iron and sulfide control, and non-oxidizing biocides don’t. Oxidizers will also improve overall water quality, making breakers more effective.
The case for oxidizers strongly outweighs the case for non-oxidizers. But what about residual disinfection? Oxidizers don’t provide residual disinfection, they react out. What we have learned after years of testing is if you kill bacteria on the surface, you don’t see it in your flowback. I personally think residual disinfection is unnecessary, but many operators don’t feel the same way.
This has led to a trend where oxidation is used to perform the bulk of the bacteria control, while providing iron and sulfide control and then in the blender, an oxidant-compatible non-oxidizing biocide is used in a low dose. The low-dose, non-oxidizing biocide becomes an insurance policy if you think the idea of residual disinfection sounds good. This way you get all the benefits of using an oxidizer, while combining the residual disinfection of a non-oxidizer. Although I think it’s unnecessary, it should not cost more than $0.02-$0.03/bbl for the low-dose, non-oxidizing biocide.
What about where do I perform my bacteria control? It’s common for bacterial disinfection to be performed at the frack site or on the fly. The problem as more produced water is used and aggregated, the quality can be
unsurmountable on the fly or so much oxidizers is needed that you will have a significant compatibility problem in your frack blender. You must also consider the formation of hydrogen sulfide if you are not performing some bacteria control method in your gathering system and during water aggregation. Wherever your water flow slows down, you will see bacteria increase. This means look at gun barrels or other oil/water separation systems and your pits. We have installed automated oxidation systems that bring this cost down below $0.05/bbl.
When it comes to produced-water storage and bacteria control, you can’t go wrong with aeration. With a bacteria-control program in your gathering system or part of your oil/water separation system, you can either add some additional oxidation prior to your pit or have a more aggressive aeration system. Aeration should not cost more than $0.01/bbl, but you must make sure you size the system appropriately. You must determine the oxidant demand of the water and how much oxygen it takes to achieve that goal, then how much air to deliver the oxygen you need. It’s not a difficult process, but we have replaced a few aeration systems only because they were severely undersized or underestimated solids burying them. We prefer mixing aerators to provide mixing and better distribution of the oxygen, while preventing stratification. Surface aerators are not bacteria control devices. They only provide an odor cap, but allow water below the surface to grow bacteria. Your aeration should always be submersed. Mixing aerators prevent the concern of being buried by solids.
A good bacteria-control program should cost you less than $0.05/bbl, while your aeration system should be less than $0.01/bbl. Now, you have taken most of the load off of your onthe-fly disinfection program allowing $0.10/bbl on-the-fly disinfection even with 100-percent produced water.
The last piece of the puzzle is solids control. This has evolved from the labor-intensive bag filters to settling tanks and back-flushable filters, allowing complete automation. Even though the cost of filtration has dropped significantly with unmanned automated filtration, micron size is still a challenge. If you goal is not TSS, but turbidity, you need to look at what micron size will achieve your goal or what combination of coagulant, settling and filtration will get you there. There are many options and no simple solution here, but we expect some of the newer filtration systems being piloted will combine the lower micron size needed in some applications with the automated unmanned features of back-washable filters. We will share our experience here in future columns as we complete some of our pilot tests.
In general, your solids-control program can be driven by your iron-control goals, and your bacteria control can be part of your ironcontrol program. Everything is related. It is important as you evaluate these areas that you look at the whole cycle and how one affects the other in your planning. We’ve only scratched the surface, but I hope this primer helps you understand your produced-water program and goals. Remember, never obstacles, only opportunities.
A good bacteria-control programshould cost you less than $0.05/bbl, while your aeration system should be less than $0.01/bbl.
Authored by Mark Patton
Mark Patton is president of Hydrozonix. He has more than 25 years’ experience in the development, design, implementation and operation of treatment technologies. Mr. Patton’s oil and gas background includes treatment systems for waters, wastewaters, drilling muds, tank bottoms and process residuals. He holds one produced-water patent with two additional patents pending.
Mr. Patton earned his B.S. in chemical engineering from the University of Southern California in 1985.