Produced water: How did we get here?

MARK PATTON, CONTRIBUTING EDITOR 

Produced water has been on an interesting journey over the last 10 years, especially in the Permian basin. Let’s talk about that evolution, but let’s also talk about fixing a broken system. Sorry, but I will nerd out a little here, so bear with me. 

Ten years ago, most produced water went to saltwater disposal wells or SWDs. A small portion went into recycling, which we called On-The-Fly (OTF) recycling, where we applied oxidizers for bacteria, iron and sulfide control, with filtration added very rarely. We were beginning to see some pits aggregating more water to increase OTF recycling. 

OTF recycling started with blends of raw, produced water and fresh water. This transitioned to raw produced water and brackish water and ultimately to just produced water. The increasing use of raw, produced water requires more oxidizer. The problem with this is, if you choose the wrong oxidizer, you will have what’s called an incompatibility problem, and your oxidizer will begin degrading your friction reducer. So, you must choose an oxidizer that is strong but has a short half-life. This is exactly why we chose ozone, it’s fit for purpose, is strong and degrades quickly. 

An alternative here is to use biocides, essentially something that is toxic to bacteria. The problem is efficacy, because most common biocides drop off at around 50,000 ppm salinity. This makes basins like the Powder River in Wyoming and Eagle Ford in South Texas ideal for this switch, but not the Marcellus shale or Permian basin. The first paper I read on this topic was about 10 years ago, where bacteria and biocide were coexisting in produced water. One of the theories presented was resistant mechanisms, which means the bacteria developed a tolerance for biocides, a not uncommon event.  

About five years later, another paper in Texas found something much different. Bacteria behaved differently in high-salinity water. The salinity was also a toxin to the bacteria, so the cell wall structure changed to prevent the salt from entering the cell. This would also prevent biocides from entering the cell wall and explains the efficacy drop-off at around 50,000 ppm salinity. 

So, back to oxidizers. Unfortunately, the idea emerged that we need to pretreat produced water to avoid oxidizers overdosing and creating a compatibility problem, instead of let’s just choose the right oxidizers. This coincided with a few pits to more and more pits and the development of the recycling facility. During this period, a few water treatment companies began marketing a produced water treatment standard. In my opinion, it’s a self-serving marketing program to increase the cost of recycling and legitimize pretreatment.  

This treatment standard didn’t include bacteria or iron, two important parameters, because they wanted a simple standard that they could use real-time testing on. The adoption of this standard grew and so did the use of treatment equipment for pretreatment. One of the biggest concerns I had with this standard was the use of Oxidation Reduction Potential (ORP) probes instead of bacteria or iron results. Now we use ORPs all the time. It is a great indicator of performance, when applied correctly. 

Probes and skids. So many chemical injection systems are built into portable skids, and the ORP probes are typically added to these skids, and this is a problem. Your probe has to be downstream enough to allow you to mix your oxidizer and water together to get accurate readings. If your probe is too close to your injection, you read pure oxidizer and report artificially high values. So, we began seeing people call and say, “how is it that my pits are going bad with bacteria growth when I’m treating to this standard?” After testing the water, we could see it had high bacteria and at the same time, we heard people touting extremely low chemical demand in their water treatment. I don’t want to imply this was done on purpose but was just a result of chemical skids being made compact without recognizing things like mass transfer efficiency or mixing. Ultimately, this led to more pit treatments. 

Inadequate water treatment. Now let’s talk a little more about these water treatment systems that are not adequately treating water. The purpose of these systems is essentially oil removal and solids removal, not bacteria or iron control; that’s a dosing problem and poor mixing. Water treatment systems like steady flow and need flow control. The solution was upstream of your water treatment system—provide storage for your produced water to control the flow into your water treatment system. This typically was two to three days of storage, based on your treatment capacity, but sometimes it was lower. 

Over 90% of the time, what we observed was the oil was separating in the tank storage, and anywhere from 70% to 90% of the solids were also settling in these same tanks. And the water treatment system, well it became a slight mixing system and where you added your liquid chemicals. We saw this repeatedly. You turned your water treatment system into a slight polishing system for solids.  

Now, this water then went into a large pit, where any solids left over would settle anyway. The bigger issue was why are you so solids-focused. This recycled water is being used to carry sand, and that’s sand with 10%-and-higher fines content. So, if the sand is properly placed in the fractures, the fines stay free-floating, and this concentration is on the order of 10 to 15 times the treatment standard for solids.  

And this is the basic problem—a standard was created, based on the performance of a treatment system, not on what created the best recycled frac or completion fluid. There are some operators, who recognize this and don’t use any treatment systems in their recycling program, but the majority use treatment systems. The only good reason I’ve heard for a treatment system is to prevent solids from filling my pits.  

So, what’s the solution? Continue using tanks, especially the large Above-ground Storage Tanks (ASTs) before your pit. Add aeration to these tanks to improve oil recovery and possibly add tanks to get better solids recovery, or add a coagulant upstream of the tanks to improve solids recovery. Use an oil skimmer and stop using vac trucks to suck up oil and water and end up with junk oil. A good skimmer will give you better than 90% oil. You just optimized your tank system to replace your treatment system. 

Then add aeration in your pits for more bacteria and iron control. Finally, pick the right oxidizer and location to make sure you’re degrading the oxidizer before you get to your frac blender. Most people set up their OTF recycling on the frac or next to it, which is a bad location for some oxidizers. Or you can use ozone, which lets you set up close, because it degrades so quickly. 

What about my treatment equipment I invested in? Well, move it upstream. Replace tank batteries with treatment systems and improve your oil cut. If done with the right oxidizer, you can eliminate emissions as well. Not to mention the huge decrease in footprint. Now, downstream of there in your gathering systems, SWDs and recycling facilities, you removed the need for oil recovery, eliminated much of your downstream maintenance expenses and pretreated your produced water for desalination or other beneficial reuse projects, but that’s a story for another time.