Bacterial disinfection is a basic objective in the management of produced water. Disinfection is accomplished with the use of oxidizing or nonoxidizing biocides. Because of the relatively slow-acting nature of nonoxidizing biocides, determining the optimum dose and performing real-time monitoring can be difficult. These monitoring difficulties, when combined with the toxic nature of nonoxidizing biocides, have swayed the industry toward using mostly oxidizing biocides. Oxidizers like ozone and chlorine dioxide (ClO2), among others, are commonly used.

Residual oxidants
Monitoring is critical for an effective bacterial disinfection program. Without proper monitoring there are no guarantees bacteria are effectively being killed. Oxidation reduction measurements and testing for residual oxidant are useful but only give a probability of disinfection. The other concern with residual oxidant measurements is that real-time methods can be affected by interference in produced water, meaning uncertainty can exist about the accuracy of these residual measurements. Nonetheless, residual oxidant monitoring has become the most common way to monitor disinfection.

Residual oxidant reacts with other additives used in fracturing, creating other concerns. Residual oxidation reacts with friction reducers (FR) and gels, effectively reducing the latter’s ability to perform. Friction reduction will decrease with increasing residual oxidation, and gel viscosity and break times can be affected. Relying on residual oxidation as a verification of bacterial reduction performance is directly accepting some level of incompatibility.

An adversarial relationship exists between residual oxidation for disinfection and compatibility with other frack additives. Unfortunately, the effects of residual oxidation on compatibility have been widely ignored and underappreciated.
In many applications, gel formulas are adjusted or stabilizers are added to improve gel compatibility, which in actuality merely treats the symptoms without addressing the root cause. FR concentrations also are normally adjusted, which again only treats the symptom without addressing the cause. It is not uncommon for more FR to be used than is necessary, masking the effect of incompatibility.

Using pump pressure to dictate dosage
A yearlong case study in the Permian Basin was completed using FR concentration directly as reported from FracFocus. The data shown in Table 1 are from the same operator in the Permian Basin using the same completion method on all wells.

Disinfectants were an oxidizing biocide, ClO2 and ozone. Instrumental in these results was the operator’s commitment to adjust FR concentration based on pump pressures. This prevented overdosing. Ozone, which uses less residual oxidation, showed decreases in friction reduction ranging from 30% to 50%.

There has to be a commitment to allow pump pressure to dictate FR dose rate. This will allow an operator to enjoy the cost benefits of lower FR use while exposing the effects of incompatibility from residual oxidation.

Incompatibility arises when there is too much residual oxidation present when the fluid reaches the blender. How each oxidizing biocide affects compatibility is a result of its oxidative strength, half-life and residual concentration (Table 2). Stronger oxidants tend to work faster, and improving mixing and mass transfer will help consume oxidation and will speed up the disinfection process. The most effective approach to bacterial disinfection is to focus on rapid disinfection, improve mixing and mass transfer and select an oxidant that has a shorter half-life. This combination provides an effective bacterial disinfection program without sacrificing compatibility.

Water treatment technology

Number of wells

FR concentration,
% in mass

Biocide

22

0.0123

Chlorine dioxide

9

0.0146

Ozone

27

0.0042

TABLE 1. A comparison is shown of FR concentrations found in Permian Basin wells over a one-year period. (Source: Hydrozonix)

Oxidant

Oxidation potential, V

Half-life @ 20 C (104 F)

Hydroxl radicals

2.8

< 1 second

Ozone

2.3

20 minutes

Hydrogen peroxide

1.8

Hours

Chlorine dioxide

1.5

93 minutes

Chlorine

1.4

140 minutes

TABLE 2. By focusing on rapid disinfection, improving mixing and mass transfer and selecting an oxidant with a shorter half-life (in red), an effective bacterial disinfection program is created that doesn’t sacrifice compatibility. (Source: Hydrozonix)

Monitoring, testing are key
In municipal water markets, residual oxidation is key. Long transit times and keeping water disinfected during transport and storage before use require strong residual disinfection. Unfortunately, this concept does not transfer to frack water reuse. The idea of needing residual disinfection to ensure there are no downhole impacts ignores the fact that additives are introduced that react with oxidizers. The chance that residual oxidation will find bacteria before FR or gels, which will be at significantly
higher concentrations, is unlikely.

The goal becomes to disinfect before the blender and have enough residual disinfection to maintain disinfection in the working tanks. An effective, quantitative real-time testing program can ensure that adequate disinfection has been achieved without excessive residual. Monitoring working tanks is critical.

In many cases, residual oxidation testing is used exclusively. There are accuracy concerns testing produced water. A test program should include a real-time bacteria quantification method.

It is difficult to test for FR in real time as a result of proprietary formulas and the lack of test methods. A field device can be used to test friction factor before and after treatment. Reductions in friction reduction after treatment will indicate an incompatibility.

Fluids can be evaluated for compatibility and FR overdosing. Eliminating overdosing is important to evaluate compatibility. Testing FR at different dose rates will allow optimization of the effective dose rate. When too much FR is used, some of it can be oxidized without a significant change in friction. When the dose rate is reduced to an optimum rate, changes in concentration will have a noticeable effect on friction reduction.

This baseline test will establish a starting point to begin evaluation. With a baseline, treated water can be compared to untreated water with the appropriate FR concentration. Figure 1 shows friction reduction between a baseline of untreated water and 0.5 gpt (gallons of FR per 1,000 gallons of water), treated water with FR and treated water without FR using ClO2. This displays a compatibility concern. It would require a decrease in ClO2 concentration and then further evaluation of disinfection to determine if optimum disinfection can be achieved while preventing a compatibility problem.

In this last example, ozone and friction reduction were monitored (Figure 2). A baseline was established, and treated water was compared with and without FR. There is an actual improvement in friction reduction. This is not unusual as a result of the mass transfer and formation of microbubbles that aid in friction reduction.

Performing these tests in the field can avoid compatibility issues in addition to helping validate the current FR dose rate. Money can be saved by avoiding compatibility issues through overdosing. Reductions greater than 50% are not uncommon and can result in significant savings.