Improving well integrity and oilfield efficiencies through biocide deployment in the Permian Basin

The fracking market is projected to be worth US$74.4 billion by 2028, up from 2023’s market value of US$52.1 billion¹. The North American market is expected to be the largest during this 2023 – 2028 projection due to the increase in unconventional reserves. The region already accounts for up to 17.9% of the world’s total oil trade today².

Fracking operations use large amounts of water to carry sand and chemicals into the rock formations to fracture the rock and release previously trapped oil and gas. Successfully stimulating and producing from a single fracked oil or gas well could potentially use upwards of 750 000 barrels of water³.

While this process is effective in accelerating the production of hydrocarbons, these large demands for water also create increasing costs and sustainability issues. Bottom line and inflationary pressures are leading industry executives, particularly those based in the United States, to predict drilling and fracking expenses will climb throughout 2024⁴. This includes the most prolific shale field, the Permian Basin, which has seen a 50% increase in costs since 2021⁵. In addition to this, the energy industry at large faces increasing pressure to improve efficiencies, in line with net zero targets, while still meeting the current demand for energy. With ongoing changes and costs mounting, the need for using chemistries to treat water used during fracking are of particular interest to operators who want to improve operating costs and sustainability.

The chemistry of water treatment used for fracking

Water is crucial to the process of fracking and serves as the basis for fracking fluid, driving pressure into the rock formation at the start of operations, right through to the clean-up of a project and the flushing out of debris. The volume of water used to complete a well has been increasing annually.

As the industry embraces methods for improving environmental sustainability, many operators have begun recycling the water used during fracking. By reusing water, freshwater can be preserved for drinking or used in industries like agriculture and eliminates the subsequent environmental impacts from its withdrawal, such as environmental damage. This also means that operations are not contributing to water scarcity, which can be impacted by the large amounts of water needed for the industry.

The practice of recycling water also reduces the infrastructure costs of sourcing, transferring and storing freshwater. However, this recycled water, combined with the mixing of surface and formation waters, can present multiple issues for operators.

These issues may include increased problems with scale formation, higher organic content, higher divalent content and higher bacteria population in the recycled water. An increased bacteria population could lead to pitting corrosion, also referred to in the industry as microbiologically influenced corrosion (MIC), which can lead to eventual production shut down and increased costs for operators. Restoration of production flow requires identification and costly replacement of the affected tubing. High bacteria population could also lead to production of hydrogen sulphide (H₂S), which is toxic to humans and is flammable, increasing the hazard to the lives of the workers supporting the project in the field.

The presence of H₂S can also cause the well to sour. Compared with sweet wells, the returns from sour wells are typically less prolific and require more intervention during downstream operations, meaning that it is more profitable and economical to ensure a sweet well stays sweet6. Another challenge with H₂S is its reaction with iron from surrounding metal or reservoir fluid to form iron sulfide (Fe₂S₃). This compound forms an emulsion with oil and water, creating a thick sludge that reduces production and adds to clean up costs. It is also suspected to form downhole and could be a factor in premature production declines.

To frack with the safest and lowest total cost of operation, it is beneficial to ensure the water entering the pipeline has the optimum balance of treatment from relevant chemistries to pre-empt these issues as reservoir souring caused by bacteria can often be irreversible.

An intricate balance

Biocides are chemicals which can be used to reduce bacteria and control growth in oil and gas production systems. There are different categories of biocides used for oil and gas projects, such as oxidisers, conventional biocides like glutaraldehyde and THPS with quat mixtures, and preservatives. Ideally, biocides should be deployed downhole from the very start of downhole operations, to avoid issues occurring down the line of production. The optimal microbial mitigation will depend on the water quality, reservoir parameters, production timeline and economics.

Testing the waters

In a recent project for an operator in the Permian Basin, ChampionX was chosen to research the benefits of deploying specific combinations of biocides and oxidisers applied for topside control.

During the initial stages of the project, a team of chemistry experts considered the benefits of the three most common oxidisers typically used for water treatment in the oil and gas industry: peracid, bleach and chlorine dioxide. Each of these oxidisers, while having many similarities, have significant differences which can drastically impact the water quality for well completion.

Bleach is the most common and readily available oxidiser. It is kept stable in a solution at pH 13 – 14. This product reacts with all organics and will not selectively target bacteria. When used in produced water that inherently carries more organic loading, much higher concentration is needed to deliver the effective kill. However, using bleach at higher volumes will increase the pH of the water, thus increasing scaling tendency, and the bleach loading needed is two-to-four times higher than other oxidisers.

Chlorine dioxide (ClO₂) is a very effective biocide gas. This biocide is generated on site and dissolved in water before dosage into the fluid to be treated. The high biocide efficacy also means high lethality for all living beings. In addition, ClO₂ gas is flammable and highly reactive. Engineering designs have come a long way to ensure safety during field usage, but in general, this product carries a higher risk to the surrounding field operators than other oxidisers.

The third option to consider is peracetic acid (PAA). This is a liquid product that naturally breaks down into its base constituents of oxygen, water and vinegar. When applied correctly, there will be no gas released, as all the oxygen will be fully consumed. PAA is more selective and will not react with the higher organic loading in produced water, resulting in significantly lower dosage. PAA also has a pungent vinegar smell that is an excellent identifier of leaks. Just like any oxidiser, it is not without hazard. The concentrate is very sensitive to contamination and will significantly exotherm with negligent handling.

Considering all the above factors, PAA was chosen to be tested as part of the project. One of the most important steps in mitigating risks associated with uncontrolled microbial growth is the detection of all microorganisms present. This measurement allows for the determination of the optimal biocide type and dosage. ChampionX uses the adenosine tri-phosphate (ATP) by enzymatic-driven bioluminescence for active bacteria method of measurement. To determine the full value of a biocide, kill studies should also account for dormant organisms.

In addition to measurement of ATP for active bacteria, a proprietary measurement technique called adenosine monophosphate (AMP), which detects a protein present in all living organisms, including those in dormant state can be used. In simple terms, the practical purpose of AMP measurement is to gauge those organisms that are weakened to a dormant state and may come back to an active state. After treatment with a biocide, both ATP and AMP measurements provide an accurate assessment of initial kill and the risk of regrowth from dormant bacteria. ATP and AMP quantification were used in a kill study to measure initial reduction and regrowth risk of planktonic bacteria by PAA, bleach and chlorine dioxide in three produced water ponds in the Permian of varying water quality.

Figure 1: Reduction of active bacteria by PAA, bleach, and ClO₂ at 15 minutes

ATP results indicate that bleach and PAA show equivalent efficacy in two of the three ponds, with ClO₂ only showing efficacy in pond one. It is unlikely that ATP measurement alone would accurately measure the efficacy of these three oxidisers, so the AMP method was also used to generate accurate representation of treated fluids for regrowth risk. There is a potential for regrowth to varying degrees in two out of three ponds for almost all three oxidisers. The conclusion from the kill study above from both ATP and AMP indicates the need for a secondary biocide to address regrowth downhole. To determine the secondary biocide, it is important to consider the antimicrobial performance of a biocide, which is dependent upon compatibility with reservoir parameters, as they are intended to provide long-term microbial control.

No loss of efficacy was observed due to adsorption for any of the tested preservatives. Incubation was extended out to nine days to account for slower kill rates associated with these biocides. In Lytic biocides (ADBAC, TTPC) adsorbed to the rock when exposed to sandstone. THPS lost efficacy over a 24-hour period, while glutaraldehyde was unaffected in the presence of sandstone. These findings are corroborated by another study comparing the performance of biocides in the presence of fine-, medium-, and coarse-ground shale.⁷

The takeaway from that work is that mild adverse effects were observed when glutaraldehyde is exposed to fine-ground shale for only four hours, but antimicrobial efficacy is otherwise unaffected, and THPS loses almost all activity when exposed to any grade of shale for four hours. As for the preservatives in the presence of shale, each saw a reduced efficacy in the presence of fine-ground shale, with CTAC and THNM showing no reduction in medium- and course-ground shale, and DMO efficacy was completely lost in the presence of medium-ground shale.

Planktonic kill studies were conducted at high temperature (75°C) against two SRB strains to validate use under reservoir conditions. Results reveal that glutaraldehyde effected short-term control, but the dosage required to maintain control rose sharply after seven days.

Preservatives THNM and DMO required high dosages to effect immediate kill but less than 40 ppm to maintain long-term control. The delayed efficacy and high initial dosages of preservatives means these types of chemistries are best used in combination with a faster acting biocide. Given the weaknesses of glutaraldehyde, quats, and THPS at downhole conditions, co-injection of an oxidiser along with a preservative provides topside and downhole microbial control.

This combination treatment of oxidiser and preservative has been validated through a study involving multiple operators over several years in more than 600 wells. Comparing wells treated with non-oxidising biocide, an oxidising biocide, and the combination of oxidiser and preservative indicated microbial control was best achieved with the combination program.

Figure 2: 30/60/90-day flowback bacteria levels in treated systems in the Permian

Securing the future

As a part of a broader initiative to validate performance of biocide program performance, produced fluid from >600 wells was analysed for bacteria (ATP) and dissolved H₂S over 30, 60 and 90-day intervals. The average value for wells treated with the oxidiser and preservative combination is lower in currently producing wells.

There are many different chemical treatment considerations to be made during well completion. It is critical that the right biocide(s) is chosen to treat produced water and therefore reduce or avoid reservoir souring and associated safety and costly production challenges like corrosion and iron sulfide build up. Compatibility of biocides for completions is especially important because they are intended to provide both short-term bacteria kill to disinfect before going downhole and long-term control under reservoir conditions. With the fracking market predicting increasing growth yet facing increasing pressure to cuts costs and improve efficiencies, treatment methods that can improve production and reduce potential maintenance and safety issues can be imperative to operators and the future of the industry.

References

1. https://www.prnewswire.com/news-releases/hydraulic-fracturing-market-worth-74-4-billion-by-2028---exclusive-report-by-marketsandmarkets-301883770.html

2. https://www.prnewswire.com/news-releases/hydraulic-fracturing-market-worth-74-4-billion-by-2028---exclusive-report-by-marketsandmarkets-301883770.html

3. https://www.nytimes.com/interactive/2023/09/25/climate/fracking-oil-gas-wells-water.html

4. https://www.bloomberg.com/news/articles/2023-09-27/us-shale-executives-bracing-for-record-costs-to-get-worse

5. https://www.bloomberg.com/news/articles/2023-09-27/us-shale-executives-bracing-for-record-costs-to-get-worse

6. https://www.investopedia.com/terms/s/sourcrude.asp#:~:text=Refineries%20generally%20prefer%20sweet%20crude,heating%20oil%20and%20jet%20fuel.

7. Moore, J., Massie-Schuh, E., Doshi, D., Schultz, C., Castillo, C., Patel, B., Moore, M., Rajan, J., and Ajayi, B. "Oilfield Biocide Performance in the Presence of Shale Formation Rock." Paper presented at the SPE International Conference on Oilfield Chemistry, Montgomery, Texas, USA, April 2017.

Read the article online at: https://www.oilfieldtechnology.com/special-reports/01112024/improving-well-integrity-and-oilfield-efficiencies-through-biocide-deployment-in-the-permian-basin/