Biocide guide: Protecting reservoirs and maximizing oilfield production

When it comes to maximizing productivity and cost in oil and gas operations, biocides matter. Although they make up less than 0.1% of hydraulic fracturing fluid, biocides play a critical role in strengthening productivity and reducing operating costs.

Hydraulic fracturing of shale requires adding large volumes of water and biodegradable additives to reservoirs, creating the ideal environment for harmful microbes to proliferate. Overcoming well contamination is not an easy—or inexpensive—task, especially with increasing costs and limitations on common oxidizing biocides, such as chlorine dioxide (ClO₂). But, by integrating a robust microbial control program, operators can effectively improve productivity, asset integrity and hydrocarbon quality, without breaking the bank.

MICROBIAL CONTAMINATION: WHAT’S AT RISK?

Without an effective antimicrobial solution in place, assets, processes and hydrocarbons are conducive to microbial damage—including biofouling, microbially influenced corrosion (MIC) and souring. When microbial contamination occurs, the operator’s bottom line is impacted. Replacing assets from MIC is likely the first expense that comes to mind. However, a less obvious challenge is the reduced flow and hydrocarbon production rate due to biofilm—a slimy film of bacteria—and plugging. In fact, modeling studies show that even minimal biofilm coverage on proppant, or sand-like material, can result in a 50% decrease in gas flowrates.¹

Additionally, reservoir souring, the generation of hydrogen sulfide (H₂S) from sulfate-reducing bacteria, presents myriad costs, including the expense of H₂S scavengers and lost value from sour hydrocarbons. At the same time, H₂S can lead to increasingly dangerous and even potentially lethal work conditions for field personnel.

H₂S is a poisonous, corrosive and potentially flammable gas produced by microbes metabolizing hydrocarbons in wells. Uncontrolled microbial growth can put worker safety at risk, due to high H₂S levels or MIC. Contaminated operations are also more susceptible to leaks and spills, which can draw unwanted attention from key stakeholders, media, government officials and the public eye. An integrated microbial control program, applying the right course of biocides, can help operators protect their workers, assets and the business’ safety profile overall.

MEET MICROBIAL MANAGEMENT

It is no secret that hydraulic fracturing is a water-intensive process, but many completion engineers may not fully understand the ongoing threat of microbial contamination. Fortunately, biocides are here to help and can be added to fracturing fluids to control microbes in water storage and transport assets. With a relatively small investment, typically less than 0.5% of overall fracturing operations, an effective biocide program can also be cost-effective.

Fig. 1. Areas where microbial control issues can develop in hydraulic fracturing.

To select the appropriate biocide, it is critical to identify application conditions for each phase of the hydraulic fracturing process. Phases can differ greatly in terms of temperature, shale type, additive compatibility, aerobic/anaerobic conditions, salinity and desired duration of microbial control. For example, before water is injected into the well, it is prepared topside, Fig. 1. Several microorganisms are often present during this phase, due to natural nutrients and the use of recycled water. Treating the water with prepare treatments, or quick-kill biocides, can help strengthen biocide performance later in the process.

It is important that biocides are integrated into the well during the decontamination stage, to control organisms introduced during drilling and completion activities. From initial production to drilling, chemistries like glutaraldehyde (glut) and glut/quaternary ammonium (quat) blends are optimal to help control microbes.

As for reservoir protection biocides, chemistries must be formulated to withstand extreme heat and saline conditions for extended periods of time. Biocides are especially vital during the final hydraulic fracturing phase, as organism growth can cause formation damage and H₂S production over time.

CHOOSING THE RIGHT BIOCIDE: PHASE 1

From bioload in topside water holding tanks to shale presence in reservoirs, conditions vary throughout the three hydraulic fracturing phases. As such, each phase requires a distinct biocide treatment. In the first phase, quick-kill biocides can be used to chemically condition the water, reducing bioload and readying the fluid for intense downhole environments. However, it is important to note that prepare treatments cannot be used alone, as they do not control downhole organisms. This is due to the chemistries’ instability to perform at high temperatures and/or interact with proppant, injection fluid additives and shale.

Many prepare treatments are oxidizing biocides that react with both biological and non-biological contaminants. These chemicals precipitate metals, while their oxide salts, such as iron, react with organic materials to make them more water-soluble. However, there are a few non-oxidizing chemistries available for water preparation, including DBNPA and surface-active solutions, such as alkyldimethylbenzylammonium chloride (ADBAC), dimethylammonium chloride (DDAC) and tributyltetradecylphosphonium chloride (TTPC).

Fig. 2. Biocide efficacy across three distinct phases of hydraulic fracturing operations.

CHOOSING THE RIGHT BIOCIDE: PHASE 2

In the second phase, the well is decontaminated from microbes found downhole and those unsuccessfully removed from the topside water tank. To successfully decontaminate wells, biocides require three key elements: stability in extreme heat and salinity, ability to control organisms downhole and compatibility with proppant/fracturing fluid additives. Prior to drilling, naturally occurring organisms exist in the well, with some as deep as 2 mi below the surface. Most testing is completed on topside water, but it is also crucial to mitigate microbes living in the downhole environment. Traditional oilfield biocides—such as glut and glut-quat—can be used to effectively decontaminate wells. These chemistries have sustained downhole activity, allowing them to eliminate microbial populations in a matter of hours to a few days.

CHOOSING THE RIGHT BIOCIDE: PHASE 3

In the final phase, biocides are tasked with protecting the reservoir. Biocides used for reservoir protection have slower kill-speeds in comparison to prepare and decontaminate treatments. Today, integrated microbial control programs often include long-term preservatives, which can control harmful bacteria for weeks to months. Sustained microbial control, in the reservoir, is essential, as there is often downtime between completion and initial production without active biocides, offering microbes a window of opportunity to proliferate. Long-term preservatives, such as dimethyloxazolidine (DMO) and tris(hydroxymethyl)nitromethane (THNM), can inhibit microbial growth and prevent the safety and economical risks associated with contamination.

Selecting the right biocide for each phase may seem like a daunting task, but it does not have to be. Biocide selection is often guided by technical data, generated under laboratory benchtop conditions. While informative, these data do not fully simulate the downhole environment created by drilling and fracturing operations, especially during a shut-in period. Advanced high-pressure, high-temperature (HPHT) Bioreactors can optimize the biocide selection process for hydraulically fractured shale reserves. This system is an industry-first technology, used to vet biocides’ ability to prevent contamination at larger experimental volumes than previously possible with field trials. HPHT Bioreactors simulate short and extended shut-in periods between completion and initial production to mitigate any souring, production and corrosion issues over time. As such, this technology is gaining vast momentum toward industry-wide adoption.

SELECTION FOR FRAC SYSTEM COMPATIBILITY

Selecting biocides, based on hydraulic fracturing phases, is just the beginning. Operators must also consider biocides’ compatibility with proppants, friction reducers and other frac fluid additives, for optimized microbial control. For example, using greater volumes of proppant can increase the potential of microbial contamination from the proppant itself. Diverse biocide solutions and dosage levels are required to both decontaminate the well and protect the reservoir.

Many hydraulic fracturing operators learn—usually the hard way—that while less-expensive options, such as ClO₂ and TTPC, are effective during topside preparation, they will not decontaminate the well or protect the reservoir. With the increasing amount of slickwater fracs used today, operators must understand that cationic surface-active chemistries, like TTPC, are often incompatible with anionic friction reducers, like hydrolyzed polyacrylamide (hPAM), and oxidizers, like ClO₂, are highly reactive when met with many frac fluid additives or other organic materials.

RESEARCH ON SHALE COMPATIBILITY

Researchers have thoroughly studied oilfield biocides’ compatibility with other frac fluid chemicals. However, there is limited research available on biocide compatibility with the reservoir shale itself. In a previous study, our experts gained further insight into shale’s impact on microbial growth and biocide efficacy in the downhole environment.

The team designed and conducted a robust study, to evaluate biocide performance in the presence of shale formation rock.² Rock and bacteria were positioned together as a model for real oilfield conditions, during the well decontamination and reservoir protection phases. Biocide efficacy was tested over 4 hr to mimic biocidal action during well decontamination. Researchers then evaluated the biocides’ efficacy after seven days, to represent long-term reservoir protection.

As a result, decontaminating biocides, such as glut, were found to be very effective, regardless of microbial presence or rock type. Quick-kill biocides, including TTPC and ADBAC, absorbed to the rock and lost efficacy entirely after a few hours. Slow-acting preservative biocides, such as DMO and THNM, performed better over an extended duration. Overall, these results revealed that shale compatibility is a critical consideration when looking to select the right biocide for an integrated microbial control program.

ONE SIZE DOES NOT FIT ALL

There is no “silver bullet” biocide capable of treating every hydraulic fracturing phase. However, there seems to be a preconceived notion that ClO₂, one of the most common oxidizers used, will address all microbial control needs. While it is an effective prepare treatment, its efficacy diminishes after initial topside decontamination. ClO₂ fails to sterilize and eliminate bacteria from fracturing operations, requiring dosage levels that could potentially cause corrosion. Furthermore, oxidizing biocides are not effective under high temperatures. In fact, when our experts explored ClO₂’s performance under extreme temperatures, they discovered that it lost efficacy within 2 hr.

Fig. 3. Biocide efficacy depends on the hydraulic fracturing phase and shale type.

Beyond efficacy in each phase, the right biocide depends on frac fluid additives—friction reducers, proppant, shale type, pressure and temperature resilience and the water used. While there is no one-size-fits all biocide, glut has proven to be one of the most versatile chemistries available. It is effective through both preparation decontamination phases, along with short-term efficacy as a protective biocide.

Furthermore, glut-quat compounds have been shown to notably increase kill speed and are well-suited to meet most oilfield application demands. Glut-quats can significantly reduce sessile microorganism populations, which are known to cause corrosion and reduce heat exchange efficiency in fracturing operations. In fact, the efficacy of glut-quat inspired the development of AQUCAR™ 714 Water Treatment Microbiocide, an aqueous biocide blend that combines advanced capabilities of glut and quat. This synergistic combination offers numerous benefits, including efficacy through extreme temperatures, increased operational sustainability, compatibility with common frac fluid additives and cost-efficiency.

BIOCIDES OR BUST

Selecting the right biocide is not a simple “check-the-box” process. Operators must take the specific hydraulic fracturing phase, fluid additives, shale type, frac system and more into account, to determine the appropriate solution. Operators should consider HPHT Bioreactors as a key resource in assessing preservative chemistries in the downhole environment and to pinpoint the appropriate biocide for oil fields. This technology is increasingly helpful for completion and production engineers to optimize their antimicrobial program, while avoiding lengthy and expensive field trials.

Collaborating closely with a reliable, established biocide supplier will equip operators with the necessary technical expertise, biocide guidance, regulatory support and long-term reservoir protection. Oilfield microbial control methods are constantly evolving, and it is critical for operators to have confidence in their integrated program to not only to protect assets, but also their personnel.

An effective microbial control program is a small upfront cost, which provides an exponential return on investment. By simply applying the right biocide, operators can maximize production and enhance hydrocarbon quality within the first six months of unconventional well production. Biocides can also help operators propel hydraulic fracturing forward by delivering high-quality hydrocarbons as quickly, sustainably and economically as possible.

REFERENCES

1. Bottero, S., C. Picioreanu, T. Heimovaara, M. Enzien, M. van Loosdrecht and H. Bruining, H., SPE, ‘Formation damage and impact on gas flow caused by biofilms growing within proppant packing used in hydraulic fracturing,” paper 128066, presented at the 2010 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Colo., Feb. 10-12, 2010.
2. Moore, J., et al., “Oilfield biocide performance in the presence of shale formation rock,” paper SPE-184583, SPE Oilfield International Conference on Oilfield Chemistry, Montgomery, Texas, April 3-5, 2017.