Addressing challenges facing the industry
In 2018 and 2019, produced-water reuse and water sourcing for hydraulic fracturing operations were key topics in the western United States. In many of these fields, water-sourcing strategies utilizing mixed water sources were implemented, but the level of treatment for the waters ranged considerably as the industry worked to optimize water sourcing and disposal costs for the frac projects.
Through our field team’s involvement in multiple unconventional projects in 2019, Water Standard and its produced-water subsidiary, Monarch Separators, observed firsthand some of the challenges that the industry is currently working to address including:
• Reducing the cost of produced-water treatment for reuse
• Reducing the environmental impact of water operations, including sourcing, treatment and disposal of hydraulic-fracturing fluids
Although there are many aspects of produced-water treatment and reuse for hydraulic-fracturing applications, the use and cost of biocides consistently emerged as a key consideration that required further attention. Therefore, the purpose of this article is to present the observations our field team experienced regarding biocide usage during our extensive testing, treatment and studies for the oil and gas industry.
Typical Frac Water and Biocide Usage
To help frame the magnitude of water usage in one of the counties our team operated in, a wealth of data from FracFocus.org was collected and analyzed. For one of the larger exploration and production (E&P) companies, a total of 96 wells were completed during the last six months. The average well required approximately 250K bbls of water to complete. Over the six-month period, the total water usage was estimated at a staggering 40MM bbls of water.
Our studies indicated that the well-completion teams generally used a combination of a quick-kill biocide (2,2-dibromo-3-nitrilopropionamide, DBNPA) to prevent biological growth in the topsides facilities followed by a slow-acting preservative (4,4-dimethyloxazolidine, DMO) to provide lasting protection of the reservoir.
In recent cases, some of the wells were completed with oxidizing biocide (chlorine dioxide) as a substitute to DBNPA for topsides biological protection. Over the studied time frame, the average maximum reported DBNPA dose was 70 mg/L, and the average reported maximum DMO dose was 90 mg/L.
A typical process flow schematic is illustrated in Figure 1.
When conducting a cost analysis, it was found that biocide costs averaged close to $250,000 per well. So even a slight reduction (5-percent optimization) could potentially save millions of dollars.
Additional considerations have been raised recently by local academic communities (Hull, et al. 2018) about the fate of biocides and disinfection byproducts and microbial conditioning in produced water and flowback from these reservoirs.
In addition to the cost benefits of biocide reduction, there is an immediate environmental benefit by reducing biocide consumption as outlined below.
Consequences of Biological Growth
Significant biological growth in frac systems can have production, financial and health, safety and environment (HSE) related impacts (Moore et al, 2019). These consequences include:
• Well souring and hydrogen sulfide (H2S) formation
• Reservoir and well damage
• Corrosion of infrastructure
Well Souring and H2S Formation
Well souring occurs when sulfatereducing bacteria (SRB) become active in the reservoir. The natural respiration and metabolism processes of the SRB lead to formation of H2S. If SRB contamination runs unchecked, the oil in the reservoir can be devalued, HSE risks associated with operating around H2S can be present at the topside facilities, and higher operating and maintenance costs are likely due to increased corrosion caused by the H2S.
The onset of reservoir souring can be through multiple processes, but is generally due to the introduction of sulfate and other sulfur-containing compounds into the reservoir. This introduction can occur during hydraulic fracturing if the source waters for the frac fluids contain elevated levels of sulfate.
Higher temperatures (above 250 degrees Celsius), which can be common in some reservoirs, or the presence of metals in the reservoir can also lead to chemical sulfate reduction. This reduction may be misdiagnosed as biological reservoir souring.
Preventing SRB growth is a critical consideration for all frac projects, and biocide is added for protection. However, biocide by itself can be ineffective if SRB is already present in the process equipment or reservoir. A key challenge is that the SRB use their environment to build protective tubercles (small iron houses with controlled internal liquid environment) that can protect them from biocides.
Once initiated, SRB and microbial induced corrosion (MIC) can be very difficult to remedy. When H2S is present in existing equipment, a more effective management strategy is generally required to prevent the formation of additional H2S by eliminating new sulfur and sulfate sources.
Reservoir and Well Damage/ Microbiologically Influenced Formation Damage (MIFD)
Reservoir and well damage occurs when biofilms develop within the proppant during hydraulic fracturing. Depending on the extent of the biological growth, a significant amount of well productivity can be lost. When caused by biological communities, formation damage is not always reversible.
Recent studies into MIFD, including well damage, provide knowledge about the speed and severity of MIFD. Decreases in permeability in wells and reservoirs can occur due to clogging from biosolids and biofilms (Bottero 2010 and Yin 2016).
Damage from biological fouling can be seen in the simulated displacement models illustrated in Figure 2, where blue illustrates the reservoir, red illustrates frac fluid and gray illustrates biological growth. The modeling work illustrates proppant displacement becomes less effective when foulant is present in the proppant material.
Preventing SRB growth is a critical consideration for all frac projects, and biocide is added for protection.
Microbial Induced/Influenced Corrosion (MIC) of Infrastructure
MIC generally causes accelerated corrosion of metal surfaces and premature asset failure. Representatives of the types of bacteria associated with metals corrosion in pipeline systems include sulfate-reducing bacteria (SRB), metal-reducing bacteria (MRB), acid-producing bacteria (APB) and metal-oxidizing bacteria (MOB).
Uncontrolled microbial growth in industrial water-treatment systems can result in extensive biofouling on critical surfaces and subsequent biodeterioration of materials and process additives.
Biological Activity of Mixed Water Sources
The development of slickwater gel chemistries has allowed a wide range of water qualities to be compatible for use as a frac fluid. Because of the high tolerance to salinity that slickwater affords, less control of blending is required during produced-water reinjection. As a result of these additional sources, the industry has made major strides in the treatment of produced water for reuse.
In addition, many frac sites are located in relatively close proximity to freshwater supplies. The fresh water can be delivered from surface water and agricultural groundwater wells to adjacent properties through lay-flat hose and in many locations, can be supplied by a large pipeline freshwater distribution system.
However, a wide range of microorganisms grow in oil, gas and water systems because the essential elements for their growth are present in these environments. Microorganisms need four elements for reproduction: a carbon source, water, an electron donor and an electron acceptor. Most microorganisms found in oil and gas systems are heterotrophs, which convert organic carbon into food sources for growth.
Heterotrophs can thrive in a diverse range of environments and because of their rapid growth rate, the biological communities quickly adapt to significant environmental changes. Different strains of heterotrophic bacteria can be aerobic (respirating with oxygen) or anaerobic (do not require oxygen).
Hydrocarbon and other organic compounds are an excellent carbon (food) source for a wide variety of heterotrophic microbes. For these reasons, oil and gas systems can be quickly overtaken by microbial communities.
Other bacteria including nitrifiers (ammonia converting), nitrate-reducing bacteria (NRB) and SRB can also grow in the presence of hydrocarbonbased food sources. These microbes can obtain energy by oxidizing organic carbon or molecular hydrogen. SRB reduces sulfate ions (SO4), which are electron acceptors, to produce H2S.
During produced-water treatment, most of the actively reproducing bacteria are generally deactivated (made sterile) using strong oxidants such as ozone, peracetic acid, hydrogen peroxide and chlorine dioxide.
However, even with effective oil separation, a large amount of dissolved organic material remains in treated produced water that can be used as food by microbes entering the system after treatment (example; windblown dirt in evaporation ponds, bird feces, etc.) or by cysts that may be present in the water.
When strong oxidizers are used, biological growth can become more severe if the dissolved organic constituents are converted to more assimilable forms during the oxidation process.
In systems where the produced water blends with raw surface water and groundwater at a frac site, the biological activity of the blend stream may be orders of magnitude higher than the disinfected produced water by itself due to the reduction in salinity that typically occurs when the fresh waters are blended with the produced water.
Challenges With Typical Topsides Equipment
Raw water is seldom disinfected during delivery. This approach is helpful from an HSE perspective because any spilled water is treated as surface or groundwater, and the spill responsibility is minimized. However, the raw water is typically filtered and disinfected at frac sites prior to storage and staging.
Considering the amount of biocide delivered during topsides disinfection, raw water should be fully disinfected. However, the sampling performed as part of our studies indicated that very high levels of biogrowth occur in the system even in the presence of relatively high doses of non-oxidizing biocide.
The benefit of the bladder tanks is that they can be collapsed and moved after the project, quickly providing a largevolume closed tank.
The reasoning behind this biogrowth can be seen in the layout demonstrated in Figure 1. The typical frac-water systems in the basin use a combination of frac tanks and large bladder tanks (Minion Tanks) for staging. The benefit of the bladder tanks, shown in Figure 3, is that they can be collapsed and moved after the project, quickly providing a largevolume closed tank.
The downside, however, is that this type of water-storage system can facilitate microbial growth. By design, these bladders are very difficult to clean and disinfect between projects, and as a result, the tanks, as well as improperly cleaned frac tanks, can seed bacteria and provide protective silt and mud deposits to allow bacteria to grow.
Over the next 10 years, microbial control during hydraulic fracturing is expected to remain a key consideration for frac reuse due to the inherent risk of well damage when microbes become established in the wells.
Other factors, including disinfection byproducts in produced water and the potential of conditioned bacterial release, are expected to become impactful to frac reuse, as well. These factors may drive the industry to reevaluate the typical ranges of biocides used for topside and downhole control as treated produced-water reuse in surface water discharge and agricultural applications become more feasible.
Presently, there is much that can be done to improve the performance of biocides or reduce biocide consumption during hydraulic fracturing including better methods for portable tank cleaning and an improved understanding of the interaction between fresh and treated produced-water systems when blended reuse strategies are employed.
Greater improvement in these areas is anticipated to have a big impact on drilling operations, and these improvements are expected to enable the industry to achieve sustainable and profitable operation in the face of consistently low oil prices.
Over the next 10 years, microbial control during hydraulic fracturing is expected to remain a key consideration for frac reuse.
Authored by Robert Boysen, P.E. + Lisa Henthorne, P.E.
References: Bottero, S., Piciorenau, C., Enzien, M., van Loosdrecht, M.C.M, Bruining H. Heimovaara, T. (2010). Formation Damage and Impact on Gas Flow Caused by Biofilms Growing Within Proppant Packing used in Hydraulic Fracturing. Presented at SPE International Symposium on Formation Damage. Lafayette, LA. February 2010. Hull, N.M., Rosenblum, J.S., Robertson, C.E., Harris, J.K. & Linden, K.G. (2018). Succession of Toxicity and Microbiota in Hydraulic Fracturing Flowback and Produced Water in the Denver-Julesburg Basin. Science of the Total Environment 644. 183-192. Moore, J., Massie-Schuch, E., Wunch, K. Manna, K., Daly, R., Wilkins, M. & Wrighton, K. (2019). Insights into Effective Microbial Control Through Comprehensive Microbiological Audit of Hydraulic Fracturing Operations. Presented at the SPE International Conference on Oilfield Chemistry. Galveston, TX, USA. April 2019. Yin, B., Williams, T. Koehler, T., Morris, B. & Manna, K. (2016) Targeted microbial control for hydrocarbon reservoir: Identify new biocide offerings for souring control using thermophile testing capabilities. International Biodeterioration & Biodegradation. 1-4.