Figure 1: Separators, flowback tanks, and water tanks on a well pad with multiple well heads

This blog describes NIOSH evaluations of worker exposures to specific chemicals during oil and gas extraction flowback and production testing activities. These activities occur after well stimulation and are necessary to bring the well into production. Included are descriptions of initial exposure assessments, findings, and recommendations to reduce worker exposures to potential hazards. Further details about these assessments can be read in a recently published peer-reviewed journal article, “Evaluation of Some Potential Chemical Exposure Risks during Flowback Operations in Unconventional Oil and Gas Extraction: Preliminary Results”.[i]

Flowback Operations

Flowback refers to process fluids that return from the well bore and are collected on the surface after hydraulic fracturing. In addition to the mixture originally injected, returning process fluids can contain a number of naturally occurring materials originating from within the earth, including hydrocarbons such as benzene. After separation, flowback fluids are typically stored temporarily in tanks or surface impoundments (lined pits, ponds) and recovered oil is pumped to production tanks, which are fixed systems at the well pad. Figure 1 shows two separators (white), six flowback tanks (tan), and multiple water tanks (yellow) arranged in a typical side-by-side manner in the background of the photo. Workers periodically gauge the fluid levels in both flowback and production tanks with hand-held gauges (sticks and tapes) through access hatches at the top of the tank. (Figure 2)

Initial Exposure Assessments during Flowback and Production Testing Operations

NIOSH exposure assessments included short-term and full-shift personal breathing zone and area air sampling for exposures to benzene and other hydrocarbons using standard methods and analyses listed in the NIOSH Manual of Analytical Methods.[ii] Real-time, direct reading instruments were also used to characterize peak and short-term exposures to workers and various workplace areas for volatile organic compounds, benzene, carbon monoxide, hydrogen sulfide, and flammable/explosive atmospheres. We conducted biological monitoring by collecting pre- and post-shift urine samples from flowback workers to evaluate exposure to benzene. Benzene metabolites found in a worker’s urine indicate some level of exposure during the work shift. Benzene is an exposure concern because the Department of Health and Human Services’ National Toxicology Program has determined that it is a known carcinogen (i.e., can cause cancer).[iii] The International Agency for Cancer Research and the EPA have also determined that benzene is carcinogenic to humans.[iv,v]

Findings

Figure 2 A flowback technician gauging a flowback tank through a hatch on top of the tank.

Workers gauging tanks can be exposed to higher than recommended levels of benzene. The average full-shift time-weighted average (TWA) personal breathing zone benzene exposure (± 1 standard deviation) for workers gauging flowback or production tanks (n=17) was 0.25 ± 0.16 parts per million (ppm). Fifteen of these 17 samples exceeded the NIOSH recommended exposure limit (REL) of 0.1 ppm (0.32 mg/m³)[vi]. (This REL is a quantitative value based primarily on analytical limits of detection. NIOSH recommends that occupational exposures to carcinogens be limited to the lowest feasible concentration). Because the flowback technicians’ work shifts were 12 hours, a reduction factor of 0.5 was calculated to modify the American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit value for benzene from 0.5 ppm to 0.25 ppm. Two of 17 samples met or exceeded the ACGIH unadjusted value of 0.5 ppm value; six of 17 exceeded the adjusted value of 0.25 ppm.[vii] Task-based personal breathing zone samples for benzene collected during tank gauging on flowback tanks exceeded the NIOSH short-term exposure limit (STEL) for benzene (1 ppm as a 15-minute TWA).[ii] At several sites, direct-reading instrumentation measurements detected peak benzene concentrations at open hatches exceeding 200 ppm.

The average full-shift personal breathing zone benzene exposures (± 1 standard deviation) for workers not gauging tanks (n=18) was 0.04 ± 0.03 ppm. The difference in mean personal breathing zone benzene exposures between those who gauged tanks and those who did not was statistically significant. Seventeen of the 18 samples were below the REL as a full-shift TWA for those not gauging tanks. None of the 35 full-shift personal breathing zone sampling results exceeded the Occupational Safety and Health Administration (OSHA) permissible exposure limit for benzene of 1 ppm for general industry (29 CFR 1910.1028) or 10 ppm for the oil and gas drilling, production, and servicing operations sector exempt from the benzene standard (29 CFR 1910.1000 Table Z-2).[viii] Exposures to other measured hydrocarbons (e.g., toluene, ethyl benzene, and xylenes) did not exceed any established occupational exposure limits.

For the biological monitoring, we used s-phenyl mercapturic acid, a specific metabolite of benzene that can be measured in urine. We compared the results to the ACGIH Biological Exposure Index (BEI) for occupational benzene exposure.[iii] The benzene BEI represents the concentration of metabolites most likely to be observed in specimens collected from healthy workers exposed to the ACGIH TLV of 0.5 ppm. None of the biological monitoring samples were found to exceed the ACGIH BEI.

Direct reading instruments identified instances of short-term flammable atmosphere measurements as high as 40% of the lower explosive limit (LEL) adjacent to separators and flowback tanks; in general, a concentration of 10–20% of the LEL is considered a risk for fires and is the typical alarm settings for direct reading personal and fixed flammable gas monitors.

Preliminary Conclusions

These findings suggest that benzene exposure can exceed the NIOSH REL and STEL and present an occupational exposure risk during certain flowback work activities. Based on these preliminary studies, primary point sources of worker exposures to hydrocarbon vapor emissions are opening thief hatches and gauging tanks; additional exposures may occur due to fugitive emissions from equipment in other areas in the flowback process (e.g., chokes, separators, piping, and valves), particularly while performing maintenance on these items. The NIOSH research found that airborne concentrations of hydrocarbons, in general, and benzene, specifically, varied considerably during flowback and can be unpredictable, indicating that a conservative approach to protecting workers from exposure is warranted. Hydrocarbon emissions during flowback operations also showed the potential to generate flammable and explosive concentrations depending on time and where measurements were made, and the volume of hydrocarbon emissions produced.

Recommendations for Protecting Workers

Based on workplace observations at the sites visited, NIOSH researchers identified a number of general recommendations to reduce the potential for occupational exposure:

1. Develop alternative tank gauging procedures so workers do not have to routinely open hatches on the tops of the tanks and manually gauge the level of liquid.
2. Develop dedicated sampling ports, other than the thief hatches, that minimize workers’ exposures to volatile organic compound emissions while manually tank gauging.
3. Provide worker training to ensure flowback technicians understand the hazards of exposure to benzene and other hydrocarbons and the importance of monitoring atmospheric conditions for LEL concentrations.
4. Limit the time spent in proximity to hydrocarbon sources.
5. Monitor workers to determine their exposure to benzene and other contaminants.
6. Establish a controlled perimeter (similar to the high pressure zone established during hydraulic fracturing) around flowback tanks. Limit entry and require that any portable tents or sunshades remain outside and upwind of the controlled area.
7. Provide workers with calibrated portable flammable gas monitors with alarms at appropriate levels. The actions to be taken if the alarm sounds should be defined before the detector system is put into use.
8. Use appropriate respiratory protection in areas where potentially high concentrations of hydrocarbons can occur as an interim measure until engineering controls are implemented. Note that OSHA regulations (29 CFR 1910.134) require a comprehensive respiratory protection program be established when respirators are used in the workplace.⁴
9. Use appropriate impermeable gloves to protect against dermal exposures during work around flowback and production tanks and when transferring process fluids.

Help Wanted

NIOSH is looking for additional partners in drilling and well servicing to work with us to further evaluate worker exposures to these chemicals and other hazards and to develop controls, as needed. To investigate whether workers are exposed to toxic chemicals at hazardous concentrations in this rapidly expanding industry and to address the existing lack of information on occupational chemical exposures, NIOSH initiated the NIOSH Field Effort to Assess Chemical Exposures in Oil and Gas Extraction Workers. NIOSH conducted a comprehensive exposure assessment to characterize worker exposure to crystalline silica in this process. The results from this evaluation and recommendations for controlling worker exposure to crystalline silica during hydraulic fracturing have been disseminated in trade association journals, a previous NIOSH science blog, an OSHA-NIOSH Hazard Alert, and a peer-reviewed publication. Furthermore, in a recent NIOSH science blog posting, we addressed reports made known to NIOSH of recent fatalities of workers who were gauging flowback or production tanks or involved in transferring flowback fluids at the well site. Other potential occupational exposures can include hydrocarbons, lead, naturally occurring radioactive material (NORM), and diesel particulate matter, which have not been fully characterized. If you have questions or wish to participate in any aspects of this effort, please contact us via the blog comment box below or by e-mail at nioshblog@cdc.gov.

Eric J. Esswein, MSPH, CIH, John Snawder, PhD, DABT, Bradley King, MPH, CIH, Michael Breitenstein, BS, and Marissa Alexander-Scott, DVM, MS, MPH.

Eric Esswein and Bradley King are with the NIOSH Western States Office in Denver, CO. John Snawder, Michael Breitenstein, and Marissa Alexander-Scott are with the NIOSH Division of Applied Research and Technology (DART) in Cincinnati, OH. The authors would like to acknowledge Belinda Johnson (NIOSH DART) for her efforts and contributions prior to and during the field studies.

Notes:

NIOSH notified company representatives of these findings and provided reports with recommendations to control exposure to benzene and other volatile materials that may be present. We provided all monitored workers with a confidential letter explaining the results of the evaluation and contact information for questions.

The objective of this blog entry is to describe the NIOSH Field Effort to characterize worker exposures in the oil and gas extraction industry, specifically during post-drilling flowback operations. To keep the blog discussion focused on worker health, we may choose not to respond to or to post comments that do not pertain to worker exposures.

References

[i] Esswein E, Snawder J, King B, Breitenstein M, Alexander-Scott M, Kiefer M [2014]. Evaluation of Some Potential Chemical Exposure Risks during Flowback Operations in Unconventional Oil and Gas Extraction: Preliminary Results. Journal of Occupational and Environmental Hygiene 11(10):D174–D184.

[ii] NIOSH [2003]. NIOSH manual of analytical methods (NMAM®). 4th ed. Schlecht P.C., O’Connor P.F., eds. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication 94–113 (August 1994); 1st Supplement Publication 96–135, 2nd Supplement Publication 98–119; 3rd Supplement 2003–154. [http://www.cdc.gov/niosh/docs/2003-154/].

[iii] NTP [2011]. Report on Carcinogens, Twelfth Edition. Research Triangle Park, NC: U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program. 499 pp. [http://ntp.niehs.nih.gov/ntp/roc/twelfth/roc12.pdf].

[iv] IARC [2012]. Chemical agents and related occupations, volume 100F: A review of human carcinogens. Benzene. Lyon, France: International Agency for Research on Cancer, pp 249–294. [http://monographs.iarc.fr/ENG/Monographs/vol100F/mono100F-24.pdf].

[v] EPA [2009]. Integrated Risk Information System (IRIS) on Benzene. Washington, DC: U.S. Environmental Protection Agency, National Center for Environmental Assessment, Office of Research and Development. [http://www.epa.gov/iris/subst/0276.htm]

[vi] NIOSH [2010]. NIOSH pocket guide to chemical hazards. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2010-168c. [http://www.cdc.gov/niosh/npg/].

[vii] ACGIH [2014]. 2014 TLVs® and BEIs®: threshold limit values for chemical substances and physical agents and biological exposure indices. Cincinnati, OH: American Conference of Governmental Industrial Hygienists.

[viii] CFR. Code of Federal Regulations. Washington, DC: U.S. Government Printing Office, Office of the Federal Register.