Microbial communities in freshwater used for hydraulic fracturing are unable to withstand the high temperatures and pressures characteristic of fractured shales

  1. Sophie L. Nixon
  2. Alvaro M. Plominsky
  3. Natali Hernandez-Becerra
  4. Christopher Boothman
  5. Douglas H. Bartlett

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Abstract

Natural gas is recovered from shale formations by hydraulic fracturing, a process known to create microbial ecosystems in the deep subsurface. Microbial communities that emerge in fractured shales include organisms known to degrade fracturing fluid additives and contribute to corrosion of well infrastructure. In order to limit these negative microbial processes, it is essential to constrain the source of the responsible micro-organisms. Previous studies have identified a number of potential sources, including fracturing fluids and drilling muds, yet these sources remain largely untested. Here, we apply high-pressure experimental approaches to assess whether the microbial community in synthetic fracturing fluid made from freshwater reservoir water can withstand the temperature and pressure conditions of hydraulic fracturing and the fractured shale environment. Using cell enumerations, DNA extraction and culturing, we show that the community can withstand high pressure or high temperature alone, but the combination of both is fatal. These results suggest that initial freshwater-based fracturing fluids are an unlikely source of micro-organisms in fractured shales. These findings indicate that potentially problematic lineages, such as sulfidogenic strains of Halanaerobium that have been found to dominate fractured shale microbial communities, likely derive from other input sources into the downwell environment, such as drilling muds.

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  1. Version published to 10.1099/acmi.0.000515.v3 on Access Microbiology
  3. Access Microbiology

    Comments to Author

    The methods used have been explained effectively and referenced for reproducibility. The underlying data are also publicly available. The results are clear and concise. The required guidelines have been followed and key findings adequately presented. This work has followed on from previous literature and assessed the effect of both high temperature and pressure on the microbial community in fracturing fluid. Previous comments raised have been addressed in this revision.

    Please rate the manuscript for methodological rigour

    Good

    Please rate the quality of the presentation and structure of the manuscript

    Good

    To what extent are the conclusions supported by the data?

    Strongly support

    Do you have any concerns of possible image manipulation, plagiarism or any other unethical practices?

    No

    Is there a potential financial or other conflict of interest between yourself and the author(s)?

    No

    If this manuscript involves human and/or animal work, have the subjects been treated in an ethical manner and the authors complied with the appropriate guidelines?

    Yes

  4. Access Microbiology

    Comments to Author

    The authors have satisfactorily addressed all of my concerns pertaining to the MS. In view of the same, I would recommend the manuscript for publication.

    Please rate the manuscript for methodological rigour

    Very good

    Please rate the quality of the presentation and structure of the manuscript

    Very good

    To what extent are the conclusions supported by the data?

    Strongly support

    Do you have any concerns of possible image manipulation, plagiarism or any other unethical practices?

    No

    Is there a potential financial or other conflict of interest between yourself and the author(s)?

    No

    If this manuscript involves human and/or animal work, have the subjects been treated in an ethical manner and the authors complied with the appropriate guidelines?

    Yes

  5. Version published to 10.1099/acmi.0.000515.v2 on Access Microbiology
  7. Access Microbiology

    Comments to Author

    1.Methodological rigour, reproducibility, and availability of underlying data The data have been deposited and are publicly available in the GenBank and methods used have been explained effectively and referenced. However, the number of replicate treatments in experimental set up does not appear to be mentioned and would be worth noting for reproducibility. 2. Presentation of results Results are clear and concise. 3. How the style and organization of the paper communicates and represents key findings The required guidelines have been followed and key findings have been adequately presented. 4. Literature analysis or discussion The review of the literature is precise and easily understood. The work has followed on from previous literature. Knowing the effect of temperature and pressure on microorganisms in injected water is important to note since previous literature have queried the effect of these parameters on microbial metabolic processes. 5. Any other relevant comments This work is relevant and provides a valuable assessment on the survival ability of freshwater microorganisms in shales and would add to knowledge in the field of fractured shales drilling. Nevertheless, the following have been observed: * Line 178: temperature and pressure values in bracket seem to have typographical error. * Line 326: 'PCR controls were omitted prior to preparation of figure' - could be explained further for better understanding. * Line 100 - 'to' missing. For microorganisms in input fluids to survive… * Line 129 - double brackets could instead read: (Kekacs et al; Mouser et al) * As storing fresh water at 4oC for 7 days prior to experiment reduced microbial count from 6.3x106 to 3.2x105, could this be a limitation to the study in terms of there being bacterial strains that could have withstood the temperature and pressure?

    Please rate the manuscript for methodological rigour

    Good

    Please rate the quality of the presentation and structure of the manuscript

    Good

    To what extent are the conclusions supported by the data?

    Strongly support

    Do you have any concerns of possible image manipulation, plagiarism or any other unethical practices?

    No

    Is there a potential financial or other conflict of interest between yourself and the author(s)?

    No

    If this manuscript involves human and/or animal work, have the subjects been treated in an ethical manner and the authors complied with the appropriate guidelines?

    Yes

  8. Access Microbiology

    Comments to Author

    The article is very well crafted, easy to read and understand. There is innovation as well as originality and it would certainly make an important contribution to the existing literature. I have highlighted a few minor concerns regarding the work presented and I would appreciate if the authors would address them. Lines 115-118: I would suggest the authors to mention the reservoir characteristics, specifically, depth of the water in the reservoir and the depth at which the samples were collected? Were they surface waters or collected from a specific depth? As microbial communities differ across the depths, it would be interesting to know more details about the samples collected. Lines 118-122: "Collection bottles were completely filled to eliminate air". Is this is to create anaerobic conditions? If so, what about the dissolved oxygen levels? Were any of the physico-chemical parameters (salinity, pH, temperature etc.) of the samples measured at the sample collection site or in the laboratory? Lines 138-141: Conditions about 'UV based sterilization' including device, light source, wave length may be mentioned. Lines 269-271: The sentences are a bit confusing. I would like the authors to mention the initial cell counts before subjecting the batches to various treatments. Is 3.2x105 the initial cell count in all the treatment batches? Line 275: DNA yields were not specified in figure 1. Only cell counts were indicated. Line 288-289: Is there any prior literature supporting the statement "high pressure or high temperature alone stimulates population growth relative to the room temperature and pressure control". If so, the same can be highlighted in discussion also stating what could be the possible reason for the same. Lines 305-318: It would be interesting if more discussion about the characteristics of dominant taxa detected in each treatment (any reports on whether they tolerate high temperature, high pressure; oxygen requirement) be added to the text. Line 321: Is there any taxa that was not detected in inoculum but detected in treatments? Line 324: What do the authors mean by below detection (any cut-off limits (ng/µL)) considering the fact that the authors could extract enough amount of DNA suitable for performing PCR? What was the DNA concentration used for performing PCR reaction? Discussion section is well written. Few sentences that are synonymous to introduction can be removed. Since very good data is generated, the discussion section can be further strengthened by comparing the data obtained with those that already exist in the literature.

    Please rate the manuscript for methodological rigour

    Good

    Please rate the quality of the presentation and structure of the manuscript

    Very good

    To what extent are the conclusions supported by the data?

    Strongly support

    Do you have any concerns of possible image manipulation, plagiarism or any other unethical practices?

    No

    Is there a potential financial or other conflict of interest between yourself and the author(s)?

    No

    If this manuscript involves human and/or animal work, have the subjects been treated in an ethical manner and the authors complied with the appropriate guidelines?

    Yes

  9. Version published to 10.1099/acmi.0.000515.v1 on Access Microbiology