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

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.

Keywords: fractured shale microbial communities; fracturing fluid additives; high pressure; hydraulic fracturing.

Fig. 1.

Cell numbers observed in high-pressure experimental end points. Details of experimental setups are given in Table 1. 1, 70 MPa (15 min)+35 MPa at 50 °C; 2, 35 MPa at 50 °C; 3, 35 MPa at 22 °C; 4, 0.1 MPa at 50 °C; 5, 0.1 MPa at 22 °C. The red dashed line indicates the cell numbers observed in the ‘no chemicals 4 °C control’, used here as a conservative baseline for starting numbers of cells in each experimental setup. These end points represent 14 days after experiments were started. Error bars represent the standard deviation of the mean of triplicate measurements. *, concentration of DNA recovered from 300 ml sample of experimental end points, where BD indicates that yields were below the limit of detection (0.5 ng ml⁻¹).

Fig. 2.

Microbial community composition in inoculum and experimental end points. Details of experimental setups are given in Table 1. I, inoculum; 1, 70 (15 min)+35 MPa at 50 °C; 2, 35 MPa at 50 °C; 3, 35 MPa at 22 °C; 4, 0.1 MPa at 50 °C; 5, 0.1 MPa at 22 °C. *, DNA yields recovered from 300 ml of experimental end points were below detection. Only prominent taxa across are shown in the legend. Lineages representing <0.1 % across all samples were omitted. ASVs identified in extraction and PCR controls were omitted prior to preparation of this figure.

Fig. 3.

Acetate concentrations in follow-on culturing of high-pressure experimental end points. The grey box indicates the experimental incubations that were subject to combined high-pressure and high-temperature conditions. All follow-on culturing was conducted anaerobically at ambient conditions.