Rates of Sulfate Reduction by Microbial Biofilms That Form on Shale Fracture Walls within a Microfluidic Testbed

Microbial sulfate reduction (biosouring) is ubiquitous in natural and engineered environments. It can be especially problematic during oil recovery from unconventional reservoirs, where the growth of sulfate-reducing bacteria (SRB) biofilms can clog fracture pathways, decrease hydrocarbon production, and produce hydrogen sulfide in the produced oil and water that corrodes pipelines and threatens both human health and the environment. Relevant experimental data on shale fractures, assessing the conditions that affect SRB growth, distribution, and activity for sulfate reduction, are sparse, and this has limited our ability to evaluate predictive models for assessing the impacts. To address this limitation, a 250 μm wide, 170 μm deep, and 2 cm long microchannel was constructed within an actual sample of subsurface core from the Devonian age New Albany Shale of the Illinois Basin to simulate a hydraulically induced fracture (fracked shale). This real rock flow-through microfluidic system, called the GeoBioCell (GBC), was designed with two inlets and one outlet. At flow rates of 3.4-80 μL/h, an SRB enrichment culture with hydrocarbon degradation products (i.e., fatty acids) was fed into one inlet, while dissolved sulfate (30 mM) was introduced into the other inlet. These were not allowed to mix until they entered the microchannel, where rapid biomass growth and sulfate reduction took place. SRB biomass was quantified as a function of both influent sulfate flux using brightfield microscopy and SRB metabolic sulfide production using an effluent zinc trap. Results indicate that biomass grows on shale fracture walls and in the channel and increases with the influent organic acid and sulfate flux, and that the downflow SRB biofilm growth on microchannel walls is limited by the upstream metabolic consumption of electron donors. An advection-diffusion-reaction model was developed to interpret these results, and a specific sulfate reduction rate of 1.0 ± 0.4 × 10-3 mmol SO42-/(mmol biomass·s) or 9 ± 4 × 10-6 mmol SO42-/(mg biomass·s) was obtained. This rate, among the highest reported to date, suggests that nearly optimal conditions for SRB sulfate reduction were attained within the microchannel that were independent of mass transfer limitations. Future tests using similar real rock microfluidic experimentation will be useful for assessing worst case scenarios for subsurface reservoir biosouring models and establishing new mitigation strategies to prevent adverse human health and environmental impacts.