(270a) Reactive Imbibition of Acidic Hydraulic Fracture Fluids into Unconventional Shales

Unconventional stimulation of gas- and oil-bearing shales by injection of acidic hydraulic fracturing fluids (HFFs) increases shale permeability/porosity and dissolves carbonates, pyrite, and other minerals, which promotes hydrocarbon extraction. In addition to the dissolution of shale minerals, fluid transport also results in the precipitation of secondary minerals (e.g., barite, celestite, gypsum, iron oxides) in the shale matrix, which ultimately reduces permeability and transport. One major consequence of the interaction of shales with HFFs is the overuse of freshwater, resulting in the production of considerable quantities of hard-to-recycle highly saline waters, which threatens watersheds. Improved understanding of the impact of unconventional stimulation on fluid transport is highly desirable as it provides one of the best ways to diminish the environmental impacts of this water-intensive practice.

Here we report unique experimental observations of fluid transport in shales by tracking the rate and extent of imbibition of a bromide tracer in acidic HFFs using synchrotron X-ray fluorescence mapping combined with compositional analysis of the bulk fluid. This approach yields a direct record of time-resolved selective ion diffusion resulting from the penetration of HFFs and attendant mineral transformations. Our experiments highlight for the first time that the penetration of acidic HFFs and the dissolution calcite occur at similar rates in carbonate-bearing shales, suggesting that porosity increases rapidly during imbibition of HFFs and thus promotes fluid penetration. We also show that the variability in mineralogy and chemical reactivity of shales can directly affect reactive imbibition. For example, the presence of major carbonates in the shale matrix reduces reactive imbibition of acidic HFFs due to a rapid neutralization of pH and inhibits fluid penetration. We also found that the presence of Ba in HFFs significantly retards reactive imbibition of acidic HFFs due to rapid precipitation of Ba-bearing minerals, which reduces pososity at the fluid-rock interface. Finally, we show the importance of high ionic strength solutions, similar to produced waters, on the release of colloids and the impact of colloids on ion diffusivity. The main finding from these experimental studies is that the chemical reactivity of unconventional shales regulates the extent and rate of imbibition of HFFs into the shale matrix.