Fluid-rock interactions between hydraulic fracturing fluid and shale: Experimental insights

Abstract:

Shales and mudstones are critical components of subsurface energy systems. These rocks are both sources and unconventional reservoirs of petroleum hydrocarbons as well as seals for hydrocarbon, carbon storage, and waste storage reservoirs. Interactions induced by anthropogenic fluids injected into shales produce distinctive and measurable geochemical signatures that constrain in-situ processes. We evaluate these interactions using batch geochemical experiments that react a broad range of shale lithologies (carbonaceous to siliceous) with stimulation fluids used in hydraulic fracturing +/- formation water +/- proppant. Experimental conditions duplicate subsurface temperature (100-130°C) and pressure (30-45 MPa). The duration of most experiments ranges from days to weeks, time frames consistent with ‘shut in’ of hydraulically-fractured wells. In all of these experiments, the stimulation fluid is out of equilibrium with the rock; these fluids are initially in equilibrium with atmospheric oxygen and possess a range of ionic strengths (I = 0.01 to 1.5 mol/kg) and pH values (2 to 10). Evolving fluid chemistries suggest incipient dissolution/precipitation reactions of carbonate and silicate minerals, but mineralogic changes are often limited to the nanoscale. With reaction progress the shale invariably reasserts primary control over the water-rock system, including the important geochemical parameters of redox and pH.