Experimental CO2 interactions with fractured Utica and Marcellus Shale samples at elevated pressure

Abstract

Experiments showed that 5% KI carbonated brine injection into fractured Marcellus and Utica Shale cores at room temperature (∼20 °C) and at 1700 psi (MPa) confining pressure resulted in dissolution along fractures, together with an increase in the total core permeability of the fractured cores. Dissolution in the more calcium-rich Utica Shale concentrated along larger aperture zones that likely channeled the bulk of the flow through the core. In contrast, dissolution in the Marcellus Shale sample was more uniform across the fracture plane and accompanied by the development of a porous, permeable zone of partial dissolution within the shale matrix that developed throughout the sample and decreased in volume along core length. X-ray fluorescence measurements in both cores revealed depletion in calcium, and in the Marcellus core the composition of the porous reaction rind was non-reactive and silica rich. The partial dissolution of the Marcellus Shale matrix adjacent to the fracture resulted in a permeable zone that contributed to the sample's increased transmissivity, as shown by fracture aperture mapping, high-resolution microtomography, and numerical simulations of flow through the fracture. Shale lithology and mineralogy were combined with the initial fracture morphology to determine where, how, and to what degree reactive fluids caused changes to fractured shales. Although the experimentally determined macroscopic permeability of both cores increased approximately one order of magnitude, the different compositions of the rocks resulted in distinct microscopic dissolution patterns. The umbrella term of “shale” encompasses a variety of lithological compositions with variable reactive potential. Our analysis demonstrates that even small lithological variations in carbonate and silicate mineral content and distribution can have an outsized impact on shale fracture behavior during reactive flow. Any field-scale assessment of the utility of shales as sealing formations or reservoirs in the presence of reactive brines must entail a thorough description of the given lithological unit and its susceptibility to reactive flow.