EXPERIMENTAL CO2 INTERACTIONS WITH FRACTURED CALCITE-RICH UTICA AND MARCELLUS SHALE SAMPLES AT ELEVATED PRESSURE

Injection of carbonated brine into fractured samples of carbonate-rich Marcellus and Utica Shale resulted in increased fracture and core permeability through dissolution of carbonate content in the matrix. The two shales show distinct differences in dissolution styles: in the more carbonate-rich Utica shale dissolution was concentrated along larger aperture zones that likely channeled the bulk of the flow through the core, resulting in fracture widening in those areas. In contrast, dissolution in the Marcellus Shale sample occurred along the entire fracture plane and was primarily in the form of a porous, permeable zone through the remaining siliciclastic portion of the matrix. The porous zone spans the length and width of the core, although it decreases in volume along core length as the carbonated brine buffered during flow. X-ray fluorescence measurements in both cores revealed depletion in calcium was most pronounced at core inlet portions of both shales, and in the Marcellus core the composition of the porous reaction rind was shown to be non-reactive and silica rich. The permeable zone in the Marcellus Shale contributed to the sample’s increased transmissivity, as is 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 shales increased approximately one order of magnitude, the different lithologies of the two rocks resulted in distinct microscopic dissolution patterns. The lithologically-determined variation in reaction potential is an important consideration when scaling up these observations to field scale for understanding both sealing formation and shale reservoir behavior during reactive fracture flow.