Shale reservoirs are characterized with having an ultra-low permeability, such that in order to produce these reservoirs economically, hydraulic fractures are created and flushed with large volumes of fracture fluid chemicals to help aid with well production. In this study, we aim to understand how fracture fluid affects the permeability of the shale matrix from either porosity enhancement or mineral precipitation due to the shale's mineral composition. We conducted laboratory permeability measurements on intact horizontal shale microcore samples from both Eagle Ford and Marcellus reservoirs before and after reaction with a dissolution-favored synthetic fracturing fluid (pH = 2). Shale reactions with fracture fluid were performed inside a batch reactor set at 80°C and 77bar for 6 days, with added BaCl2 and Na2SO4 salts in place of barite-rich drilling mud to promote barite precipitation. Our results show a permeability reduction for the clay-rich Marcellus, while permeability was significantly enhanced for the Eagle Ford sample. Additionally, SEM image observations and energy diffusive spectroscopy (EDS) provided supporting evidence for mineral composition changes post fracture fluid reaction. The occurrence of barite precipitation in the Marcellus sample and accumulation inside the limited microcracks, resulted in occlusion of the main transport flow paths and a reduction in permeability. As for the Eagle Ford sample, even though barite precipitation also occurs in the microcracks, the rate of carbonate dissolution is far greater, creating a large amount of secondary porosity and an order of magnitude increase in permeability. By demonstrating the complex interactions of fracture fluid chemistry with the shale matrix and its effect on permeability, we suggest that fracture fluid recipes could be tailored specifically to the mineral composition for each shale reservoir in order to enhance production of stimulated shale volumes.
During hydraulic fracturing, large volumes of fracture fluids are flushed into the reservoir to help enhance permeability and well production. Combined with the technological advancement of hydraulic stimulation along horizontal wells, unconventional shale reservoirs, characterized with having an ultra-low intrinsic permeability, are now able to be extracted and produced economically. In spite of this progression, unconventional shale reservoir production remains highly inefficient, with industry reported recovery factors of around 25% for gas (Rassenfoss, 2018) and rapid rate declines a few years after initial production (Valko and Lee, 2010), suggesting that a large portion of the resource is still left trapped, as adsorbed gas, inside the nanopores of the shale matrix (Ambrose et al., 2010). Although shale mineral composition is considered to be a main factor controlling shale permeability (Ismail and Zoback, 2016; Chalmers et al., 2012), there is a shortage of experimental work looking at fracture fluid chemical reactions on shale matrix permeability of intact cores with varying mineral composition. In this study, our goal is to understand how fracture fluid affects the permeability of the shale matrix from either porosity enhancement or mineral precipitation due to the shale's mineral composition. The main chemical reaction we focused on is barite scale formation from drilling mud, which can reduce permeability and hinder well production (Kan and Tomson, 2012). Deconvolving the factors that influence permeability (fluid vs. chemical effects) is crucial for optimizing production strategies.
