Lawrence Berkeley National Laboratory

In this paper, we report research findings on geochemical rock-fluid interactions between Caney Shale samples and completion fluids used for hydraulic fracturing, and how these reactions impact reservoir productivity. Hydraulic fracturing of shales triggers a series of dissolution, precipitation reactions and ion exchange in clays, resulting in the alteration of reservoir petrophysical properties, including conductivity of hydraulic fractures. These reactions are largely dependent on the chemical compositions of the fluids and reservoir rock mineralogy. In this study, static batch reactor experiments are employed where rock powders are reacted with produced brine. The reactions are conducted at temperature of 95oC in sealed pyrex reaction bottles, with sampling at the beginning and after four 4 weeks. There was no external pressurization applied to the reaction bottles. Mineralogical characterization of Caney Shale samples before and after reaction are conducted using x-ray diffraction (XRD) whilst microstructural and elemental analyses are studied using scanningelectron microscopy (SEM) and energy dispersive spectroscopy (EDS) respectively. The results show changes in the elemental and mineralogical composition of rock-slabs and powdered rock samples respectively, therefore confirming geochemical reactions occurred during the experimental period. XRD measurements of rock powders before and after reaction show decreases of pyrite, feldspar, and carbonate contents whiles quartz and illite contents either remained the same or increased. In terms of elemental compositions on the surface of rock-slabs, there is significant change to the pyrite elemental composition due to oxidation-induced dissolution. Precipitation of new minerals and amorphous entities is also observed on the surface of rock-slabs. These results reveal that geochemical reactions following hydraulic fracturing persist in the long-term. The breakdown of pyrite by dissolved oxygen or breakdown of oxidants can lead to higher localized acidity which can dissolve more minerals resulting in higher elemental ion concentrations in the system. The dissolved elemental ions subsequently form new mineral precipitates which can occlude hydrocarbon flow paths and lead to loss of permeability and productivity decline from a reservoir. This study provides an appreciation of the post-fracturing geochemical rock-fluid reactions and the impact of these reactions on long-term permeability of shale reservoirs. The study also shows the initiation of geochemical reactions during shut-in where hydraulic fracturing fluids are imbibed by the reservoir rock This study can therefore serve as a basis from which composition of hydraulic fracturing fluids are fine-tuned to mitigate adverse geochemical rock-fluid reactions in the subsurface, post-hydraulic fracturing.