Lawrence Berkeley National Laboratory

Interactions between rock minerals and hydraulic fracturing fluids directly impact the geochemical and geomechanical properties of shale formations. However, the mechanisms of geochemical reactions in shale unconventional reservoirs remain poorly understood. To investigate the geochemical reactions between shale and hydraulic fracturing fluids, a series of batch reactor experiments were undertaken. Three rock samples with different mineralogical compositions and three fluid samples of different compositions [deionized water, deionized water + 2% potassium chloride (KCl), and deionized water + 0.5% choline chloride (C5H14ClNO)] were used. Experiments were undertaken at reservoir temperature and atmospheric pressure. Elemental compositions of effluents after 1, 3, 7, 14, and 28 days were analyzed using inductively coupled plasma mass spectrometry. Medical computed tomography scanning and X-ray fluorescence spectroscopy were conducted on the entire core to help upscale results obtained from rock-fluid interaction experiments. Geochemical modeling using a reactive simulator, TOUGHREACT, was undertaken to corroborate experimental results. Results show that a lower pH triggered high dissolution rates in the rock samples, especially the carbonate components. As the pH increased, the rate of dissolution declined significantly, though for most cases dissolution still continued. The observed dissolved silica concentrations were much higher than the quartz solubility, suggesting that much of the silica originated from more soluble silica polymorphs and possibly desorption from clay mineral exchange sites. The concentration of most elemental species in solution increased, but aluminum (Al) and magnesium (Mg) concentrations declined rapidly following initial entry into solution. Geochemical modeling corroborated the conclusions regarding mineral dissolution and precipitation observed from experiments, notably the dissolution of calcite and pyrite in the reacted shale samples, the likely presence of silica polymorphs such as opal, chalcedony, or amorphous silica in these samples, and the reduction of Al and Mg concentrations in solution by precipitation of secondary aluminosilicate phases. The de-flocculation of clay minerals during reaction implies fines migration after hydraulic fracturing. This is detrimental to reservoir productivity as clay fines are displaced and lodged within the micro- and nanofractures created during fracturing. The immediate consumption of Al and Mg also has implications on blockage of hydrocarbon pathways due to precipitation of new minerals in these locations.