UNCONVENTIONAL MINERALOGY: INTERACTIONS OF HYDRAULIC FRACTURING FLUIDS WITH MINERALS AND ORGANIC MATTER IN UNCONVENTIONAL AND TIGHT OIL FORMATIONS

Production of oil and gas from hydraulic fracturing of unconventional hydrocarbon reservoirs has dramatically improved the energy landscape of the United States. However, current hydrofracking technologies result in recovery of about 5% of the oil and 25% of the gas present. Furthermore, hydrocarbon production drops dramatically within several months of initial hydraulic fracturing. A potential explanation of this decrease is precipitation of minerals that occlude pores, pore throats, and fractures, resulting in a decrease in porosity and permeability and thus in hydrocarbon production. Pyrite, which is common in many shales, can undergo dissolution when it interacts with oxic hydraulic fracturing fluids (HFF), including an initial 15% hydrochloric acid spearhead. The released Fe²⁺ can redistribute, undergo oxidation to Fe³⁺, and precipitate as Fe(III)-oxyhydroxides. To test this hypothesis, we carried out experimental studies of the interaction of HFF with the Marcellus fm, Barnett fm, Eagle Ford fm, and Green River fm, with and without HCl, followed by characterization of the reacted shales using XRD and synchrotron-based Fe K-edge X-ray absorption spectroscopy (XAS) and m-X-ray fluorescence (mXRF) imaging. These shales contain variable abundances of clays, carbonates, and total organic carbon. We found that solution pH is the most important factor impacting release of Fe from pyrite into solution. Clear evidence of precipitation of Fe(III)-oxyhydroxides was shown by a significant decrease in solution Fe concentration for reactors with an initial solution pH 2.0 as well as by mXRF and mXAS analysis. In experiments on shales with high pH buffering capacity and no added HCl (initial pH 7.1) no Fe was released into solution, indicating little precipitation of Fe-bearing phases. In a separate study of barite scaling in unconventional shale reservoirs, we examined the efficiency of anti-scaling chemicals on barite precipitation. Barite is added to drilling muds as a weighting material. We found that organics in the HFF had little impact on barite scale formation, that pH ≥ 5 increases barite precipitation, and that ionic strength ≥ 1 M inhibits precipitation. We also found that replacement of HCl by H₂SO₄ in the initial acid spearhead results in a major reduction in barite dissolution and barium scaling.