Imaging Pyrite Oxidation and Barite Precipitation in Gas and Oil Shales

The complex suite of chemicals that comprises fracture fluid may result in chemical alterations of shale matrices, which can in turn affect gas and oil transport through the matrices and across fracture-matrix interfaces. In this study, we examined chemical reactions in carbonate-poor Marcellus and carbonate-rich Eagle Ford matrices upon reaction with synthetic fracture fluid (initial pH = 2) at 77 bar and 80 °C for three weeks. A subset of the experiments was conducted with additional Ba²⁺ and SO₄^2- ions in the fracture fluid to promote barite precipitation. Micro-CT and synchrotron x-ray fluorescence microscopy were used to identify the reaction zones in shale. In contrast with previous studies that focus on chemical reactions at the shale-fluid interface, we observe that the chemical reactions can penetrate a considerable distance into the shale matrix unless the matrix is devoid of any microcracks. The results show that, for shale cores with microcracks, both pyrite oxidation/dissolution and barite precipitation can extend millimeters into the matrix, depending on mineral compositions of the shale. Following 3 weeks of reaction, the carbonate-poor Marcellus system had final pH of 4. The altered zone of the shale core showed a pyrite oxidation zone > 5 mm into the matrix, while barite precipitation was limited to the surface (≤ 45 μm into the matrix). Conversely, the solution of the carbonate-rich Eagle Ford cores was completely neutralized during reaction. When compared to Marcellus, the Eagle Ford shale had a reversed trend with limited pyrite oxidation, and extensive barite precipitation (several millimeters into the matrix) in microcracks and in matrix pores. This comparison is consistent with earlier findings that compared to low pH, near-neutral pH promotes barite precipitation and Fe(II) oxidation. Our results also suggest that scale precipitation, even if formed only at the fracture-matrix interface, can limit transport of dissolved oxygen across the interface, and may also affect transport of gas and oil from shale matrices to fractures for recovery.