USING SR ISOTOPES TO TRACK THE DISSOLUTION OF MARCELLUS SHALE BY HYDRAULIC FRACTURING FLUIDS
A core sample of Marcellus Shale from Greene County, PA was crushed and placed into a high P and T reaction vessel. Solutions were added in two different experiments: one with synthetic brine, and another using brine+HF fluid. The HF fluid was made up of components listed on fracfocus.org. Experiments were run for ~16 days at 27.5 MPa and 130 oC. Aqueous samples were periodically removed for analysis and Sr isotope ratios were measured by MC-ICP-MS.
Using just brine, the pH decreased from 7.6 to 5.3 after 24 hrs, but then became steady at ~6.1. Sr (aq) concentrations started at 2.5 mmol/L after 24 hrs and then rose to ~3 mmol/L over ~380 hours. During this time only 6% of the total inorganic carbon (TIC) dissolved from the shale. The εSr values (deviation of the 87Sr/86Sr ratio from that of seawater in parts per ten thousand) started at +43.2 and decreased to +42.4. In the experiment using brine+HF fluid, the pH rose from 1.8 to 5.6 by day six. Sr concentrations were higher after ~24 hours (3.9 mmol/L) and increased to 4.5 mmol/L by the end on day 16. Over 60% of the TIC dissolved from the shale during the reaction. The εSr started at +36.5 at 24 hours and decreased to +35.5 by the end.
Differences in the Sr isotopic results between the experiments, in combination with the TIC data, suggest the presence of at least two distinct Sr reservoirs in the sample: 1) water-soluble or exchangeable Sr, and 2) Sr incorporated into carbonate minerals. The lower εSr values using brine+HF fluid may be explained by the increased dissolution of carbonates. These results suggest that carbonate minerals within the Marcellus Shale may help control Sr isotopic composition of produced waters from these formations.