USING SR ISOTOPES TO TRACK THE DISSOLUTION OF MARCELLUS SHALE BY HYDRAULIC FRACTURING FLUIDS

Hydraulic fracturing (HF) has allowed for the production of formerly inaccessible shale gas resources. However, fluid/rock reactions during HF may lead to changes in produced water chemistry such as an increase in total dissolved solids (TDS). Sr isotopes are an effective tool for differentiating Marcellus Shale derived-fluids from other high TDS sources (Chapman et al. 2012, doi: 10.1021/es204005g), but questions remain as to what controls Sr concentrations and isotopic values in Marcellus Shale fluids. Here, we present Sr isotopic data from Marcellus Shale dissolution experiments using synthetic brines and HF 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.