THE EVOLUTION OF SALINITY IN FLUID ROCK REACTIONS WITH THE MARCELLUS SHALE

A series of experiments aimed at investigating the changing chemistry of fluids in contact with the Union Springs and Oatka Creek Formations of the Marcellus Shale has been completed. Thirty shale samples used in the study include both surface exposures and cores from PA and NY. Benchtop batch reactions were completed at standard temperature and pressure using samples ground finer than 250 µm. Experiments tested the effects of starting solution concentration and composition as well as solid to volume ratios. Solution pH, conductivity, and composition were monitored as a function of time over a period of 6 months. The mineralogy and trace metal concentrations of the same samples had been determined in an earlier study.

Results show that fluids in contact with Marcellus shale acquire and maintain a neutral to slightly basic pH. In this study only one outcrop sample with very low carbonate content produced an acidic pH after 4 months of monitoring. Fluid conductivities increased 50-500% after 4 hours of contact with the shale and continued to increase after 4 months. Conductivities were a function of the starting composition of the solution as well as the solid to volume ratio. The major, minor, and trace metal concentrations in batch solutions are still being analyzed.

To understand the processes involved during fluid evolution, cation exchange capacities (CEC) of the shale samples were measured via exchange with copper complex of ethylenediamine [Cu(en)₂]²⁺. CEC values were low, generally less than 10 meq/100g. CEC measurements on clay mineral separates from the shale were generally not much higher. These data indicate that cation exchange does contribute to the high conductivities in batch solutions, but cannot completely account for the fluid compositions.

As part of the National Energy Technology Laboratory's Regional University Alliance (NETL RUA), a collaborative initiative of the NETL, this technical effort was performed under the RES contract DE-FE0004000.