A new method for characterization of porewater chemistry in low-permeability sedimentary rocks

Abstract:

Measurements of spatial variability of naturally-occurring tracers in porewater from low-permeability rocks can provide insight into porewater residence time and the nature of solute transport. These system characteristics, in turn, are required to assess the long-term integrity of geologic systems for waste management. However, the measurement of porewater chemistry in low‑permeability rocks is challenging because of the small fluid volume and the difficulty of extracting representative samples. Several techniques for porewater characterization are available, but the results they provide can be affected by ion exchange and mineral dissolution, and they may require independent porosity measurements. The objectives of this work are to develop and test a method of extracting representative samples of in-situ porewater from low‑permeability rocks, and to accurately quantify solute concentrations in the extracted porewater. The method involves extraction of porewaterby absorption into hydrophilic cellulosic membranes. The masses of solutes extracted with the porewater are measured by inductively coupled mass spectrometry (ICP‑MS), and the mass of extracted water is measured by near infrared (NIR) spectrometry. In-situ porewater solute concentrations are obtained by normalizing solute mass to water mass. Laboratory experiments using controlled additions of brine to cellulosic membranes confirm that the method is capable of providing porewater solute concentrations with precision and accuracy that are within the limits suggested by USEPA Method 6020A for analysis of saline water samples by ICP-MS. This presentation will provide a detailed description of the absorption method and show the results of a field trial in which concentrations of major porewater ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Cl^-, Br^-) in a low-permeability shale (K <10^-12 m/s, porosity < 10 %) were quantified and compared to results from the crush-and-leach method obtained from paired samples. The results of the comparison demonstrate that the absorption method provides precise porewater chemistry data that are free from ion-exchange and mineral-dissolution artefacts.