An absorption method for extraction and characterization of porewater from low-permeability rocks using cellulosic sheets

Abstract

Characterization of porewater chemistry in low-permeability rocks can provide insight into the origin and residence time of porewater, the history of fluid movement and the nature of transport and reaction processes. 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 are available, but the results they provide can be affected by ion exchange and mineral dissolution, and they may require independent porosity measurements. The objective of this work is to develop a method of extracting representative samples of in situ porewater from low-permeability rocks and accurately quantify solute concentrations in the extracted porewater. A preliminary trial demonstrated the feasibility of extracting porewater by absorption into hydrophilic cellulosic membranes from low-permeability shale (Georgian Bay Formation, Michigan Basin, southwest Ontario, Canada). Solute concentrations are calculated from independent measurements of solute mass quantified by Inductively-Coupled-Plasma Mass Spectrometry (ICP-MS) and water-content quantified by Near Infrared (NIR) spectrometry. Sorption experiments indicate that there is no preferential sorption of solutes (Na, Cl, Mg, Ca, K, Sr, and Br) to the cellulosic membrane. The results indicate that the method is capable of determining solute concentrations on absorbed porewater with analytical precision that is within the margins of error recommended by EPA Method 6020a for analysis of saline water samples.