Design and implementation of resistive grout for cross-borehole electrical imaging

Electrically resistive grout mixtures were evaluated for cross-borehole electrical resistivity tomography (ERT) applications, which are increasingly used for environmental investigations. Grout is typically used to fill the annular space between the electrodes and the surrounding formation and therefore plays a critical role in electrically connecting the electrodes to the surrounding formation. The ideal grout for cross-borehole ERT applications has a resistivity that is higher than the surrounding formation so that the injected current is channeled within the formation instead of along the wellbore. Bentonite and Portland cement are typically used as materials to fill the annulus in wellbore installations; however, the resistivity of these materials is too low for high-quality ERT imaging. In this study, we tested grout mixtures that combine Portland cement with additives to develop resistive grout options to optimize ERT imaging while maintaining acceptable construction properties. Additives were chosen based on known chemical reactions, verified using powdered X-ray diffraction on select samples, whereby pore water and dissolved ions would be reduced, resulting in an increased resistivity. Additives include pozzolans such a Class F fly ash and Hess pumice. Other additives include slag, aluminum sulfate, and gypsum. Grouts were tested in three groups by additive, with one grout from each group with the highest resistivity selected for an X-ray diffraction analysis and other physical characteristic testing (strength, viscosity, and set time). The resistivity of pozzolan grouts can be predicted using the calcium to silicate and aluminate ratio with resistivities ranging from 76 to 680 Ω-m in 90 days. Grouts using sulfate minerals increase in resistivity more quickly than grouts only using pozzolan additives and have a resistivity range from 970 to 1820 Ohm-m in 90 days. Grouts containing pumice had reduced strength and increased viscosity, but grout containing fly ash and slag maintained similar physical properties to neat Portland cement. This study provides quantitative results that can be used to aid in selection of grout formulations that have a high likelihood of success in terms of construction and ERT image quality.