Micro Computer Tomography Study of Potassium Iodide Precipitation in Bentheimer Sandstone Caused by Flow-through CO2 Drying

Abstract

Storage of CO2 in depleted gas reservoirs and saline aquifers is being considered to prevent its emission to the atmosphere and thus the subsequent climate changing effects. The injection of large amounts of anhydrous carbon dioxide in the under- ground will lead to salt precipitation as connate water evaporates in the CO2 flux. This precipitation leads to decreased porosity and permeability of the formation in the near wellbore region and may have a significant negative influence on CO2 injectivity and thus the economics of the operation.The temporal and spatial extension of the drying zone are controlled by the mass flow and characteristics of the reservoir. The reduction of permeability of a porous medium caused by the precipitation of salt is currently being investigated. However, the application of these results to the reservoir remain troublesome as it is unclear how to translate and up-scale these relations to in-situ conditions. An improved understanding of the relevant physics regarding salt precipitation would be beneficial for ex- trapolation of the laboratory permeability effects. Therefore, a laboratory investigation of salt precipitation in a heterogeneous porous medium at the micro-scale has been performed.In this study, the precipitation of salt in a Bentheimer sandstone (approx. 125 cm3) induced by CO2 through-flow of 0.265 cm3/(cm2h) is investigated at 27±1 bar and 306±1 K. The spatial and temporal development of salt crystals are monitored intermittently using micro CT analysis over a period of 160 days (700 g, 400 pore volumes). The sandstone is initially saturated with potassium iodide brine just below its salt saturation limit. A data analysis method is developed method that is able to distinguish the precipitated salt in the sandstone for volumes in excess of 1.8×103mm3. In addition, the decreasing water saturation of the core could be qualitatively tracked with the micro CT scans: a preferred CO2 pathways through the core was present.The observations regarding salt precipitation are (1) a 10 mm high region in the scan showed slightly more salt precipitation, (2) the salt precipitation is distributed evenly in the horizontal plane, (3) the observed maximum size of the salt crystals is between 0.04 and 0.45 mm3 and the probability distribution of the larger crystals is smooth.