Direct assessment of porosity and permeability in unconventional reservoirs such as shales and tight gas sands using imaging technologies is a challenge because of small pore throats, a broad pore size range, and sample heterogeneities with respect to mineralogy and grain size. This contribution presents the outcome of a range of Liquid Metal Injection (LMI) experiments in which we injected various fusible alloys into unconventional reservoir type of rocks, followed by Broad Ion Beam (BIB) polishing and SEM imaging to visualize and to better understand the pore connectivity and transport mechanisms in these tight rocks.
In a series of experiments liquid metal was injected into a variety of organic-rich shales and other tight rocks at elevated temperature (approximately 90 °C) by increasing the pressure stepwise, up to a maximum of 500 MPa. Once the desired pressure was reached, the experiment was completed by turning off the heating and letting the samples cool down while maintaining the pressure. After solidification of the alloy, the sample was argon-ion polished to create a smooth surface for SEM imaging. Imaging sister samples on which we applied different intrusion pressures enables insight into how these tight rocks are progressively filled with the alloy.
Firstly, injection of a non-wetting alloy followed by BIB surface polishing and SEM imaging allows comparison with Mercury Intrusion Porosimetry and visualizing which parts of the pore space are filled during intrusion of a liquid metal. SEM enables identifying alloy-filled pores below ten nanometers in size. High-resolution SEM images provide insight in preferred transport pathways and the variability of pore throat sizes. The experiments show that the non-wetting alloy does not intrude into samples in a uniform way. The relatively lowest porous sample investigated displayed the intrusion of alloy only into fractures, interpreted as induced micro-fractures, while others showed alloy within parts of the in-situ pore space. In a Posidonia Shale sample, intruded and non-intruded layers exist within the same sample, while the layers show no significant difference in mineral fabric at the large scale. At higher resolution, porous calcite fossils and framboidal pyrites were mostly filled with the alloys as was the clay-rich matrix. Injection of a sister sample with a more wetting alloy showed uniform intrusion of the alloy throughout the sample in the in-situ pore space and micro-fractures. The segmented SEM images allow quantification of the connected porosity. The question, which of the identified micro-fractures are induced or not, is subject of current research.
Recent advances in LMI in combination with BIB milling technologies and SEM facilitate imaging connected porosity, both induced and in-situ, down to the nanometer scale in representative 2D cross-sections. Hence, LMI-BIB-SEM is a relatively easy method to verify whether unconventional reservoir samples contain good poro-perm and fracability properties.