The original work on 3-D X-ray microtomography (μCT) by Flannery et al.  states in conclusion: “we believe that it will be possible to study contained systems under conditions of temperature, pressure, and environment representative of process conditions.” Since this seminal work, there has been an explosion in the use of μCT with the rapid development of bench-top scanners and huge interest in the study of porous rocks—as in the original paper—with application to hydrocarbon production and carbon dioxide storage. μCT is now the foremost method for the noninvasive imaging of rock cores at ambient conditions, applied to modeling and experimental interpretation [Berg et al., 2013; Blunt et al., 2013; Dann et al., 2011; Feali et al., 2012; Wildenschild and Sheppard, 2013]; however, imaging under conditions representative of flow and transport deep underground, including effects due to chemical equilibrium, has remained a challenge [Silin et al., 2011]. In this paper, we present the first in situ images of multiple phases in the pore space at elevated temperatures and pressures, representative of an aquifer at around 1 km depth, while maintaining mutual chemical equilibrium between the fluid phases and the rock.
 The application of this study is for geological carbon dioxide (CO2) storage, where the concern is to design injection such that the CO2 remains underground for hundreds to thousands of years. Sedimentary basins that are potentially suitable carbon storage sites include deep carbonate aquifers [Bachu, 2003]. An important mechanism that limits the spread and potential escape of CO2 is capillary trapping, where CO2, displaced by aquifer brine, is stranded as pore-scale droplets (ganglia) [Juanes et al., 2006]. Under favorable conditions, this process can, in theory, render the vast majority of the CO2 immobile [Ennis-King and Paterson, 2002; Golding et al., 2011; Qi et al., 2009]. The average amount of trapping can be measured in core flood (cm scale) experiments [Akbarabadi and Piri, 2013; Bennion and Bachu, 2010; El-Maghraby, 2013; El-Maghraby and Blunt, 2013; Okabe and Tsuchiya, 2008; Pentland et al., 2011] and has been imaged at the pore scale in a sandstone [Iglauer et al., 2011]. However, in typical deep storage sites, the CO2 will be in a supercritical (sc) phase in mutual chemical equilibrium with the host brine and the rock: dissolved CO2 forms an acid that can react with many rock minerals, including carbonates. We demonstrate that, locally, substantial quantities of scCO2 can be trapped in the pore space of the limestone studied, at representative in situ conditions where the scCO2, brine, and rock are in mutual chemical equilibrium. We demonstrate that the trapped clusters have an approximately power law distribution of size, consistent with the predictions of percolation theory assuming that CO2 is the nonwetting phase [Blunt and Scher, 1995; Dias and Wilkinson, 1986]. It is also consistent with measurements of trapping on analogue systems at ambient conditions [Iglauer et al., 2012; Iglauer et al., 2010].