Micro-computed tomography (micro-CT) is increasingly utilized to image the pore network and to derive petrophysical properties in combination with modelling software. The effect of micro-CT image resolution and size on the accuracy of the derived petrophysical properties is addressed in this study using a relatively homogenous sandstone and a heterogenous, highly porous bioclastic limestone. Standard laboratory procedures including NMR (nuclear magnetic resonance) analysis, micro-CT analysis at different image resolutions and sizes and pore-scale flow simulations were used to determine and compare petrophysical properties. NMR-derived pore-size distribution (PSD) was comparable to the micro-CT-derived PSD at a resolution of 7 µm for both the rock types. Porosity was higher using the water saturation method as compared to the NMR method in both rocks. The resolution did not show a significant effect on the porosity of the homogeneous sandstone, but porosity in the heterogeneous limestone varies depending on the location of the sub-sample. The transport regime in the sandstone was derived by simulations and changed with the resolution of the micro-CT image. The transport regime in the sandstone was advection-dominated at higher image resolution and diffusion-dominated when using a lower image resolution. In contrast, advection was the dominant transport regime for the limestone based on simulations using higher and lower image resolutions. Simulation-derived permeability for a 400 Voxel³ image at 7 µm resolution in the Berea sandstone matched laboratory results, although local heterogeneity within the rock plays an integral role in the permeability estimation within the sub-sampled images. The simulation-derived permeability was highly variable in the Mount Gambier limestone depending on the image size and resolution with the closest value to a laboratory result simulated with an image resolution of 2.5 µm and a size of 300 Voxel³. Overall, the study demonstrates the need to decide on micro-CT parameters depending on the type of petrophysical property of interest and the degree of heterogeneity within the rock types.

Many metals of economic value are commonly recovered from low grade ores through heap leaching at mine sites. The recovery of metals in heaps is often limited for multiple reasons including mineral overgrowth of targeted minerals resulting in suboptimal mineral dissolution rates. Chalcopyrite is a major copper iron sulfide mineral found in copper sulfide ores and its dissolution is controlled by multiple dissolution reaction mechanisms and potential mineral overgrowth. Traditional continuum scale reactive-transport models, including those applied to simulate heap leaching, do not account for local, mineral surface-controlled processes which may have implications for large-systems such as copper recovery in heaps. A new finite element, multicomponent reactive-transport model was developed and enabled the simulation of concurrent reactions at the discrete chalcopyrite surface and its surrounding gangue mineral and the fluid phase. The model predicts the rate of chalcopyrite dissolution controlled by different reaction mechanisms (proton- and ferric-iron promoted) as well as the rate of secondary mineral formation (jarosite) at the mineral surface. The latter may increasingly inhibit copper mobilization over time through surface passivation. The onset and growth of the surface passivation layer is largely controlled by the surrounding fluid composition and ion transport through the gangue mineral. The reduction in porosity of the surface passivation layer leads to the breakdown of ionic transport to the chalcopyrite surface and associated copper mobilization. Maximum copper recovery is achieved by high ferric iron concentrations and low pH and by preventing the formation of a passivation layer by keeping the sulfate concentration low.