Mercury porosimetry as a tool for improving quality of micro-CT images in low porosity carbonate rocks

• We compared low-porosity carbonate rocks μCT images before and after Hg intrusion.
• Hg intrusion improves quality of μCT images of very low porous (< 1%) rocks.
• Hg metal effect strongly affects μCT image quality of more porous (4–8%) rocks.
• Hg filled pores 1–2 order of magnitude smaller than voxel size are imaged in μCT.
• μCT quantitative image analysis is affected by partial volume effect.

The combined use of Hg-impregnation and microCT scanning is a powerful tool for detecting structural and textural features in low porosity massive carbonate rocks. Hg-impregnation of carbonate rocks can improve extremely the quality and resolution of microCT images, because of the high density contrast between Hg and the surrounding rock. The success of the combined used of these two techniques depends mainly on two different rock characters:1) Shape of the pores and characteristics of the porous system. Ink-bottle shape pores and tortuous complex porous systems prevent Hg from flowing out of the sample at the end of Hg-porosimetry test, producing reliable microCT images of the porous system itself. On the other side low tortuous cylindrical pores allow Hg flowing out, resulting in not completely true microCT images.2) Effective rock porosity. Hg-impregnation of carbonate rocks with very low porosity (< 1%) and tortuous or ink-bottle pores improves significantly the analysis and quantification of their porous system. On the contrary, for carbonate rock with higher porosity (> 4%), although with ink-bottle pores, microCT images are undetectable, as the relatively high quantity of Hg entrapped in the sample creates artefacts (metal effect) that completely obliterate the rock structure. Intermediate porosity (about 2%) can behave either as low porosity (< 1%) or as higher porosity (> 4%) depending on pore shape. A prevalence of tortuous ink-bottle pores results in undetectable microCT images, due to widespread metal effect. A small amount of cylindrical pores, reducing the quantity of entrapped Hg, results in a good quality of microCT images.The resolution of the technique depends on the rock porous system. For low porosity (< 1%) rock, due to high density contrast between Hg and rock, pores with radius one order of magnitude smaller than the voxel size are clearly imaged by microCT and pores with radius two order of magnitude smaller than the voxel size are still detectable. Hg-impregnated pores smaller than the voxel increase the voxel density, as a function of the relative volume of Hg and rock. As a consequence, pores smaller than the voxel size appear as large as the voxel itself and bigger pores appear larger than they really are (partial volume effect). Although Hg-impregnation improves 3D qualitative analysis of porous system and its relationships with rock texture, quantification of the porous system through segmentation of microCT images is strongly affected by intrinsic error in pore dimensions, caused by partial volume effect. Quantification error is a function of the shape of the porous system, being lower for rounded pores and higher for complex tortuous pore system.