Microtomographic Characterization of Dissolution-Induced Local Porosity Changes Including Fines Migration in Carbonate Rock

Abstract

Summary The microstructure of carbonate rocks experiences substantial changes under reactive processes, in particular chemical dissolution and deposition, including dissolution-released-fines migration occurring during acidizing. A better understanding of such changes at the pore scale and their influences on rock properties is of great value for the effective design and implementation of reactive processes in carbonate reservoirs. In this work, we demonstrate the use of X-ray micro-computed tomography (micro-CT) to quantitatively investigate the local porosity changes in a meso-/microporous carbonate core sample during chemical dissolution. A reactive flooding experiment in a core sample by a nonacidic solution is designed such that changes in pore space from before to after the reactant injection could be imaged in exactly the same locations with micro-CT at a resolution of less than 5 μm. A methodology with three-phase segmentation and 2D histograms of image intensity is used to quantify distributions of the evolution of each image voxel. This technique allows the incorporation of microporosity into the calculation of the evolution regions, including the migration of fines, to accurately quantify the evolution scenarios. The micro-CT images reveal a quasiuniform dissolution pattern and allow characterizing the accompanying migration of fines within the core sample. The 3D pore networks are derived from the image data, which quantify changes in network structure and the pore geometry. The 2D histograms of image intensity derived from the pre- and post-dissolution images show quantitatively how macro- and micropores are enlarged by dissolution close to the inlet, whereas the deposition of fines mainly occurs in pores far from the inlet boundary. These results can explain why permeability of the sample initially decreases and then increases when injection time increases. Pore-surface area between each region is computed on the basis of the spatially resolved voxel evolution scenarios. This allows calculation of local distribution of reactive surface area, which, in turn, will assist in the prediction of local reaction rates in reactive flow simulators.

Anti-correlated Porosity–Permeability Changes During the Dissolution of Carbonate Rocks: Experimental Evidences and Modeling

“…Few recent publications document a typical φ − k relationships with transient negative correlations (Mangane et al 2013;Qajar et al 2013;Luquot et al 2014). Limestones are formed by the aggregation of grains spanning a large range of sizes and degrees of induration and cementation among particles at the submicron level.…”

Characterization of reactive flow-induced evolution of carbonate rocks using digital core analysis- part 1: Assessment of pore-scale mineral dissolution and deposition

“…Details of subresolution pores in voxel-by-voxel analysis of successive images of a sample have been the subject of several studies. Qajar et al (2013) examined dissolution-induced local porosity changes of a carbonate core using 3D microtographic data of pre-and post-alteration sample. They showed that ignoring microporosity leads to inaccurate quantification of the fractions of the medium evolved or unchanged.…”

“…Recently, a few studies have addressed quantifying local fines migration and deposition using XMT images. Qajar et al (2013) reported temporal permeability reduction during high-pH reactive flooding of a carbonate core. They showed evidence for particle mobilization from μ-CT images of the pre-and post-dissolution sample and developed a method to quantify the fraction of solid phase locally migrated within the sample.…”

“…In our previous study (Qajar et al, 2013), local (voxel-based) reactive flowinduced evolutions of rock microstructure was quantitatively investigated using gray-scale attenuation data provided from 3D µ-CT images in pre-and post-alteration states. In this respect, a method based on three-phase segmentation of images and 2D histogram of intensity was proposed to quantify voxel-wise structural evolution in terms of a few general evolution scenarios, including mineral dissolution/detachment and particle deposition regions.…”

“…Reactive coreflood in two carbonate cores with different rock types are considered. For the first rock, we used the experimental and imaging results published by Qajar et al (2013) which showed a quasi-uniform dissolution regime. A similar reactive core flood was conducted in the second rock which resulted in wormhole-like dissolution regime.…”