Observations and modeling studies have shown that during CO₂ injection into underground carbonate reservoirs, the dissolution of CO₂ into formation water forms acidic brine, leading to fluid-rock interactions that can significantly impact the hydraulic properties of the host formation. However, the impacts of these interactions on the pore structure and macroscopic flow properties of host rock are poorly characterized both for the near-wellbore region and deeper into the reservoir. Little attention has been given to the influence of pressure drop from the near-wellbore region to reservoir body on disturbing the ionic equilibrium in the CO₂-saturated brine and consequent mineral precipitation. In this paper, we present the results of a novel experimental procedure designed to address these issues in carbonate reservoirs. We injected CO₂-saturated brine into a composite core made of two matching grainstone carbonate core plugs with a tight disk placed between them to create a pressure profile of around 250 psi resembling that prevailing in reservoirs during CO₂ injection. We investigated the impacts of fluid-rock interactions at pore and continuum scale using medical X-ray CT, nuclear magnetic resonance, and scanning electron microscopy. We found that strong calcite dissolution occurs near to the injection point, which leads to an increase in primary intergranular porosity and permeability of the near injection region, and ultimately to wormhole formation. The strong heterogeneous dissolution of calcite grains leads to the formation of intra-granular micro-pores. At later stages of the dissolution, the internal regions of ooids become accessible to the carbonated brine, leading to the formation of moldic porosity. At distances far from the injection point, we observed minimal or no change in pore structure, pore roughness, pore populations, and rock hydraulic properties. The pressure drop of 250 psi slightly disturbed the chemical equilibrium of the system, which led to minor precipitation of sub-micron sized calcite crystals but due to the large pore throats of the rock, these deposits had no measurable impact on rock permeability. The trial illustrates that the new procedure is valuable for investigating fluid-rock interactions by reproducing the geochemical consequences of relatively steep pore pressure gradients during CO₂ injection.

The late Proterozoic Ntaka Ultramafic Complex is a body of dominantly pyroxenitic cumulate rocks containing cyclic alternations of olivine–orthopyroxene cumulates. Chemical zoning in the pyroxenes has been imaged at 25–40 µm resolution using desktop microbeam X-ray fluorescence mapping followed up with laser ablation–inductively coupled plasma mass spectrometry analysis for minor and trace elements on selected samples. Poikilitic and granular harzburgites are finely intermingled, in some cases on a centimetre scale in the same thin section. Poikilitic varieties display spectacular textures, ranging from isolated equant orthopyroxene oikocrysts within olivine-rich heteradcumulate harzburgites to rocks composed entirely of interlocking centimetre-sized anhedral orthopyroxene oikocrysts containing sharply bounded idiomorphic Cr-enriched cores. The poikilitic harzburgites are interlayered with cumulate pyroxenites in which orthopyroxene grains show a variety of zoning patterns: Cr-rich cores similar to those in the oikocrysts; sharply bounded oscillatory zoned cores; and reverse zoning with Cr-poor cores and Cr-enriched rims. A further variation is the presence of a mingled harzburgite lithology in which dunite or poikilitic harzburgite is invaded on a centimetre scale by diffuse vein networks or patches of coarse orthopyroxenite. This range of textures and lithologies attests to a more complex set of processes than implied by the standard cumulus theory model in which oikocrysts are considered to have crystallized from intercumulus liquid within a permeable crystal mush. A range of hypotheses is proposed, including infiltration metasomatism of original olivine cumulates by migrating orthopyroxene-saturated pore fluid; however, the textural relationships, whole-rock chemistry and Cr zoning within the grains can best be explained by a model in which the orthopyroxene oikocrysts form in part or whole as mechanically accumulated cumulus grains. The complexity of zoning patterns is attributed to stirring of entrained olivine and orthopyroxene crystals within a heterogeneous flowing crystal mush, where the transporting magma has a wide range of silica contents owing to poorly stirred incorporation of siliceous country-rock material. The Cr-rich orthopyroxenite component grew from Si-enriched chromite-saturated magma. Mingled lithologies developed after accumulation as a result of percolation and infiltration metasomatism by Si-enriched liquid derived by melting of xenoliths within the crystal pile. The model may be more generally applicable: dunite–harzburgite cycles, common in many layered intrusions, may reflect variable degrees of contamination rather than cycles of fractional crystallization and replenishment.