Abstract. The aim of this article is to describe and interpret qualitative and quantitative changes at rock matrix scale of lower–upper Cretaceous sandstones exposed to supercritical (SC) CO₂ and brine. The effects of experimental injection of CO₂-rich brine during the first injection phases were studied at rock matrix scale, in a potential deep sedimentary reservoir in northern Spain (Utrillas unit, at the base of the Cenozoic Duero Basin).

Experimental CO₂-rich brine was exposed to sandstone in a reactor chamber under realistic conditions of deep saline formations (P ≈ 7.8 MPa, T ≈ 38 °C and 24 h exposure time). After the experiment, exposed and non-exposed equivalent sample sets were compared with the aim of assessing possible changes due to the effect of the CO2-rich brine exposure. Optical microscopy (OpM) and scanning electron microscopy (SEM) aided by optical image analysis (OIA) were used to compare the rock samples and get qualitative and quantitative information about mineralogy, texture and pore network distribution. Complementary chemical analyses were performed to refine the mineralogical information and to obtain whole rock geochemical data. Brine composition was also analyzed before and after the experiment.

The petrographic study of contiguous sandstone samples (more external area of sample blocks) before and after CO₂-rich brine injection indicates an evolution of the pore network (porosity increase  ≈ 2  %). It is probable that these measured pore changes could be due to intergranular quartz matrix detachment and partial removal from the rock sample, considering them as the early features produced by the CO₂-rich brine. Nevertheless, the whole rock and brine chemical analyses after interaction with CO₂-rich brine do not present important changes in the mineralogical and chemical configuration of the rock with respect to initial conditions, ruling out relevant precipitation or dissolution at these early stages to rock-block scale. These results, simulating the CO₂ injection near the injection well during the first phases (24 h) indicate that, in this environment where CO₂ enriches the brine, the mixture principally generates local mineralogical/textural re-adjustments on the external area of the samples studied.

The application of OpM, SEM and optical image analysis have allowed an exhaustive characterization of the sandstones studied. The procedure followed, the porosity characterization and the chemical analysis allowed a preliminary approximation of the CO₂–brine–rock interactions and could be applied to similar experimental injection tests.