Qualitative and Quantitative Changes of Carbonate Rocks Exposed to SC CO2 (Basque-Cantabrian Basin, Northern Spain)

Abstract

Abstract:This study aims at the qualitative and quantitative determination of porosity, mineralogical and textural changes in carbonate rock samples after injection of (i) supercritical CO 2 -rich brine and (ii) dry supercritical CO 2 , under similar experimental conditions (P ≈ 75 bar, T ≈ 35 • C, 970 h exposure time and no CO 2 flow). The studied rocks were sampled in the western Basque-Cantabrian Basin, North Spain, and consist of vuggy carbonates ("Carniolas") of the Puerto de la Palombera formation (Hettangian). Mineralogical and pore space characterization is completed using optical microscopy, scanning electron microscopy and optical image analysis. In addition, X-ray fluorescence analyses are performed to refine the mineralogical information and to obtain whole rock geochemical data and the brine composition is analysed before and after the experiment. Mineralogical and chemical results indicate that the carbonate rocks exposed to supercritical CO 2 in dry conditions do not suffer significant changes. However, the injection of supercritical CO 2 -rich brine induces chemical and physical changes in the rock due to the high reactivity of calcite at the low pH conditions produced by the acidified brine. Numerical modelling validates the experimental observations. These results can be used to characterize the behaviour of carbonate rocks under conditions similar to the vicinity of a CO 2 injection well. The results should be considered only at the scale of the studied samples and not at reservoir scale.

Semi-automated procedure of digitalization and study of rock thin section porosity applying optical image analysis tools

“…The comparison of samples before and after their exposure to SC CO 2 revealed a 0.34% absolute increase in porosity (that is 8.15%, in relative terms) and, with regard to pore shape, a mean decrease of Ro of 2.24% and a mean increase of As of 1.94%. These values agree with the results obtained in a previous study (Berrezueta et al, 2017b), in which 0,32% absolute porosity increase (7.77% in relative terms), 2.40% decrease of Ro and 2.26% increase of As were measured using an OIA-HRS system with a resolution of 4.97 µm/pixel. The difference in the optical resolution of the employed devices (Epson 850 Pro versus Cannon FS 4000 Us) is the principal cause of the deviation in the obtained results since the scanner of the present study (Epson 850 Pro) has a resolution that is 62% higher (Haines et al, 2015;Pirard.…”

“…The measurements showed up the alterations that took place in the pore system of the vuggy carbonate samples due to rock+CO 2 +brine interactions and which were previously observed by OpM and SEM (Berrezueta et al, 2017b). The results harmonize with chemical and mineralogical processes observed by other authors under similar conditions (André et al, 2007;Kaszuba et al, 2003;Luquot and Gouze, 2009); however, the numerical values could be affected by sample variation (3.82% of the porosity) and operator error (1.25%).…”

“…8b). A previous study of the vuggy carbonates (Berrezueta et al, 2017b) applying OIA-HRS (with scanned images of 4000 DPI) demonstrated changes in the pore space configuration of the sample surfaces due to rock-CO 2 interactions (Fig. 8b).…”

“…The previous quantitative petrographic study by OpM and SEM (Berrezueta et al, 2017b) demonstrated variations in the pore system of the vuggy carbonates after their exposure to SC CO 2 rich brine. In particular, dissolution textures could be observed including (i) the apparition of secondary pores on the surface of calcite minerals and (ii) a relatively rough surface after CO 2 -brine-rock interaction.…”

Homogenization of Dissolution and Enhanced Precipitation Induced by Bubbles in Multiphase Flow Systems

“…We have no indication of the time required for nucleation in these regions, but the dependence of induction time on saturation index, which also depends on the precipitation mechanism (Prasianakis et al, 2017), suggests that it took on the order of day(s). Note that precipitation rate, that is, growth rate, is also expected to increase with saturation index and therefore with time (Berrezueta et al, 2017).…”