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 CO2-rich brine and (ii) dry supercritical CO2, under similar experimental conditions (P ≈ 75 bar, T ≈ 35 °C, 970 h exposure time and no CO2 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 CO2 in dry conditions do not suffer significant changes. However, the injection of supercritical CO2-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 CO2 injection well. The results should be considered only at the scale of the studied samples and not at reservoir scale.
Highlights
Introduction and ObjectivesReduction of greenhouse gas concentration in the atmosphere has been in the focus of scientific research over the last decades
This study focuses exclusively on the textural-mineralogical changes produced by chemical interactions by X-ray fluorescence (XRF), disregarding physical alterations that may appear under dynamic conditions due to the flow of CO2 and brine through the rock
The optical porosity measured by optical image analysis (OIA) on thin sections is ≈4.1 ± 0.15% and the average pore size is (≈20 um)
Summary
Introduction and ObjectivesReduction of greenhouse gas concentration in the atmosphere has been in the focus of scientific research over the last decades. One of the possible approaches is to capture CO2 from large industrial sources and to storage it in deep geological formations [1,2,3,4]. During this process, the CO2 is injected into a formation of high porosity and permeability (reservoir), which is overlaid by an impermeable formation (seal). The CO2 injection is intended in a dense phase or in supercritical conditions (SC CO2 : P > 73.8 bar, T > 31.7 ◦ C), which results in a significant volume reduction of the gas [1,6]
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