Study on the Dissolution of Sandstones in Gyeongsang Basin and the Calculation of Their Dissolution Coefficients under CO₂Injection Condition

Abstract

Lab scale experiments to investigate the dissolution reaction among supercritical CO 2 -sandstone-groundwater by using sandstones from Gyeongsang basin were performed. High pressurized cell system (100 bar and 50 o C) was designed to create supercritical CO 2 in the cell, simulating the sub-surface CO 2 storage site. The first-order dissolution coefficient (k d ) of the sandstone was calculated by measuring the change of the weight of thin section or the concentration of ions dissolved in groundwater at the reaction time intervals. For 30 days of the supercritical CO 2sandstone-groundwater reaction, physical properties of sandstone cores in Gyeongsang basin were measured to investigate the effect of supercritical CO 2 on the sandstone. The weight change of sandstone cores was also measured to calculate the dissolution coefficient and the dissolution time of 1 g per unit area (1 cm 2 ) of each sandstone was quantitatively predicted. For the experiment using thin sections, mass of Ca 2+ and Na + dissolved in groundwater increased, suggesting that plagioclase and calcite of the sandstone would be significantly dissolved when it contacts with supercritical CO 2 and groundwater at CO 2 sequestration sites. 0.66% of the original thin section mass for the sandstone were dissolved after 30 days reaction. The average porosity for C sandstones was 8.183% and it increased to 8.789% after 30 days of the reaction. The average dry density, seismic velocity, and 1-D compression strength of sandstones decreased and these results were dependent on the porosity increase by the dissolution during the reaction. By using the first-order dissolution coefficient, the average time to dissolve 1 g of B and C sandstones per unit area (1 cm 2 ) was calculated as 1,532 years and 329 years, respectively. From results, it was investigated that the physical property change of sandstones at Gyeongsang basin would rapidly occur when the supercritical CO 2 was injected into CO 2 sequestration sites.

The Use of the Surface Roughness Value to Quantify the Extent of Supercritical CO2 Involved Geochemical Reaction at a CO2 Sequestration Site

“…The scCO2-sandstone slabgroundwater reaction in the cell was executed at 100 bar and 50 °C for 150 days and the apparatus of the slab experiment is shown in Figure 2. According to previous studies related to geochemical reactions, the weathering process of the reservoir rock after scCO2 injection was closely related to the surface roughness change of the rock because dissolution and precipitation occur simultaneously and lead to surface roughness change until the geochemical system turns back to a status of equilibrium [22,49]. To observe the surface roughness change of sandstone slabs during the geochemical reaction, seven minerals, quartz, perthite, plagioclase, muscovite, calcite, biotite, and chlorite for each slab surface were determined According to previous studies related to geochemical reactions, the weathering process of the reservoir rock after scCO 2 injection was closely related to the surface roughness change of the rock because dissolution and precipitation occur simultaneously and lead to surface roughness change until the geochemical system turns back to a status of equilibrium [22,49].…”

“…According to previous studies related to geochemical reactions, the weathering process of the reservoir rock after scCO2 injection was closely related to the surface roughness change of the rock because dissolution and precipitation occur simultaneously and lead to surface roughness change until the geochemical system turns back to a status of equilibrium [22,49]. To observe the surface roughness change of sandstone slabs during the geochemical reaction, seven minerals, quartz, perthite, plagioclase, muscovite, calcite, biotite, and chlorite for each slab surface were determined According to previous studies related to geochemical reactions, the weathering process of the reservoir rock after scCO 2 injection was closely related to the surface roughness change of the rock because dissolution and precipitation occur simultaneously and lead to surface roughness change until the geochemical system turns back to a status of equilibrium [22,49]. To observe the surface roughness change of sandstone slabs during the geochemical reaction, seven minerals, quartz, perthite, plagioclase, muscovite, calcite, biotite, and chlorite for each slab surface were determined using a reflecting microscope (Nikon eclipse LV100N, Nikon, Fukasawa, Japan), and three locations for each mineral were randomly selected to acquire 3-dimensional surface images.…”

“…Geological map around the drilling site ( ) and photographs of sandstone cores and a slab for the experiment (modified from [22]). …”

“…was identified by modal analysis and more than 500 locations on each thin section surface were observed using the point counter attached in a polarizing microscope. Geological map around the drilling site ( ) and photographs of sandstone cores and a slab for the experiment (modified from [22]). …”

“…The previous research over the last two decades revealed that the injection of supercritical CO 2 (scCO 2 ) into a reservoir rock initiates a geochemical reaction in the scCO 2 -rock-groundwater system, but it does not threaten the storage safety of CO 2 reservoir formations because of its long reaction time [18][19][20]. However, in the past five years, several studies have cast doubts on whether the rate of the scCO 2 -rock-groundwater reaction is too slow to affect the change of CO 2 storage capacity and leakage safety after scCO 2 injection [21][22][23][24][25][26].…”