Porosity changes due to a nalcime in a basaltic tuff from the Janggi Basin, Korea: experimental and geochemical modeling study of CO_2-water-rock interactions

摘要

This study aimed to identify the geochemical reactivity of basaltic tuff as a cap rock in the Janggi Basin, Korea, and its effect on porosity changes by CO2-water-rock interactions. The basaltic tuff used in this study is composed of mainly smectite, zeolite, plagioclase, and pyroxene and contains high amounts of divalent cations that are required to form carbonate minerals. Batch experimental and modeling studies were carried out for CO2-water-rock interactions over 84 days at 50 degrees C and 100 bar. In this study, increases in porosity are observed from 14.9 to 18.7% by mercury intrusion porosimetry (MIP) and from 15.1 to 20.7% by images from 3D X-ray CT scans (XCT) because of the predominant dissolution reactions of the primary minerals. Among the primary minerals, analcime dissolves the most from fitting geochemical modeling to experimental measurements of cation concentrations and backscattered electron (BSE) analysis. Notably, the dissolution of analcime contributes more than 80% of the total reduction in rock volume according to the change in volume of each mineral calculated using the results of geochemical modeling. This outcome indicates that the dissolution of analcime among primary minerals contributes the most to the increase in porosity of the basaltic tuff. However, long-term research is required to discuss the role of analcime in sealing capacity because analcime is only 6.3% by weight in the basaltic tuff.