Meitan xuebao (Jul 2023)
Mineralogical erosion and precipitation characteristics and their effects on adsorption property of shale during scCO2-H2O-shale interaction
Abstract
Understanding the material composition and pore structure variation of shale gas reservoirs during supercritical CO2-shale interaction is of essence to achieve CO2 sequestration and enhanced natural gas production. The storage space evolution associated with mineral corrosion has been well reported, but the studies on the secondary precipitation and plugging characteristics are still insufficient, and there is particularly a lack of theoretical and experimental investigations. To analyze mineralogical corrosion/precipitation characteristics and its effects on adsorption capacity with different reaction time (6‒30 days) and thus demonstrate the fluid-solid interface kinetics and evolution laws of pore structure, the supercritical CO2 (scCO2)-water-shale reaction, scanning electron mi-croscope (SEM), low-pressure N2 adsorption and isothermal adsorption experiments were primarily conducted using the Lower Silurian Longmaxi shales in the southern Sichuan Basin. The results reveal that the content of calcium and potassium minerals in the sample after the reaction gradually decreases with time, leading to an increase of Ca2+ and K+ concentration in the solution. Calcite minerals are notably corroded across the reaction, accompanied by a number of carbonate precipitates that converge on the surface. Corrosion generally expands the initial pore space, resulting in a volumetric increase of pores ranging from 3.29 to 4.50 nm. But the precipitation process may lead to pore space plugging and correspondingly shrinks the increments of the pore volume. As a result of corrosion and precipitation, the surface fractal dimension D1 increases slightly, while the structure fractal dimension D2 shows a reducing trend, which intensifies pore heterogeneity. In general, mineralogical corrosion can enlarge the samples’ storage space, initiate more CO2 molecules to be polarized, and thus strengthen interactions between scCO2 fluids and shale. These are the main reasons for the enhancement of adsorption capacity and adsorption potential. There is no remarkable change in the adsorption and adsorption potential of the sample before and after precipitation, indicating that the precipitation effect has a minor impact on adsorption capacity. Insights into corrosion and precipitation mechanisms and their effects on storage space are a matter of enlightenment for high-efficiency and long-term CO2 geo-sequestration.
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