Temporal Evolution of Shale Mechanical Properties and Pore Characteristics Under ScCO2 Action

Abstract

As shale oil exploration extends to low-permeability systems, understanding mechanical evolution under supercritical CO2 (ScCO2) is crucial for fracturing design. Compressive strength and fracture toughness are key parameters defining rupture tendency. This study aims to systematically reveal the time-dependent evolution of mechanical properties and damage modes of Gulong shale under ScCO2-water-rock interaction. The objective is to establish a correlation between microscopic pore structure changes and macroscopic mechanical degradation to guide engineering applications. Typical Gulong shale samples from the Songliao Basin were selected for ScCO2 soaking experiments ranging from 0 to 16 days. A comprehensive testing workflow was established involving scratch tests, fracture morphology analysis, and Nuclear Magnetic Resonance (NMR) measurements. Continuous scratch tests were conducted to determine the temporal variations in hardness and fracture toughness. Simultaneously, optical and electron microscopy were employed to quantify surface cracks, while NMR tracked the evolution of porosity and pore size distribution, constructing a time-series dataset of mechanical-structural evolution. The experimental results demonstrate a significant degradation in the overall mechanical performance of the shale as the soaking duration increases. Specifically, both hardness and fracture toughness exhibit a continuous downward trend. The damage morphology evolves from predominantly brittle fracture in the early stages to a mixed mode of intergranular and transgranular failure. This transition reflects the weakening of the rock matrix and changes in energy dissipation. Concurrently, NMR measurements reveal that effective porosity and micro-fracture volume increase significantly over time. These observations confirm that ScCO2 interaction facilitates mineral dissolution and promotes the initiation and connectivity of micro-cracks, which directly corresponds to the measured decline in mechanical integrity. This paper presents novel experimental evidence quantifying the time-dependent mechanical degradation of Gulong shale. Unlike traditional bulk tests, it integrates micro-scale scratch mechanics with pore structure evolution to reveal the underlying damage mechanisms. These insights offer a robust theoretical basis for fracture propagation prediction, parameter optimization, and reservoir stimulation evaluation during ScCO2-assisted fracturing and CO2 acidification operations.