Localized episodic deformation events in mudstone suggest limited pathways for gas breakthrough

Understanding the deformation behavior of mudstones induced by gas invasion is important to discuss the mechanical response of cap rock for geological sequestration of carbon dioxide (CO₂), especially for evaluating possible CO₂ leakage through cap rock. In this study, gas injection experiments were conducted by using a water-saturated mudstone core sample under relatively low excess gas pressure conditions, and the observed strain behaviors were compared between experiments with and without continuous gas invasion to identify characteristic deformation signals associated with gas invasion. In the experiments, the lateral surface of the sample was sealed with silicone rubber, then, the confining pressure was maintained to be 0.50 MPa and the pore water pressure to be 0.20 MPa as an initial condition. The air pressure was applied at the bottom of the sample, and was increased stepwise from 0.25 to 0.40 MPa with 0.05 MPa increments. Each condition was kept until a steady-state condition was achieved. Axial and circumferential strains at half the height of the sample were monitored using four cross gauges, and water discharge at the sample top was also observed. The measured water discharge indicated a very small amount of air invasion at the bottom air pressures of 0.25, 0.30, and 0.35 MPa, with eventual cessation of invasion. Notably, at 0.35 MPa, gas invasion persisted for a longer duration compared to the other two pressure conditions. At 0.40 MPa, the water discharge increased, and air breakthrough was observed. The measured cumulative water discharge at air breakthrough divided by the sample pore volume was 1%, indicating very limited air pathways in the sample. Under the condition that the bottom air pressure was 0.30 MPa or less, the measured strains showed initial possible poroelastic induced axial contraction and gradual extension, followed by gradual contraction, reaching to a steady-state condition. In the case where the bottom air pressure was 0.35 MPa or higher, early-stage strain behavior was similar, however, from the middle stage of the contraction phase, the strains showed a number of episodic sudden extensions and subsequent gradual contractions. Furthermore, the magnitude of the extensions differed significantly from gauge to gauge, and two of the gauges showed no extension. The observed localized episodic strain behaviors are attributed to air migration through limited pathways. When air invaded into part of the pore network filled with water, pore water pressure increased locally nearby the invaded pore, which should be close to the capillary pressure of the invaded pore. Strain gauges closer to the invaded pore then showed sudden extension and subsequent gradual contraction due to pore water pressure diffusion, while no strain was detected by other gauges located far from the invaded pore. The localized episodic sudden extensions followed by gradual contractions observed in our study strongly suggest very limited pathways for gas breakthrough in mudstones.