Measurement of Threshold Capillary Pressure Using Data of CO₂ Injection under Triaxial Compression Test on Mudstone

Abstract

Elucidating the sealing capacity of caprock is very important because CO₂ sequestered in deep aquifers can remain there for several hundred years in the CO₂ sequestration. Threshold capillary pressure is a key property affecting sealing capacity, and therefore, the ability to measure the threshold pressure of a target rock conveniently is highly sought after. In this study, data collected during the CO₂ injection process in triaxial compression tests of mudstone supposed to be a caprock were used to measure threshold pressure via the dynamic method. These triaxial tests were conducted to study the mechanical properties of mudstone below depths of 1000 m. The threshold pressure for 23 out of 26 test samples was successfully measured. Measured values ranged from 0.54 to 1.57 MPa, which was within a reasonable range compared to the results reported by three other studies that used similar mudstone specimens. A small amount of shrinkage induced by a pore pressure decrease, which was caused by a threshold pressure, was observed in the experiment. The threshold pressure showed a positive correlation with the shrinkage, implying that this pressure might be estimated from the shrinkage. Thus, a volumetric elastic constant was calculated from the shrinkage assuming that the decrease in mean pore pressure was half of the threshold pressure. The mean volumetric elastic constant was 0.92 GPa, which was about 40% lower than that of the rocks from which the test samples were obtained. These results indicate that the shrinkage value cannot be directly used to estimate the threshold pressure, and that a correction of this shrinkage value is necessary.