抄録
The long-term promise of geological sequestration of CO2 is dependent on capillary sealing and slow flow mechanisms for low-permeable rocks such as caprocks. The purpose of this study is to investigate threshold pressure of Namihana (NMH) Fm. and Ohara (OHR) Fm. mudstones depending on effective confining stress at 40℃ and effective pressures up to 20 MPa. The mudstone specimens tested were taken from Kazusa group of Boso peninsula in Japan. Two support experiments of water permeability and porosity measurements were performed before the threshold pressure test to assess mechanical behaviors of OHR and NMH mudstones. After the threshold pressure experiment, porosities of OHR and NMH mudstone specimens were measured using Mercury Intrusion Porosimetry (MIP) method. Our results demonstrated that OHR mudstone exhibited a steep decreasing trend in water permeability with increasing effective pressure at around 5 MPa, while water permeability of NMH mudstone decreases monotonously as effective pressure increases. It has been reported by previous studies that the observed significant reduction in water permeability for OHR mudstone could be attributed to the presence of crack-like voids in rocks. Thus, it was suggested that there were a rich crack-like voids in OHR mudstone compared to NMH mudstone. The critical pressure inferred from the obtained compression curves for OHR and NMH mudstones, corresponding to transient from elastic to elasto-plasticity deformations was around 5 MPa and 7 MPa, respectively. These values were fairly consistent with the MIP data. All the data for threshold pressure as a function of effective pressure exhibited a linear correlation with water permeability except for OHR mudstone in lower effective pressure range. It can, therefore, be concluded that the presence of such crack-like voids in mudstones has a significant effect on threshold pressure and will require an additional model to be capable of having the relationships between threshold pressure and water permeability.
