Spontaneous Alteration of Rock Permeability under Constant Confining Pressure and Temperature Conditions

Yasuhara, Hideaki (Ehime University) | Takahashi, Manabu (AIST) | Kinoshita, Naoki (Ehime University) | Nakashima, Shinichiro (Yamaguchi University) | Kishida, Kiyoshi (Kyoto University)

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ABSTRACT

In this study, a series of permeability experiments on both intact rock and a single fracture in sandstone and on a single fracture in siliceous mudstone has been conducted under confining pressures of 3 – 15 MPa, and at temperatures of 20 – 90°C for several hundred days in each experiment. Spontaneous alteration of permeability and dissolved mass fluxes have been observed. In sandstone, the permeability of intact rocks little changed until a couple of hundred days, then it started to increase with time. In mudstone, the permeability in a single fracture monotonically decreased with time and reached a quasi-steady state within one month. However, it started to increase with time after ~100 days. This augmentation in the permeability in sandstone and mudstone should be attributed to mineral dissolution within the void spaces. In contrast with the sandstone results, the fracture permeability in mudstone resumed decreasing with time throughout the rest of the experiment. A coupled chemo-mechano conceptual model, accounting for pressure and free-face dissolutions, is utilizedto follow the evolution of the permeability observed in the experiments. The model predictions show a relatively fair agreement with the experimental measurements, although further considerations and modifications are found to be required.