Investigation on gas migration in saturated bentonite using the residual capillary pressure technique with consideration of temperature

Abstract

Determination of parameters for description of gas migration in saturated bentonite is of great importance for the design and construction of artificial barriers in the geological repository for the disposal of high-level radioactive nuclear waste. In this paper, temperature-controlled gas injection tests were conducted on initially water-saturated bentonite specimens using the residual capillary pressure (RCP) technique. Effective gas permeabilities in low injection pressures and the gas breakthrough pressures at temperatures 20, 40 and 60°C were obtained. Results show that: (i) for all the temperatures tested, the intrinsic water permeabilities (kin) range between 3.2 × 10−20 and 5.72 × 10−20 m2, furthermore, those values increase with rising temperature. Meanwhile, the intrinsic water permeabilities derived from the steady state tests are lower than those obtained from the non-steady state tests; (ii) for all the temperatures tested, the effective gas permeabilities (keff) corresponding to the viscous gas flow before gas breakthrough range between 4.81×10-24 and 2.74×10-22 m2, with slight fluctuations with time. The maximum effective gas permeabilities measured at the occurrence of gas breakthrough on the initially water-saturated bentonite specimens extend from 2.27×10-18 up to 3.32×10-17 m2 and increase as temperature increases, while the time required for gas breakthrough decreases as temperature increases; (iii) for all the temperatures tested, the gas breakthrough pressures measured on the initially water-saturated bentonite specimens vary from 2.74 to 4.08 MPa, while the residual capillary pressure differences, also denoted as snap-off pressures (Psnap-off), range from 0.2 to 0.38 MPa. Additionally, the gas breakthrough pressures and the residual capillary pressures decrease as temperature increases.