Swelling and Fluid Transport of Re-sealed Callovo–Oxfordian Claystone

Several experimental methods are used to investigate the swelling capacity and transport properties of re-sealed macro-cracked Callovo–Oxfordian (COx) claystone, particularly in the absence of confining pressure. Six COx claystone samples from four different geological locations of the Bure basin (France) are tested, five of which are macro-cracked and one remains intact. Sample swelling occurs, during re-hydration with liquid water, leading to the measurement of an apparent swelling pressure. The latter is continuously recorded with a dedicated device. The asymptotic apparent swelling pressure of macro-cracked UT (transitional unit) COx is approximately 1 MPa, while it varies from 3 to 5 MPa for macro-cracked UA (clayey unit) COx. Quantitative X-ray diffraction (QXRD) analysis demonstrates that the amount of smectite, which is a swelling clay, is weakly correlated with apparent swelling pressure. Surprisingly, the interstratified illite/smectite with lower smectite content is highly correlated to apparent swelling pressure. Nitrogen isotherms data imply that the Gurvich total pore volume (V_Gurvich) and specific surface area (SSA) are highly linearly correlated with the low smectite content interstratified phase. This means that the distribution of smectite strongly affects the swelling capacity of COx. Moreover, nitrogen sorption is an easier and more effective technique than QXRD for assessing COx swelling capacity, since both V_Gurvich and SSA have been proven as effective indicators. For both UT and UA COx, self-sealing can cause significant reductions in water permeability (Kw). In particular, UA COx shows higher sealing efficiency and faster kinetics compared to UT COx. After sealing, the equivalent crack aperture (calculated from Poiseuille’s law) decreases from tens of microns to less than 1 micron. According to the gas breakthrough tests, the gas breakthrough pressure (GBP) of re-sealed macro-cracked COx is of the same order of magnitude as the equivalent capillary pressure of residual crack. This indicates that the gas migration in the re-sealed cracked COx claystone mainly occurs through the residual crack and is ‘a priori’ controlled by capillary processes.