| Popis: |
The role of shale behaviour in borehole instability is classically explained by the high reactivity with water, resulting in swelling, slaking, weakening, etc. However, laboratory experiments are carried out on shale samples whose total saturation is never ensured, in spite of all the care taken for their preservation. The presence of air in the porous network induces: 1. (1) capillary suction, which, in turn, increases apparent cohesion; and 2. (2) gas entrapment and rise in capillary pressure upon exposure to fluid. It is shown that wet shale weakening can be attributed to release of capillary suction between grains, which is linked to pore size distribution. This study also indicates how gas entrapment may occur and how gas pressurization can induce tensile stresses in the sample and be responsible for swelling pressures and apparent slaking of the shale. Finally, rehydration experiments conducted on shale samples are described and a method indicated for complete saturation and long term preservation of samples. |
