GAS-DRIVEN CRACKING IN WIPP ROCK-SALT

Earlier nonlinear finite element analyses based on the local continuum models, and ignoring fracture mechanics, have indicated that the gas pressure in the Waste Isolation Pilot Plant (WIPP) may exceed the lith ostatic pressure of 14.8 MPa by 10MPa or more. Rock salt cannot sustai n high tensile stress, and therefore such a high gas pressure cannot i n reality exist. This high predicted pressure indicates the possibilit y that large gas-driven hydrofractures will form. Analytical calculati ons show that, using the assumptions of linear elastic fracture mechan ics, a horizontal penny-shaped fracture several kilometers in radius i s possible. Also, it is possible that after growing horizontally in th e weaker anhydrite layer, the crack could break out of this layer and propagate into the halite and toward the ground surface at an inclined angle of 53 degrees above horizontal. To prevent this latter scenario , it is shown that the anhydrite must have a fracture toughness less t han 0.5590 times the fracture toughness of the halite.