The effects of porosity on the brittle‐ductile transition in sandstones

Sandstones with porosities of 6.0%, 13.9%, 19.9%, 20.9%, 22%, and 27.6% were deformed under triaxial conditions either as whole core samples or 35° sawcut samples at confining pressure conditions of 0.1, 5, 10, 30, 50, 70, 90, 110, and 130 MPa. Whole cores exhibited three types of deformational behavior: (1) thoroughgoing shear fractures, (2) brittle‐ductile transitional behavior characterized by cataclastic shear zones, and (3) pervasive ductile cataclastic flow. Sawcut cores exhibited either (1) brittle behavior accomplished by reactivation of the sawcut or by the development of a crosscutting shear fracture, or (2) penetrative ductile cataclastic flow which obliterated the sawcut. The 35° sawcut cores lack the development of multiple shear zones associated with transitional brittle‐ductile behavior in whole cores. The brittle‐ductile boundary in both sawcut and whole cores occurs at similar confining pressures and is dependent upon initial porosity of the sandstone. Increasing the confining pressure has the effect of shifting the brittle‐ductile boundary to lower porosities. A comparison of frictional strengths and fracture strengths confirms Byerlee's (1968) observation that the change from brittle to ductile behavior is related to frictional properties of the material.