Effects of Mechanical and Frictional Rock Properties on Hydraulic Fracture Growth Near Unbonded Interfaces

摘要

Experiments are being performed to study the growth of hydraulically driven cracks in the vicinity of an unbonded interface in rocks. The materials on either side of the interface may be the same or different. The materials used in these studies are Nugget sandstone from Utah (3-6% porosity) and Indiana limestone (12-15% porosity). The fracturing fluid is oil which is injected into the test specimens through high pressure steel tubing. Blocks of the rock materials which are to be studied are held adjacent to one another in a hydraulic press so that a normal stress is set up across their mutual interface. A hydraulically driven crack is initiated in one of the blocks. It is found that the major parameters which affect hydraulic crack penetration into the adjacent block are the relative values of the mechanical strength and moduli of the rocks on either side of the interface, the finish of the interface surfaces, and the magnitude of the normal stress across the interface. To gain a better understanding of the mechanisms which control the growth of cracks across the interfaces, experiments are being performed to study the frictional properties of these interface surfaces. The experiments involve the simultaneous measurement of the normal force across the interface and the applied force necessary to overcome friction to initiate displacement. These experiments indicate that in addition to the normal force, the frictional force depends upon the surface finish and the presence of water in the rock. 10 figures. (ERA citation 04:054159)