ABSTRACT
A comprehensive series of fracture toughness tests were performed on oil shale obtained from the Anvil Points Mine, Rifle, Colorado. Three-point-bend fracture specimens were used and testing techniques closely conformed to ASTM standardized methods for metals [ASTM designation EB99-72T]. Since oil shale is layered and transversely isotropic, specimens were tested in the three principal crack orientations--divider, arrester and short transverse. Specimens representing two nominal grades of oil shale, 80 and 160 ml/kg (20 and 40 gal/ton) were tested. The specimens were fatigue-cracked to produce a sharp natural crack in a stable manner by means of loading between fixed limits of the crack opening displacement. Crack front position was marked by immersing the specimen in a penetrating dye so that the crack length could be determined after final failure. The average length of the fatigue crack was determined later by digitizing the position of the crack front from a photograph of the fracture surface and by computer reduction of this digitized data. Fracture toughness was found to decrease by approximately 40 percent for an increase in kerogen content from 80 to 160 ml/kg. Highest values of fracture toughness were found for the divider geometry, lowest for short transverse, and intermediate for arrester with the actual values varying from 0.3 to 11 MNm -3/2 Records of load vs. crack opening displacement showed evidence of crack surface interference or crack closure. Additional tests on larger specimens demonstrated a slight effect of crack length on the apparent fracture toughness. Tension tests were also performed in order to evaluate the ASTM size criterion as specified for metallic materials. Also, a series of stress corrosion cracking tests were conducted to assess the effects of the environmental factors of distilled water, laboratory air, and dry argon. These tests indicate that water induces subcritical crack growth by stress corrosion in oil shale at a stress intensity level 90 percent of the fracture toughness.
N
In situ retorting of tight oil shale deposits will require that the beds be fragmented or rubblized in order to create the permeability necessary to sustain a burn. The mechanics of the fracture processes in oil shale is clearly important from an engineering standpoint if one is to understand and predict the degree of rubblization and resulting permeability from the various bed preparation schemes now proposed. Optimization of bed preparation techniques is possible with adequate knowledge of how crack propagation depends on kerogen content, in situ stress state, crack tip environment, and propagation direction with respect to bedding planes. Use of linear elastic fracture mechanics, LEFM, for modeling fracture in rock is receiving rapid acceptance. LEFM is based on the stress intensity factor, K, which quantifies the intensity of the stress singularity at a crack tip. Fracture mechanics states that a crack will advance when its stress intensity reaches a critical value, Kic, for the case where the crack tip is in a state of plane strain. This critical value, known as plane strain fracture toughness, has been shown to be a material constant for a vast number of metal alloys, ceramics, polymers and even some organic materials such as wood, paper, and rubber.
