MECHANICAL PROPERTIES AND BRITTLENESS EVALUATION OF LAYERED SHALE ROCK

The mechanical properties of shale gas reservoir have great influences on fracturing effect. It is of great necessity to investigate damage mechanism, mechanical properties and brittleness evaluation of shale rock and then to provide technical support for large-scale volume fracturing of shale gas exploitation. This paper conducts uniaxial compression tests on black carbonaceous shale of Lungmachi Formation. The test results indicate the follows. The black shale rock has obvious layered flake mineral structure. Its main mineral components are quartz and calcite, which causes the structure surface having high strength. Due to the effects of strong structure surface, shale rock with inclined bedding plane has larger values of elastic modulus and uniaxial tensile strength and the minimum total volumetric strains. However, when the bedding plan is parallel or vertical to the loading direction, the shale rock has smaller values of elastic modulus and uniaxial tensile strength and the maximum total volumetric strain. When the bedding angle β is increasing, the trend of crack initiation index behaves like a U-shaped curve, which is to say, the initiation crack indexes have smaller values at the β of 30° or 60° whereas bigger values at the β of 0° or 90°.It can be seen that brittleness is the key factor to rock fracturing. At the β of 30° or 60°, the rock has larger brittleness index with high fracturing variations of brittleness behave as an inverted U-shaped curve. The study of evaluating shale rock fracturing capability and improving the fracturing effect has important practical significance and application value.