THE MECHANISM OF LOCAL HIGH-TEMPERATURE INDUCED CRACKING OF HETEROGENEOUS GRANITES

The slow drilling speed and high drilling cost in deep strata are the bottleneck problems encountered in drilling speed improvement at present stage. Traditional mechanical rock breaking is difficult to greatly improve the speed, so a new efficient rock breaking method is in urgent need. To this end, this paper analyzes local high temperature under the action of granite crushing mode through indoor test, and then establishes the local high temperature induced crack granite thermal-mechanical coupling model based on the discrete particle flow yuan PFC2D heterogeneity equivalent of granite rock mass model and considering the formation pressure, fluid column pressure effect on local high-temperature crack induced granite. The micro and macro fracture processes of heterogeneous granite are reproduced, and the relationship between temperature, pressure, and rock micro damage is summarized. The experimental results show that under the premise of non-producing molten granite, the effect of crushing granite at high temperature is very significant, and the broken debris is flaky, which is conducive to cuttings removal. There are mainly two types of cracks in granite under local high temperature, intragranular shear crack and intergranular tensile crack. The liquid column pressure inhibits the crack induced by high temperature, and the lateral pressure promotes the crack propagation. The obvious damage of granite caused by local high temperature only occurs in shallow strata and has little effect on deep strata, so it is recommended to combine with PDC bit for cutting or impact. When the granite is heated at different locations, the intragranular shear cracks only exist at the center of heating, while the intergranular tensile cracks expand in a wider range, and the crack density and crack distribution are related to grain properties. The research results are helpful to understand the mechanism of high-temperature thermal fracturing rock and provide a new idea for deep drilling speed.