The Damage Research Of Granite With Thermal Elastic Brittle Quality And The Analysis Of Borehole Wall Stability

Stratum temperature is an important factor affecting the stability of the wall inhigh temperature geological drilling well. Formation rock internal structure willchange under the action of temperature. It also affects the strength. Base on deepgeological drilling in granite formation. This paper discusses the thermal elastic andbrittle damage failure mechanism and characteristics of borehole wall instability. Theresearch work include following areas:According to failure mechanism of granite at different temperature. to get thegranite model of thermal elastic and brittle damage. The damage defined with thestatistical is introduced. Which is used to describe the changing trend of isotropy rockstructure at both temperature effect and pressure effect. And then contrasting thedamage defined with modulus of elasticity to verify it.In order to discuss the heat damage granite characteristics under the influence oftemperature, the granite is heated to different temperatures and cooling, repeatedcycles, and then tested the rock mechanics, the regularity test results about parametersof the rock mechanical properties with temperature changes are obtained, to providethe parameter of material for the thermal damage theory model. Results show that thethermal expansion coefficient is different at350℃to50℃, it is an important factorthat cannot be ignored in the mechanical analysis. Elastic modulus, poisson’s ratiowith the temperature change regularly, obvious effects on the thermal damage of therock.Based on the established thermal elastic and brittle damage model and theexperimental results, this paper established the effective stress expression around theborehole.which the influence of temperature and pressure are comprehensivelyconsidered. The wellbore instability formulas about collapse pressure due tocompression damage and burst pressure due to tensile failure are derived. Finally getthe safe drilling fluid density are increased with the borehole wall temperatureincreasing. It reveals that the rock thermal expansion coefficient of borehole wallstability is bad, the greater the damage variables significantly influence on stress of shaft wall fracture.The result provides a theoretical basis which can accuratelydetermine the high temperature geological drilling wellbore stability.