Study on hydraulic cracking mechanism and propagation characteristics of ice hole based on Peridynamics

Abstract

Drilling of polar deep ice cores is an important means to study the evolution of the earth's climate. When drilling into complex formations such as brittle ice layer, warm ice layer, basement meltwater layer, or ice-rock interlayer, drilling problems frequently occur, especially the hydraulic cracking of ice holes. This paper studies the dynamic propagation process and the influence mechanism by theory and simulation to solve the problem of cracks initiation and propagation around the ice hole. Based on continuum mechanics, a stress model is established to obtain the stress state around the borehole well. In local continuum mechanics, the stress state is represented by spatial partial differential equations, and its spatial partial derivatives cannot describe the discontinuous situation. Peridynamics (PD) use integral equations to solve this problem effectively. The PD model of the ice hole is established to realize the unified description of the continuous and discontinuous space around the ice hole. The dynamic cracks propagation process, the influence mechanism of hydraulic pressure and fracture toughness are analyzed. The loading rate of ice in the paper and experiment is 0.008 mm/s, the elastic modulus is 9 GPa, and the basic fracture toughness is 175 kPa·m0.5. Case study shows that hydraulic pressure promotes cracks propagation, the hydraulic pressure increases by 1 MPa, the volume ratio of cracks increases from 7.67% to 14.93%. The volume ratio of cracks is defined: the ratio of the cracks volume to the total model volume. Fracture toughness hinders cracks propagation, which increases from 150 kPa·m0.5 to 175 kPa·m0.5, the proportion of cracks volume decreases from 13.85% to 7.67%. When drilling into the formation with prominent ductile-brittle transition, the mud density shall be adjusted in time to control the hydraulic pressure in the hole to reduce cracks propagation and ensure the safety and efficiency of ice drilling.