📄 earth_science

Fracture development mechanism of borehole walls in deep drilling

This study integrates numerical simulations, theoretical analysis, and laboratory experiments to reveal that thermal damage, horizontal in-situ stress, and drilling fluid pressure collectively govern the evolution and morphology of borehole wall fractures in deep granite drilling, providing a theoretical basis for enhancing wellbore stability and optimizing drilling fluid technologies.

Original authors: Lichang Wang, Qinfu Qian, Longjun Dong, Wei Long, Hanyang Zhu, Lu Wang, Wenxiang Peng, Jian Zhou

Published 2026-08-04

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CC BY 4.0

Original authors: Lichang Wang, Qinfu Qian, Longjun Dong, Wei Long, Hanyang Zhu, Lu Wang, Wenxiang Peng, Jian Zhou

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Earth as a giant, multi-layered cake, but instead of delicious frosting, the layers are made of incredibly hard rock. Deep down, where the heat is intense and the weight of the mountains above is crushing, humans are trying to drill tiny holes to find water, energy, or minerals. But here's the catch: rock isn't just a solid block; it's a mosaic of different minerals glued together, and when you drill into it, you change the temperature and pressure around the hole. If the rock gets too hot, too stressed, or the pressure from the drilling fluid (a special liquid used to cool the drill and carry away rock dust) gets out of balance, the walls of the hole can crack, crumble, or collapse. This is a huge problem because if the hole collapses, the drill gets stuck, the project fails, and we lose access to the resources hidden deep below. Scientists have been trying to figure out exactly how and why these cracks start and spread in these extreme conditions, because knowing the "recipe" for a collapse helps engineers build safer, deeper holes.

This paper is like a high-tech detective story where the authors use a super-powerful computer simulation to watch what happens inside a tiny piece of granite rock when it gets drilled. They didn't just look at the rock from the outside; they built a virtual model made of millions of tiny digital "particles" representing different minerals like quartz and feldspar. They then subjected this digital rock to the triple threat of deep drilling: extreme heat, massive squeezing pressure from the surrounding earth, and the push-and-pull of the drilling fluid. By watching how these digital particles interact, they discovered the secret life of cracks.

First, they found that heat is a troublemaker. Granite is made of different minerals that expand at different speeds when they get hot, kind of like how a metal bridge expands more than the concrete pillars holding it up on a hot day. In their simulation, the quartz particles (which expand a lot) and the plagioclase particles (which also expand quite a bit) started pushing against each other. This mismatch created tiny cracks right at the boundaries where they touched. The hotter it got, the more these cracks multiplied and raced through the rock, especially once the temperature passed 300 °C and hit a major phase change for quartz around 573 °C. It's as if the rock was holding its breath until the heat made it snap.

Next, they looked at how the rock reacts to being squeezed. The Earth pushes on the rock from all sides, but not equally. The simulation showed that when the rock is squeezed, the cracks love to form in the direction where the squeezing is the weakest. If the rock is being squeezed hard from the left and right, but less from the top and bottom, the cracks will run vertically, following the path of least resistance. This confirmed that the direction of the cracks is a direct map of the invisible forces pressing on the hole.

Finally, they tested the drilling fluid, which acts like a hydraulic brace holding the hole open. They found that the fluid's pressure is the boss of the crack's shape. When the fluid pressure was too low (weaker than the rock's internal squeeze), the cracks formed in a circle around the hole, creating a "V" shape that looked like a bite taken out of the wall. But when they cranked up the fluid pressure to be higher than the rock's squeeze, the cracks stopped circling and shot straight out from the center, like spokes on a wheel. The paper suggests that finding the perfect balance for this fluid pressure is key to keeping the hole stable.

The authors are careful to note that these findings come from their computer simulations and lab tests on specific granite samples, so while the patterns are very clear in their model, the real world might have extra surprises like pre-existing cracks or different rock types. However, by understanding these rules of heat, stress, and fluid pressure, engineers can better predict where a borehole might fail and design their drilling fluids to keep the walls standing tall, even in the deepest, hottest parts of the Earth.

Technical Summary: Fracture Development Mechanism of Borehole Walls in Deep Drilling

Problem Statement
Deep drilling is critical for accessing deep Earth resources, yet its feasibility is frequently compromised by borehole wall instability, including collapse, lost circulation, and borehole enlargement. These instabilities are closely linked to the rapid development and heterogeneous evolution of fractures under complex deep-earth conditions characterized by high temperatures, large in-situ stresses, and significant pore water pressure. While previous studies have addressed rock mechanics and borehole stability through theoretical analysis, experiments, and numerical simulations, a systematic understanding of the Thermo-Hydro-Mechanical (THM) coupling mechanisms in deep granite environments remains insufficient. Specifically, there is a need to elucidate the meso-mechanisms of fracture initiation and evolution under the combined effects of thermal damage, in-situ stress states, and drilling fluid column pressure.

Methodology
This study integrates laboratory experiments, the Discrete Element Method (DEM), elastic theory, and THM coupling theory to investigate fracture development in granite.

Model Calibration: Granite samples from Zhangzhou, Fujian, were analyzed for mineral composition (33% plagioclase, 25% quartz, 15% K-feldspar, 27% biotite). A DEM model was constructed using Particle Flow Code (PFC) with distinct particle types representing these minerals. The model was calibrated against uniaxial compression and Brazilian splitting tests, achieving deviations of less than 5% in compressive strength, elastic modulus, and tensile strength.
THM Coupling Framework: A 2D plane-strain DEM model was established incorporating:
- Thermal Effects: Mineral-specific thermal expansion coefficients were assigned to simulate thermal damage. Temperatures ranging from 25°C to 600°C were applied to simulate non-uniform heating.
- Mechanical Effects: Horizontal in-situ stresses ( $\sigma_H$ , $\sigma_h$ ) and vertical stress ( $\sigma_v$ ) were applied under three distinct stress regimes.
- Hydraulic Effects: A fluid grid was embedded to simulate drilling fluid seepage and column pressure ( $P_w$ ), allowing for the calculation of pressure changes based on domain volume variations.
Validation: The DEM results were cross-validated against analytical solutions derived from Kirsch equations and Hoek-Brown strength criteria, as well as against existing experimental observations of borehole breakouts and thermal fracturing.

Key Contributions and Results
The study establishes a visualized, traceable, and quantitative simulation framework for borehole wall fracture development, yielding the following specific findings:

Mechanism of Thermal Damage: Thermal damage-induced fractures primarily originate from the differential thermal expansion between mineral grains, specifically at the boundaries of quartz and plagioclase, which possess higher thermal expansion coefficients. Elevated temperatures significantly accelerate fracture propagation. The distribution of these fractures is heterogeneous and random, coinciding with zones of concentrated force chains. The model successfully reproduced the "X"-shaped fracture patterns and the transition from tensile to shear failure observed in high-resolution CT scans of thermally treated granite.
Influence of In-Situ Stress: Horizontal in-situ stress predominantly governs the secondary development of shear fractures. Fractures are most densely distributed in the direction of the minimum horizontal principal stress ( $\sigma_h$ ). As the difference between maximum and minimum horizontal stresses increases, the borehole undergoes compression along the maximum stress direction and tension along the minimum stress direction, leading to fracture concentration in the $\sigma_h$ direction. This aligns with field observations of borehole breakouts.
Role of Drilling Fluid Column Pressure: The drilling fluid column pressure ( $P_w$ $P_{w}$ ) markedly influences fracture morphology and stress redistribution:
- $P_w < \sigma_{in-situ}$ : The borehole wall is primarily compressed, leading to the development of circumferential fractures with a "V"-shaped profile. These fractures initiate as circumferential cracks that interconnect, causing particle detachment.
- $P_w > \sigma_{in-situ}$ : The stress state shifts, favoring the formation of radial fractures that extend outward from the borehole wall.
- $P_w \approx \sigma_{in-situ}$ : Fracture density reaches a minimum, indicating a state of relative stability.
Model Validation: Comparative analysis between the DEM simulations and elastic theory analytical solutions showed general consistency in stress distribution patterns, particularly in the circumferential and radial directions, though the DEM model exhibited slightly more conservative stress predictions under high fluid pressures.

Significance
This research provides a theoretical basis for evaluating borehole wall stability in deep crystalline formations. By clarifying the meso-mechanisms of fracture evolution under THM coupling, the study offers insights for optimizing drilling fluid loss prevention and plugging technologies. The developed framework allows for the quantitative characterization of fracture development under multi-field coupling conditions, aiding in the prediction and mitigation of instability risks during deep drilling operations. The authors note that while the model is robust, future work should address limitations regarding 3D fracture network complexity and the integration of pre-existing fracture systems.

Technical Summary: Fracture Development Mechanism of Borehole Walls in Deep Drilling

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