How to deal with high-hardness rock formations during horizontal directional drilling?

Today, Drillto Trenchless Co., Ltd., a horizontal directional drilling manufacturer, will introduce you to horizontal directional drilling. High-hardness rock formations are a common challenge in horizontal directional drilling operations. Their compressive strength can reach hundreds to thousands of kilograms per square centimeter. Traditional drilling methods are prone to inefficiency and severe drill tool wear. To address this challenge, a systematic solution must be developed, combining engineering geological characteristics with equipment performance, encompassing drill tool selection, process optimization, and auxiliary measures.

1. Drill Tool Selection and Combination Innovation

For high-hardness rock formations such as strongly and moderately weathered granite, a composite drilling system is required. For example, in a river crossing project, where the saturated uniaxial strength of larger rocks reached 125 MPa, the construction company employed a combination of a pneumatic down-the-hole hammer and a super-cone reamer. The pneumatic down-the-hole hammer uses compressed air to drive a piston-driven high-frequency impact drill bit, crushing the rock. Combined with the cutting power of the cone reamer, this allows for efficient drilling in hard rock formations. In addition, the drill pipe must be made of high-strength alloy and equipped with front and rear stabilizers to reduce torque fluctuations during long-distance drilling and prevent drill pipe swing from causing hole collapse.

II. Dynamic Adjustment of Construction Processes

Drilling in hard rock formations requires strict control of drilling parameters. During the pilot hole phase, PII direction control software and a ground beacon system are used to accurately position the magnetic field to ensure the drilling trajectory conforms to the designed curve. During reaming operations, step-by-step hole cleaning (for example, from 300mm to 1200mm) is used to reduce the resistance of each reaming operation, and larger-diameter rock reamers are used to adapt to formation variations. For example, when encountering alternating soft and hard formations, one project effectively avoided mud loss and stuck pipe by adjusting the mud pressure (maintained at 1.5-2.0MPa) and drilling speed (0.5-1.0m/min). Furthermore, before pulling back the pipeline, a squeeze expander is used to test the hole depth, and a trial pullback verifies formation stability to ensure the success rate of the pullback. III. Auxiliary Measures to Strengthen Support
Geological Improvement Technology: For fractured rock formations with developed fractures, high-pressure jet grouting is used to reinforce the structure, creating a "casing-in-pipe" structure and improving borehole wall stability. In one project, this method reduced the hole collapse rate by 80% when the bottom of the casing encountered strongly weathered granite at the exit point and pulverized it with water.
Emergency Response Mechanism: Backup drilling rigs and salvage tools (such as salvage cones) are available to handle emergencies such as drill pipe breakage. For example, after a drill pipe breakage occurred on one project, extensive excavation at the entry point combined with salvage cones enabled the broken pipe to be recovered in just six hours, avoiding project delays.
Slurry Performance Optimization: Specialized bentonite slurry is formulated based on the characteristics of the formation, and polymers are added to enhance wall protection. During construction in gravel layers, increasing the slurry viscosity (to 60-70s) and solids content (to 15%) effectively reduced the risk of hole wall collapse.