Key Takeaways & Executive Findings
- •• A novel reinforcement fluid that undergoes liquid-gas phase transition in vacuum is proposed for lunar borehole stabilization. • The developed reinforcement liquid increases the cohesion of simulated lunar soil from 2 to 800 kPa, significantly enhancing borehole stability. • High-vacuum (5 Pa) and low-temperature (−30 to 50 °C) coring experiments confirm spontaneous release and effective borehole reinforcement. • Optimal solute concentration for borehole reinforcement is between 0.15 and 0.25 g/mL.
Abstract
Lunar core samples are the key materials for accurately assessing and developing lunar resources. However, the difficulty of maintaining borehole stability in the lunar coring process limits the depth of lunar coring. Here, a strategy of using a reinforcement fluid that undergoes a phase transition spontaneously in a vacuum environment to reinforce the borehole is proposed. Based on this strategy, a reinforcement liquid suitable for a wide temperature range and a high vacuum environment was developed. A feasibility study on reinforcing the borehole with the reinforcement liquid was carried out, and it is found that the cohesion of the simulated lunar soil can be increased from 2 to 800 kPa after using the reinforcement liquid. Further, a series of coring experiments are conducted using a self-developed high vacuum (vacuum degree of 5 Pa) and low-temperature (between −30 and 50 ℃) simulation platform. It is confirmed that the high-boiling-point reinforcement liquid pre-placed in the drill pipe can be released spontaneously during the drilling process and finally complete the reinforcement of the borehole. The reinforcement effect of the borehole is better when the solute concentration is between 0.15 and 0.25 g/mL.
1. Introduction
Since the 1960s, humans have embarked on the journey of exploring the moon. The geological core of the moon is considered a key material for accurately evaluating lunar mineral resources and inferring historical evolution. To date, lunar exploration projects such as the Apollo series of the United States, the Luna series of the Soviet Union, and the Chang’e series of China have successfully retrieved some core samples. Through artificial stratum drilling, Apollo 17 obtained the deepest core to date, with a depth of 3.05 m [1]. Luna 24 acquired a core sample with a maximum depth of 2.25 m via mechanical automatic drilling [2]. With the help of mechanical automatic salvage and drilling, Chang’e-5 obtained core samples at a depth of 1 m [3]. These core samples provided us with a wealth of information on the shallow lunar strata. Overall, the depth of the obtained rock cores is relatively shallow and is mainly concentrated in the lunar soil layers.
With the development of lunar exploration technology and the continuous pursuit of drilling depth by various countries, more challenges have been posed to lunar core drilling technology. Difficulties in borehole stability and drill bit heat dissipation may affect the drilling depth [4]. A thick soil layer covers the lunar bedrock, with an average thickness of more than 4 m in the lunar sea region and approximately 12 m in the highland region [5]. Thus, the drill bit inevitably needs to pass through the soil layer to obtain the bedrock core. According to the relevant theories of soil mechanics, the lunar soil is in a mechanical equilibrium state before drilling. However, after the lunar soil in the borehole is removed and the hollow drill bit passes through the lunar soil, a free surface appears around the borehole. Despite the relatively large internal friction angle of the lunar soil and the relatively high self-stabilizing height of the borehole, the risk of borehole collapse remains a significant concern.
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Dingqiang Mo, Tao Liu, Zhiyu Zhao, Liangyu Zhu, Dongsheng Yang, Yifan Wu, Cheng Lan, Wenchuan Jiang, Heping Xie (2025). Borehole reinforcement based on polymer materials induced by liquid-gas phase transition in simulating lunar coring. SinoTechIntel Verified Research. https://doi.org/10.1016/j.ijmst.2025.02.001
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Frequently Asked Questions
What is the main challenge in lunar coring addressed by this research?
The main challenge is maintaining borehole stability during lunar coring, which limits the depth of core sampling. The research proposes a reinforcement fluid that undergoes phase transition in vacuum to stabilize the borehole.
How does the reinforcement fluid work?
The reinforcement fluid is a high-boiling-point liquid that spontaneously undergoes liquid-gas phase transition in a vacuum environment. It is pre-placed in the drill pipe and released during drilling, penetrating the lunar soil and increasing its cohesion, thereby reinforcing the borehole.
What were the key experimental conditions in this study?
The experiments were conducted using a self-developed high-vacuum (5 Pa) and low-temperature (between −30 and 50 °C) simulation platform to mimic lunar conditions.
What is the optimal concentration of the reinforcement liquid?
The optimal solute concentration for borehole reinforcement is between 0.15 and 0.25 g/mL, as this range provides the best reinforcement effect.
How much did the cohesion of simulated lunar soil increase?
The cohesion of the simulated lunar soil increased from 2 kPa to 800 kPa after using the reinforcement liquid, demonstrating a significant improvement in soil stability.
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