Xianbo Su

Coal seams in China are typically characterized by high coal seam gas content and low permeability, posing challenges for efficient coal seam gas extraction. However, achieving successful boreholes, especially in soft coal formations, remains a challenge. The mechanisms underlying permeability improvement in different coal structures need further exploration. Therefore, this paper is focused on the fundamental principles of permeability improvement in soft coal through hydraulic punching, and in hard coal via hydraulic jet fracturing. Firstly, borehole instability results from a dynamic interplay of four factors: in situ stress, coal structure, mechanical properties of coal with fluid, and drilling technology. While borehole instability is inevitable, enhancing drilling tools, drilling media, and drilling processes can mitigate risks associated with buried and stuck drill pipes by ensuring effective discharge of drill cuttings through critical flow velocity and displacement. Secondly, permeability improvement in soft coal through hydraulic punching aims at pressure relief and capacity increase, while in hard coal, hydraulic jet fracturing induces crack formation within the coal seam. Finally, this study illustrates the dynamics of the granular arch in soft coal after hydraulic punching, shedding light on the complex processes involved.

To enhance the efficiency and cost-effectiveness of coal seam gas extraction in coal mines, this study introduces an integrated approach combining drilling, hydraulic punching, and fracturing technologies. This method optimizes drilling media tailored to specific coal seams and fine-tunes technical parameters during drilling and permeability improvement processes. Furthermore, it clarifies the relationships among the coal breaking rate, discharge of drill cuttings, jet velocity, rotation speed, and displacement. This ensures precise and effective permeability improvement in both soft and hard coal formations. To implement this technology, a set of high-performance drilling tool with a large inner diameter, high pressure resistance, and superior sealing capabilities has been developed. Consisting of a water braid, drill pipe and hydraulic ejector, this tool meets the requirements of substantial displacement and high pump pressure. Field test results demonstrate remarkable enhancements in borehole formation, coal breaking and slagging efficiency. Specifically, the operation time is reduced by 60%–80%, the coal output is increased by 1–1.5 times, the pure coal seam gas extraction yield is increased by 1–2 times, and the extraction radius expands by more than 100%. These results highlight significant advancements in operational efficiency and permeability improvement effects, paving the way for efficient and economical coal seam gas extraction practices.

The sealing quality of the gas extraction holes determines the extracted gas concentration. Based on this, the paper reveals the basic principle of hole sealing by analyzing the gas leakage mechanism of the borehole. The hole sealing technology of solid-liquid materials with three pluggings and two injections for the gas extraction hole is proposed, and the hole sealing device and material are developed. Through testing the granularity distribution of the solid material, as well as the surface tension and contact angle of the slurry, the hole sealing material that can meet the requirements of accessible, sticky, and anti-deformation is selected. The sealing material enters microcracks and bonds coal rock more easily. First, the solid material is injected for hole sealing. Second, the liquid material can be injected repeatedly to maintain a high concentration for holes with poor sealing and gas concentration attenuation in the late stage of gas extraction. Field tests show that the gas concentration of solid material is 1.3 times that of the conventional material after 30 days of sealing. The liquid material injected after the concentration decline enables the gas extraction concentration to be recovered at 85%.

To characterize the coal measure shale pore structure of the Xiashihezi Formation in the Sunan Syncline block, a series of experiments were conducted. Results show that the main components of the shales are clay minerals and quartz, and organic matter, fluid escaping, interlayer, intercrystalline and interparticle pores are well-developed. The shale pore specific surface area (SSA) is concentrated in the pores with the size of <50 nm and is significantly higher in the pores with the size of < 5 nm, indicating that these pores have an important effect on gas adsorption. The shale pore volume is mainly concentrated near the pore size of 10 nm, followed by 10,000–100,000 nm, indicating that in addition to the fractures, the matrix pores with a size of about 10 nm contribute the most to pore volume and free gas accumulation. The volume and SSA of the micropore, small pore, and mesopore increase with the TOC content when it is lower than 6%, leading to higher porosity and total pore SSA of the shales. However, this positive correlation disappears under the compaction effect when the TOC content is higher than 6%. The clay mineral content has negative correlations with the volume and SSA of the mesopore and macropore under the compaction and pore blockage effects, but it correlates poorly with those of the micropore and small pore, causing a decrease in the shale porosity as it increases, while the brittle mineral can promote mesopore and macropore development and is conducive to micropore and small pore maintenance when its content is higher than 45%. As a result, the shale porosity tends to increase with the brittle mineral content. This study is helpful in clarifying the coal measure shale gas enrichment law in the block and in providing fundamental support for coal measure gas efficient development.