Theory and technology of enhanced coal seam gas production by integrated drilling, punching and fracturing in coal mine: Part 1 theory

Abstract 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.