Instability-negative pressure loss model of gas drainage borehole and prevention technique: A case study

Abstract

The internal collapse of deep seam drainage borehole and negative pressure loss represents a serious technical problem affecting gas drainage. To address this problem a creep model of coal around borehole was established based on the plastic softening characteristics of coal. The final collapse time of the borehole was determined and used to derive the three stages of the borehole collapse process. The model of negative pressure loss in drainage borehole was established according to the theory of fluid dynamics, the model of methane gas flow and the creep model of the coal around the borehole. The relationship between the negative pressure loss of drainage and the change of borehole aperture was derived, thereby revealing the main influencing factors of the negative pressure loss in the borehole. A drainage technique named “Full-hole deep screen mesh pipe” was introduced and tested to prevent the collapse of borehole and reduce the negative pressure loss. The result shows that after the borehole was drilled, the borehole wall was affected by the complex stress of the deep coal seam, the coal surrounding the borehole collapsed or presented the characteristics of creep extrusion towards the borehole. The “Full-hole deep screen mesh pipe drainage technology” could effectively control the collapse as well as the deformation of the borehole and reduced the negative pressure loss. Compared with the traditional drainage technology, the methane gas drainage concentration was increased by 101% and the gas flow was increased by 97% when the methane gas was drained for 90 days, the gas drainage efficiency increased significantly.

Experimental investigation of the creep damage evolution of coal rock around gas extraction boreholes at different water contents

“…Under the action of long-term axial loading, the porous specimens were subjected to internal sprouting, expansion, evolution of microfractures, and finally formation of penetration cracks leading to damage. Based on the results of the graded loading creep experiments on porous specimens in this paper, it is found that there is a relationship between creep damage and creep strain in coal rock materials [ 29 , 30 ]. According to the definition of the damage variables, the creep strain at each stage of the creep process is taken as the damage variable of the specimen, and assuming that the damage variable is 1 when the specimen is damaged and the coal rock is in a non-damaged state when the water content is 0%, the creep strain damage variable of the coal rock material is written as [ 31 , 32 ].…”

Controlled reconstruction and application research of lateral overlying strata structure in high-gas mines with thick-hard roofs for a fully-mechanized top-caving face

“…He JF [ 45 ] conducted research on large-diameter boreholes and demonstrated their effectiveness in gas extraction from deep soft coal. Qi QJ [ 46 ] addressed the issues of borehole collapse and negative pressure loss in deep gas drilling by proposing a technique that used a full-hole deep-screen wire mesh for extraction.…”

Study on Instability Characteristics of the Directional Borehole on the Coal-Seam Roof: A Case Study of the Tingnan Coal Mine

“…Di et al [35] proposed the regional solidification pore formation method for soft coal seams, which can solidify the strength of the rock surrounding the borehole and improve the pore formation rate. Qi et al [36] tested the full-hole deep screen mesh tube drainage technology to solve the problem of internal collapse and negative pressure loss of deep coal seam drainage boreholes, which can effectively control the collapse and deformation of boreholes and reduce the negative pressure loss. Compared with conventional drainage, after 90 days, the gas drainage concentration increased by 101% and the gas flow increased by 97%, so the gas drainage rate increased significantly.…”