Abstract
Hydraulic flushing drilling technology can not only improve the efficiency of high-pressure and low-permeability geological reservoir coalbed methane drainage but also effectively reduce the probability of coal and gas outburst disasters through pressure relief. The main mechanism of this technology is to expand the borehole diameter through hydraulic flushing measures, increase the strain of the coal around the borehole, and increase the development of cracked pores, to improve the permeability of the coal seam and realize the dual reduction of ground and gas pressure. However, in the actual application process, the interaction mechanism among the stress field, the structure field, and the seepage field is still not clear, and there is no clear method to accurately determine the pressure relief range based on the pressure relief mechanism in order to carry out reasonable drilling arrangements. Therefore, this article comprehensively uses laboratory experiments, numerical simulations, and field practices to fully explain the hydraulic flushing pressure relief mechanism and proposes a method to accurately determine the pressure relief range based on the radial line strain law. The results based on radial line strain showed that the effective relief radius expands to 0.86 m once adopting the Φ579 mm hydraulic flushing borehole compared to Φ160 mm; the borehole’s equivalent diameter of drilling field #11 is 2 to 3 times than that of #10 and 1.2 times the average CBM extraction amount. Therefore, as the borehole diameter increases, the permeability and radial line strain of the coal around the borehole increase significantly, but the tendency of the increase in permeability decreases with increasing vertical stress. The findings of this study can help for a better understanding of the pressure relief and permeability enhancement mechanism of hydraulic flushing, and the method of determining the pressure relief range based on radial strain can also provide a new way for other mines to practice ideas.
Highlights
In recent years, with the increasing depth of coal seam mining in China, the physical and chemical properties and the mechanical environment of the coal seam have undergone major changes [1,2,3]
The true benefit of thermal stimulation lies in the enhanced rate of recovery, which results in improving the net present value (NPV) of the produced gas; [15] introduced cryogenic technology into the coalbed methane (CBM) fracturing and puts forward a new cryogenic volume fracturing technology, whose mechanism is alternative injecting water and cryogenic fluid by small rate to freeze the water in the fracture and cleat, temporary plugging fracture to divert, and forming complex fracture networks to increasing coal bed methane (CBM) production
On the basis of Lin et al, [23] demonstrated a novel enhanced coal bed methane recovery (ECBM) extraction technology, which involves the integration of hydraulic slotting (HS) and hydraulic fracturing (HF), and the efficiency of the method is evaluated by a field test
Summary
With the increasing depth of coal seam mining in China, the physical and chemical properties and the mechanical environment of the coal seam have undergone major changes [1,2,3]. In terms of promoting the development of the coal fracture structure, [8] proposed a 3D drainage technique based on the development of fissures during the mining process and the flow of depressured gas to improve the permeability of soft coal seams, thereby improving the efficiency of CBM extraction. In terms of hydraulic antireflection technology, hydraulic fracturing technology is a traditional approach to significantly improve the CBM reservoir permeability and CBM extraction rate by injecting high-pressure water into the coal body to promote the fissure development, increase the strain in the coal seam during the mining process, and further make the coal body looser [19,20,21]. There is a lack of numerical simulation methods that can be directly used to determine the field parameters in the actual application process
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