Study On The Mechanism Of Homogeneous Permeability Enhanced By Tree-Type Borehole In Coal Seam

Enhancing the permeability of coal reservoirs over large ranges is the crucial to realize high-efficient exploitation of coalbed methane.Conventional technologies such as dense boreholes or hydraulic slotting cannot enhance coal seam permeability uniformly.The extension of the enhanced-permeability zone produced by a borehole or slot is limited and the borehole drainage efficiency is low.Hydraulic fracturing commonly leaves unstimulated zones in the seams because the hydraulically generated fractures mainly propagate along the direction of maximum principal stress.Methane in the unstimulated zones will cause a relative high risk of methane-related hazards during mining.How to form a large-scale and uniform fracture network in coal seam is the key to achieving comprehensive and high-efficient developemt of coalbed methane.To address above problems,a novel tree-type borehole method to homogenously enhance coal seam permeability and increase the methane drainage capacity of boreholes has been developed.Taking full advantage of the capabilities of water jet drilling,this technique carves long radial sub-boreholes that extend outward from an existed drainage borehole,thereby relieving stress and increasing its drainage length.Studing the evolution of fracture field and the law of methane flow in coal seam stimulated by tree-type borehole method,as well as the methane drainage capacity of tree-type borehole to reveal the mechanism of homogeneous permeability enhanced by tree-type borehole in coal seam is of great significance to the development of related technologies and the design of construction parameter.Therefore,this paper has carried out researches on the evolution mechanism and distribution of fracture field in a coal seam after the excavation of sub-boreholes from a main borehole.The multi-scale model under thermo-hydro-mechanical coupling conditions for methane drainage from coal seam is established and used to analyze the characteristics of methane flow during the process of tree-type borehole drainage.The lab experiments for the simulation of tree-type borehole drainage are conducted to investigate the relationships between different tree-type borehole properties and methane drainage performance.The main innovative achievements are as follows:(1)The evolution mechanism of fracture field induced by sub-boreholes in coal seam was revealed.The basic principle of tree-type borehole to relieve stress,induce fracturing,and thereby enhance the permeability in coal reservoirs was clarified.Based on the quasi-continuum theory,the effect of the geometry of tree-type borehole,crustal stress and key physical parameters of coal on the evolution and distribution of fracture field were investigated numerically.It was found that the homogeneity coefficient and the number of induced fracture were obviously affected by sub-borehole length,the number of sub-boreholes,crustal stress,tensile strength of coal and frictional angle of coal.Meanwhile,the formulas for calculating for the area fractured,homogeneity coefficient,the density and the number of induced fracture was obtained through these parameters.(2)A coupled multi-scale model for methane drainage from coal seam was developed to reveal the characteristics of methane flow in dual-porosity coal seams considering the effect of sub-boreholes and induced fractures.After stimulating the coal seam through the excavation of multiple sub-boreholes,the area of meathane flow was affected by the sub-boreholes and fracture field.When the influence of fracture field was neglected in the simulation analysis,the drainage area will be underestimated.The influence law of numerous factors such as induced fracture aperture and coal seam gas pressure on the tree-type borehole drainage range was obtained and the sensitivity of methane fow range to Langmiur volume constant,the initial coal seam permeability and other parameters in different drainage stages was quantitatively analyzed.(3)A true triaxial thermo-fluid-solid coupling test system was developed and used to reveal the the effect mechanism of sub-borehole layout on drainage capacity of tree-type borehole.During the tree-type borehole draiange expreiments,the evolution law of methane pressure and flow at different stages were analyzed and the relationship between the increased production coefficient ζ and the length and number of sub-boreholes were clarified.The longer the total length of sub-boreholes in a tree-type borehole,the greater the increased production coefficientζ.When the total length of sub-boreholes in a tree-type borehole was constant,the arrangement of a large number of long sub-boreholes in a tree-type borehole can achieve better borehole stimulation and methane drainage effect compared with a smaller number of short sub-boreholes.(4)A field trial was performed in a coal mine in Henan province to compare the permeability enhancement effects of tree-type borehole method and dense boreholes technology.The field test results showed that a gas pressure reduction area where the gas pressure was decreased by 75% could be formed around the tree-type borehole,which was much larger than the drainage influence range of a conventional borehole.Compared with dense boreholes drainage mode,the gas production rate from a single tree-type borehole was 3.4～5.9 times faster.In addition,after 35 days of extraction,the total volume of gas production produced from 238 tree-type boreholes in a 200 m section of roadway was 1.4 times the methane extracted by 938 conventional boreholes in an adjoining 200 m section of the same roadway.The field test results suggested the reliability and effectiveness of tree-type borehole method to enhance the coal seam permeability homogeneously.