Investigation On Fracture Network Extension And Permeability Enhancement In The Thin And Composite Coal Seam

As the composite thin coal seam is extracted for gas,the coal-gangue interbedding leads to low gas-extraction efficiency.Hydraulic fracturing is widely utilized to improve the gas-extraction efficiency.Specifically,hydraulic fracturing activates the primary joint fissures of the coal seam while generating hydraulic fractures.Thus a complex fracture network is formed to improve the permeability of the coal seam.However,characterizing HF propagation and permeability evolution of in the composite thin coal seam remains challenging that need further research.In this work,a systematic research is carried out on the law of propagation and permeability evolution in the composite thin coal seam fracture network.This study mainly includes four aspects: the formation mechanism and the network of hydraulic fracture propagation law in the composite coal seam,hydraulic fracture propagation characteristics of the composite coal seam under the perturbation of level effect,the law of permeability enhancement with HF extension and field applications using hydraulic fracturing to improve the permeability in the composite thin coal seam.For the coal seam with developed cleats,micron CT is utilized to observe the evolution of macroscopic fracture of coal samples before and after fracturing.The formation mechanism of fracture network in coal body is revealed.Due to the gradient decreasing of water pressure in the fracture caused by filtration loss and flow resistance,the main hydraulic crack opens non-uniformly which is similar to a flat ellipsoid.Then,the local stress field around the fracture is disturbed and deflected by the gradient pore pressure,inducing a new crack to initiate to fracture and propagate from the original fracture surface that meets breaking conditions to form a branch hydraulic fracture.Afterwards,the primary laminae and cleat surface disturbed by the matrix stress opens by the lateral pore pressure distributed along the gradient of the crack length,causing the hydraulic fracture to propagate.Initial fissures form stress-concentrated rock bridges with hydraulic fractures under the matrix stress of the coal body,inducing local deflection of hydraulic fractures.Then,a meso-micro hydraulic fracture network in the coal body is formed with main hydraulic fractures,branch fractures,open laminae and cleats,and local hydraulic fractures penetrating with the initial fissures.The physical experiments of hydraulic fracturing in coal were conducted and the propagation law of macroscopic networks of hydraulic fracture was proposed.There are micro fracture networks near the macro main hydraulic fracture,which make the macro hydraulic fracture more complex,increasing the permeability of coal seam by fracturing.Under experimental conditions of relatively small injection ratio,the acquired energy of AE signal is weak,the water pressure fluctuates gently,and most of the fractures are made up of main hydraulic fracture rather than secondary fracture.However,under experimental conditions of relatively large injection ratio,the acquired energy of AE signal is strong,the water pressure decline sharply,there are more secondary factures in addition to main hydraulic fracture,making the hydraulic fracture more complex.When the kinetic energy caused by the increase of fracture propagating rate is larger than that of fracture energy,the branching fractures are easier to generate and the fractures is more complex.The fracture energy of cleats and natural cracks is more less than that of coal matrix,thus the natural joints is easier to become the dominant rupture surface promoting the formation of a more complex hydraulic fracture networks.When the borehole for fracturing is perpendicular to the interface of cleats,the propagation of main hydraulic fracture is mainly influenced by networks of cleats leading to the main rupture surface parallel to the interface of cleats;meanwhile,when the confining pressure is relatively small,the priority of factors influencing rupture in the growth of hydraulic fractures is: face cleats > confining pressure > butt cleats bedding planes.Given that it is difficult to achieve multi-layer penetration of composite thin coal seam and the fracturing zone is limited,several groups of simulation tests with different discharge volumes and different crack initiation points of composite coal seam were carried out to explore the best application conditions for forming fracture network.The study shows that the hydraulic fracturing of the composite coal seam is divided into three main stages: water injection and pressure boosting,crack initiation and fracture propagation,stopping water injection and pressure release.The fracture water pressure exhibits a tendency to increase as the discharge volume increases during the fracturing process.Affected by the cleats and bedding of the composite coal seam,the hydraulic fracture will deflect,branch and propagate along the weak surface during the expansion.Additionally,the connection between hydraulic fractures and surface creates a complex network of cracks.In the comparison of hydraulic fracturing of composite coal seam with different displacements and crack initiation points,the fracturing effect of coal seam fracturing under high displacement is better.Aiming at the question of permeability increase by hydraulic fracturing,the numerical modelling for the evolution law investigation of permeability in rock during the initiation and propagation by a classical fluid-structure coupling model which considering the influence of damage and stress change on the permeability,and a self-developed program is used to characterize the evolution feature of pore pressure filed of seepage in composite thin coal seam considering the fracture propagation and heterogeneity of permeability.The propagation of hydraulic fracture network in composite thin coal seam includes the extension of micro fracture zone which continues growing under the driving of water pressure from injection resulting in the crack of coal seam the gauge layer.The zone with high permeability distributed around the hydraulic fracture zone,whose area is larger than that of zone treated by hydraulic fracturing(hydraulic micro fracture zone),and the volume increase of hydraulic micro fracture zone is helpful to increase the permeability of composite coal seam.As the continuous of water injection,the permeability significantly increase,and the permeability can be increased by 1-2 orders of magnitude after hydraulic fracturing.As the increase of half-length of hydraulic fracture,the influenced zone of pressure decline significantly increase,namely,the coverage of methane drainage significantly increase.In the numerical example,the average pressure in borehole without fracturing treatment is 106.3 k Pa,and change to 19.2k Pa with the fracture half-length of 25 meters,whose range of discount is 81.9 percent.For effectively decrease of drainage blind area to increase the productivity,the borehole layout spacing needs to be decreased toward the zone with relatively low permeability for the coal mass with great heterogeneity(the heterogeneity coefficient is greater than 5 in this example).The field experiments of hydraulic fracturing in composite thin coal seam with four layers of coal and three layers of gangue for increase of permeability conducted.It is found that the high injection rate is helpful to generate dense networks of hydraulic fracture and the conventional hydraulic fracturing with high pressure is helpful to further extend the complex hydraulic fracture networks.The combination of high injection rate fracturing with conventional hydraulic fracturing with high pressure is able to increase the volume covered by hydraulic fractures and permeability of coal seam,which significantly improve the drainage efficiency of coal bed methane and promote the safety and productivity of coal mining face.This thesis includes 85 figures,11 tables and 127 references.