Proppant Transport And Embedment Mechanism In Coal Seam Hydraulic Fractures

The abundance of oil and gas resources in China is low,and the contradiction between supply and demand is prominent.Considering the sustainable development strategy,the development and utilization of coalbed methane are essential measures for the comprehensive utilization of resources and energy conservation and one of the reliable and vital alternatives to clean energy in the 21 st century.The permeability of coal seams in China is relatively low.Fractures are often formed through hydraulic fracturing while exploiting coalbed methane to improve the permeability of coal seams.However,the closure stress causes the fracture to close slowly,so it is necessary to add proppants to support the fracture and maintain its conductivity.Proppant transport and embedment are the main controlling factors of fracture conductivity.Therefore,in this thesis,the methods of physical experiment,theoretical analysis,numerical simulation,and on-site monitoring are used to reveal the proppant transport and embedment mechanism in coal seam fractures.The following innovative results have been achieved:(1)The basic law of proppant transport in horizontal fractures is along the gradient dispersion,reverse gradient or valley sedimentation.Proppant’s deposition area includes peak climbing,ring peak,and convection-diffusion deposition.The surface roughness of horizontal fracture is one of the critical factors affecting the proppant transport.The fracture entrance roughness determines the transport direction.The overall roughness of the fracture surface determines the proppant distribution.The transport rule in vertical fractures is a large amount of proppant in the parallel area of the open hole section and a small amount of proppant in the closed hole section.The stress concentration at the hole bottom forms the horizontal wing branch crack,and a small amount of proppant is deposited in the crack.Generally,the stress required for proppant transport in horizontal fractures is smaller than that in vertical fractures.(2)The stress required for proppant transport in fractures is high pressure and low injection<same pressure and same injection<low pressure and high injection.The greater the proppant concentration,the more proppant at the fracture entrance,and the closer the proppant transport distance.The smaller the particle size is,the easier proppant is to enter into micro-cracks and reach the crack tip.During multiple sandinjection,the proppant injected the previous time is embedded into the fracture surface under the closure stress.The proppant injected in the subsequent time needs greater stress for transport,and the proppant settlement at the fracture entrance is greater.The proppant transport characteristics in the corner area of the fracture network can be divided into the corner anomaly area,the buffer area,and the stable area.The influence of corner on proppant transport is within the range of four times the fracture width after turning.(3)A theoretical model of the single proppant embedded fracture is established,and the formation mechanism of the embedment pit and embedment cracks are revealed.The single proppant is embedded in the coal fracture surface,forming four areas: central compaction area,layered fracture area,fracture development area,and original state area.In the energy release of coal and proppant,the energy of proppant is released first,and the power of coal is released later.The surface failure forms of coal fracture include pressure failure(forming embedment pit),tensile shear failure(forming circumferential crack),and tensile failure(forming radial crack).When the contact radius is more than three times,the effect of the single proppant on the surface deformation of coal fractures can be ignored.(4)According to Hertz contact theory and based on the embedment characteristics of the single proppant,a theoretical model of the embedment depth of the proppant considering the effective influence height is proposed,and a surface deformation calculation model of multiple proppants is established based on the superposition principle.This model is helpful to capture the influence of uneven particle size and spatial distribution on proppant embedment.The theoretical model was verified by four experiments of monolayer and multi-layer proppants embedded under different closure stresses.The calculation results can well describe the variation trend of the embedded depth of monolayer and multi-layer proppants.Under the same closure stress,the more discrete the monolayer proppant,the larger the particle size range,and the greater the proppant embedment depth.When the multi-layer proppant is embedded,the larger the particle size range is,the larger the range of proppant embedment depth is,but the average embedment depth is the same.From the perspective of reducing proppant embedment at the project site,proppant with uniform particle size should be used to support fracture.If particle size uniformity cannot be guaranteed,more proppant shall be injected to form a multi-layer proppant placement form to reduce the impact of particle size non-uniformity.(5)The creep model of coal with fracture-proppant coal is established,which considers the strain hardening,elastic damage,and proppant compression viscosity damage of coal.The fracture mechanism of the coupling effect of bedding and proppant on coal bodies is clarified.The larger the angle between the fracture and bedding is,the smaller the coal’s elastic modulus with fracture-proppant coal is,and the smaller the initial fracture stress is.When the bedding surface is parallel to the fracture surface,the proppant insertion increases the fracture surface pore diameter,but no macro-cracks are formed.When the bedding aperture is large,the proppant will be embedded to reduce the bedding aperture.The proppant embedment promotes the fracture aperture when the bedding aperture is small.After the proppant is injected into the fracture parallel to the bedding,the larger fracture aperture and higher fracture conductivity will be maintained to the maximum extent.Through the orthogonal test,the influence of each parameter on the proppant-embedded fracture surface can be effectively analyzed.(6)Based on the theory of dual porosity medium,a theoretical model of CBM exploitation with different proppant distribution forms in hydraulic fractures considering proppant compaction and embedment is established,and on-site CBM exploitation results verify the model.The laying of proppant and the high permeability of the empty area between proppant columns make the extraction efficiency of CBM from high to low in the order of intermittent injection of proppant>continuous injection of proppant>no injection of proppant>no fracturing.It is suggested that continuousinjection of proppant should ensure that the placement rate is greater than 60%.When proppant is injected pulsed,the residual gas stress is distributed in a ladder shape due to the high conductivity of the proppant void.The length of the proppant column is 0.2m～0.3 m,the length of the empty area is 0.5 m,the placement rate of the proppant column area is 40%～50%,and the CBM extraction efficiency is the highest.(7)During proppant transport in coal seam hydraulic fractures in the project site,the sand ratio impacts water pressure more.A large sand ratio may lead to the plugging of fractures and the rapid increase of water pressure.Hydraulic fracturing forms horizontal fractures in coal seams.The proppant distribution in the fracture is elliptical.The maximum horizontal principal stress direction is the short axis of distribution,and the intermediate principal stress direction is the long axis of distribution.Fracture roughness greatly influences proppant migration in horizontal fractures,with less proppant at the high and proppant deposition at the low level.The smaller the proppant column spacing,the greater the fracture aperture between columns.In the multi-layer proppant laying fracture,there is a large area of continuous proppant embedment pit on the fracture surface.The damage degree of coal in the upper part of the fracture surface is less than that in the lower part of the fracture face.The coal body below the fracture surface forms a network of embedded fractures.At the proppant placement tip,there is a gradual transition from multi-layer proppant to single-layer proppant until it becomes a sand-free fracture zone.In the single layer proppant placement fracture,the proppant embedment depth is larger,the fracture aperture is smaller,or even completely closed.There are 147 figures,26 tables and 221 references in this study.