Study On The Mechanism Of Hyraulic Fracture Network And Propagation In Coalbed Methane Reservoir

Hydraulic fracturing is an effective way to enhance coalbed methane well production and increase coalbed methane recovery.The fracturing crack propagation,especially the mechanism of map cracking,in coalbed during hydraulic fracturing process still are not well known,which restricts the efficient implementation of hydraulic fracturing technology for coalbed methane(CBM)exploitation.Therefore,fully understanding the hydraulic fracture propagation of coalbed,revealing the map cracking mechanism have great significance to speed up the CBM exploration and development of China.Taken Jiaozuo coal mine as the study target for the study of hydraulic fracturing,the hydraulic fracture propagation,mechanism of map cracking and coalbed gas well productivity prediction after fracturing are investigated.Experiment,theoretical analysis and numerical simulation are used.The main conclusions are as follows:(1)Uniaxial compression,triaxial compression,Brazilian splitting,three point bending and seepage characteristic tests are carried out on the samples with different bedding angles.The mechanical parameters and their change rules are obtained.The effects of anisotropy characteristics on the failure modes and their mechanisms of samples under different test conditions are given.The influences of bedding on the failure model of coal rock are preliminarily discussed,which is the key factor.(2)Physical simulation experiments of hydraulic fracturing are carried out on the large size samples,which are prepared by the in-situ coal rock.The rules of the hydraulic fracture initiations and propagations are monitored.The mechanism of map cracking is preliminarily revealed.There are four basic modes for the hydraulic crack initiation and propagation.When the hydraulic fracture is perpendicular to bedding,it will change its propagation direction and fork.Many secondary cracks are formed.These secondary cracks,hydraulic fractures,natural fractures and beddings compose a map cracking system.In-situ stress and fracturing fluid rate are the key factors of forming the map cracking system.(3)The numerical model used to analyze the micro-fracture initiation and propagation in the hydraulic fracturing is established,which includes the effects of anisotropic characteristic of coalbed.The mechanism of forming map cracking is given.The effects of different factors on the hydraulic fracture shapes and dimensions are discussed.Calculating results show a map cracking is more likely to be formed under perforation completion condition.Crack initiation and extension are mainly tensile crack instability extension.The fracture toughness of bedding is the key factor of bifurcation and steering of fracture.In-situ stress has effects on the extension distance of crack.Fracturing fluid rate has great influence both on fracture morphology and extension distance.(4)Hydraulic fracturing and volume fracturing are simulated respectively based on the formation characteristic of Jiaozuo coal mine.The three-dimensional geometries of fractures at different fracturing conditions are obtained.Calculating results show that volume fracturing in Jiaozuo coal mine has good feasibility.The volume fracturing can be beneficial to form the map cracking system and improves the flow conductivity of the fractured zones.The relations between geometry dimensions of main fracture,fracture number,and volumes of the fractured zones with time are analyzed.(5)The mathematic model of the water-gas flow in the map cracking is built up based on Buckley-Leverett equation.A calculating program is developed based on the mathematic model.The factors affecting production of the CBM well after hydraulic fracturing are analyzed.Calculating results show that permeability,matrix radius and thickness of coalbed are the key factors of gas supply ability of substrate.To improve the efficiency of hydraulic fracturing in JZ-B well,the main fracture flow conductivity and fracture space are proposed as 200 mD.m and 20~30m respectively.The parameters of the map cracking have great effect on the fractured well production.Volume fracturing can greatly improve the production of the CBM well.