| At present,in the process of coalbed methane exploitation,hydraulic fracturing is a major measure to increase production of the gas.Because of the special structure of the coal seam,there are a lot of problems such as difficulty of crack extension,pollution of coal seam and great loss of fracturing fluid.Indirect fracturing is a method that it can connect the top and bottom floors and this technology can effectively solve the problems.This method has been used in the exploitation of coalbed methane.With the unclear understanding of the fracture crack propagation,the indirect fracture application is easy to fail.In this thesis,the rock mechanics parameters and reservoir parameters of the coal seams and tight sandstone in Linxing district are tested.By comparing with districts including Qiyuan、Xinjing district,and the characteristics of coal rock in Linxing block are obtained.The finite element model of a well in Linxing district is established with ABAQUS and the indirect fracture propagation rule is obtained.(1)The coal rock in Linxing block has relatively high poisson ratio,low porosity and permeability,and it belongs to low-porosity and low-permeability coal seam.(2)When the stress difference between the tight sandstone on the roof and floor is small,the shielding effect of the stress on the roof and floor of the coal seam is not obvious and the seam height is easily out of control.The ideal stress difference should be greater than 4MPa.(3)When the difference in elastic modulus between the roof and floor and the coal reservoir is relatively small,the stress difference is the main controlling factor of fracture propagation.With the increase of modulus difference,the influence of modulus on fracture propagation is strengthened and the fracture tends to expand in the bottom layer of brittle roof with high elastic modulus.By the time,the fracture height is difficult to control.The ideal difference of elastic modulus should be less than 23 GPa.(4)With discharge of fracturing increasing,seam pressure and pressure at the junction between roof and floor and coal seam is also increasing.The speed of fracture propagation between roof and floor to connect with coal seam is accelerated and the overall fracturing construction efficiency is improved.(5)The perforation cracking position should be within 10 m of the vertical distance from the coal seam.Above the range,the crack only expands on the top and bottom floor and cannot communicate with the coal seam.Using the research results of this thesis,the top floor fracturing is carried out in a well in Linxing district,and a good fracturing effect is obtained. |
