By integrating laboratory physical modeling experiments with machine learning-based analysis of dominant factors,this study explored the feasibility of pulse hydraulic fracturing(PHF)in deep coal rocks and revealed th...By integrating laboratory physical modeling experiments with machine learning-based analysis of dominant factors,this study explored the feasibility of pulse hydraulic fracturing(PHF)in deep coal rocks and revealed the fracture propagation patterns and the mechanisms of pulsating loading in the process.The results show that PHF induces fatigue damage in coal matrix,significantly reducing breakdown pressure and increasing fracture network volume.Lower vertical stress differential coefficient(less than 0.31),lower peak pressure ratio(less than 0.9),higher horizontal stress differential coefficient(greater than 0.13),higher pulse amplitude ratio(greater than or equal to 0.5)and higher pulse frequency(greater than or equal to 3 Hz)effectively decrease the breakdown pressure.Conversely,higher vertical stress differential coefficient(greater than or equal to 0.31),higher pulse amplitude ratio(greater than or equal to 0.5),lower horizontal stress differential coefficient(less than or equal to 0.13),lower peak pressure ratio(less than 0.9),and lower pulse frequency(less than 3 Hz)promote the formation of a complex fracture network.Vertical stress and peak pressure are the most critical geological and engineering parameters affecting the stimulation effectiveness of PHF.The dominant mechanism varies with coal rank due to differences in geomechanical characteristics and natural fracture development.Low-rank coal primarily exhibits matrix strength degradation.High-rank coal mainly involves the activation of natural fractures and bedding planes.Medium-rank coal shows a coexistence of matrix strength degradation and micro-fracture connectivity.The PHF forms complex fracture networks through the dual mechanism of matrix strength degradation and fracture network connectivity enhancement.展开更多
Variable frequency,a new pattern of pulse hydraulic fracturing,is presented for improving permeability in coal seam.A variable frequency pulse hydraulic fracturing testing system was built,the mould with triaxial load...Variable frequency,a new pattern of pulse hydraulic fracturing,is presented for improving permeability in coal seam.A variable frequency pulse hydraulic fracturing testing system was built,the mould with triaxial loading was developed.Based on the monitor methods of pressure sensor and acoustic emission,the trials of two patterns of pulse hydraulic fracturing of single frequency and variable frequency were carried out,and at last fracturing mechanism was analyzed.The results show that the effect of variable frequency on fracture extension is better than that of single frequency based on the analysis of macroscopic figures and AE.And the shortage of single frequency is somewhat remedied when the frequency is variable.Under variable frequency,the pressure process can be divided into three stages:low frequency band,pressure stability band and high frequency band,and rupture pressure of the sample is smaller than that of the condition of single frequency.Based on the Miner fatigue theory,the effect of different loading sequences on sample rupture is discussed and the results show that it is better to select the sequence of low frequency at first and then high frequency.Our achievements can give a basis for the improvement and optimization of the pulse hydraulic fracturing technology.展开更多
基金Supported by the National Natural Science Foundation of China(52274014,52421002).
文摘By integrating laboratory physical modeling experiments with machine learning-based analysis of dominant factors,this study explored the feasibility of pulse hydraulic fracturing(PHF)in deep coal rocks and revealed the fracture propagation patterns and the mechanisms of pulsating loading in the process.The results show that PHF induces fatigue damage in coal matrix,significantly reducing breakdown pressure and increasing fracture network volume.Lower vertical stress differential coefficient(less than 0.31),lower peak pressure ratio(less than 0.9),higher horizontal stress differential coefficient(greater than 0.13),higher pulse amplitude ratio(greater than or equal to 0.5)and higher pulse frequency(greater than or equal to 3 Hz)effectively decrease the breakdown pressure.Conversely,higher vertical stress differential coefficient(greater than or equal to 0.31),higher pulse amplitude ratio(greater than or equal to 0.5),lower horizontal stress differential coefficient(less than or equal to 0.13),lower peak pressure ratio(less than 0.9),and lower pulse frequency(less than 3 Hz)promote the formation of a complex fracture network.Vertical stress and peak pressure are the most critical geological and engineering parameters affecting the stimulation effectiveness of PHF.The dominant mechanism varies with coal rank due to differences in geomechanical characteristics and natural fracture development.Low-rank coal primarily exhibits matrix strength degradation.High-rank coal mainly involves the activation of natural fractures and bedding planes.Medium-rank coal shows a coexistence of matrix strength degradation and micro-fracture connectivity.The PHF forms complex fracture networks through the dual mechanism of matrix strength degradation and fracture network connectivity enhancement.
基金Financial support for this work,provided by the National Basic Research Program of China(No.2011CB201205)the Natural Science Foundation of Jiangsu Province(No.BK2012571)+1 种基金the Program for New Century Excellent Talents in University(No.NCET-120959)the"Qing-Lan Project"and Collegial Graduate Research and Innovation Program of Jiangsu Province(No.CXZZ13_0955)
文摘Variable frequency,a new pattern of pulse hydraulic fracturing,is presented for improving permeability in coal seam.A variable frequency pulse hydraulic fracturing testing system was built,the mould with triaxial loading was developed.Based on the monitor methods of pressure sensor and acoustic emission,the trials of two patterns of pulse hydraulic fracturing of single frequency and variable frequency were carried out,and at last fracturing mechanism was analyzed.The results show that the effect of variable frequency on fracture extension is better than that of single frequency based on the analysis of macroscopic figures and AE.And the shortage of single frequency is somewhat remedied when the frequency is variable.Under variable frequency,the pressure process can be divided into three stages:low frequency band,pressure stability band and high frequency band,and rupture pressure of the sample is smaller than that of the condition of single frequency.Based on the Miner fatigue theory,the effect of different loading sequences on sample rupture is discussed and the results show that it is better to select the sequence of low frequency at first and then high frequency.Our achievements can give a basis for the improvement and optimization of the pulse hydraulic fracturing technology.
