Experimental study on fracture propagation and stress evolution characteristics of segmented hydraulic fracturing in coal strata
[Objective]With the increasing mining depth of coal seams in China,issues related to gas extraction efficiency owing to local seam permeability have become increasingly severe.Conventional gas extraction methods are no longer sufficient.Hydraulic fracturing technology offers a promising solution to improve coal seam permeability and enhance gas extraction.However,crushed soft coal seams in China present challenges such as low strength,low permeability,significant gas content,and significant gas pressure,which result in poor drilling outcomes and limit control over gas disasters.Standard hydraulic fracturing techniques often result in short pressure cracks.To address this,long drilling stage hydraulic fracturing technology is used to create fractures that extend from the coal seam roof into the coal seam,forming a complex fracture network that improves gas migration channels and gas control.[Methods]To investigate the development and expansion of fracture networks during segmented hydraulic fracturing,a physical similarity simulation experiment was conducted using triaxial coal and rock assemblages.Acoustic emission monitoring technology was used to monitor the dynamic process of fracture expansion,and strain bricks were embedded in samples to monitor stress during and after fracturing.The study focused on understanding the fracture development and stress change laws of samples generated during subsection hydraulic fracturing under varying ground stress conditions,coal-rock interface characteristics,and the effect of pump displacement on the formation of new fractures.The goal was to determine whether new cracks generated in the roof rock stratum during segmented hydraulic fracturing can smoothly enter the coal seam and provide efficient gas extraction channels to increase the efficiency of gas extraction in the coal seam.[Results]The research results show that,in the process of segmented hydraulic fracturing of coal strata,the hydraulic fracturing effect of the second stage is affected by the fracture network generated during the first.Increasing the pumping displacement improves the development and expansion of new fractures and the degree of connectivity,thereby enhancing coal seam permeability and gas extraction efficiency.The stress change curve during fracturing can be divided into three stages:the pressure holding stage,the fracture expansion stage,and the stress recovery stage.The pumping displacement in segmented hydraulic fracturing will significantly affect the role of fracture-induced stress.Under low pumping displacement and low stress conditions,the stress change amplitude of the hydraulic fracturing during fracture expansion and recovery stages is relatively weak.At the same time,the stronger the coal-rock interface,the more favorable it is for the expansion of the new fractures into the coal seam,which limits changes in coal seam permeability.[Conclusions]The study provides valuable insights into the induced stress action law and fracture propagation mechanism during segmented hydraulic fracturing.It highlights the importance of factors such as ground stress,coal-rock interface characteristics,and pump displacement.The findings offer a theoretical basis for optimizing the complex fracture network in segmented hydraulic fracturing of coal and rock masses in soft crushed coal seams.When implementing antireflection measures of segmented hydraulic fracturing,it is recommended to select regions with a strong coal-rock interface to maximize fracturing effectiveness.To ensure that the fractures in the rock can expand into the coal seam,it is essential to increase the pumping capacity as much as possible during fracturing.The research findings offer valuable insights for designing technical parameters for segmented hydraulic fracturing in deep coal seams.These insights have significant implications for the efficient development of deep coal bed methane and the prevention and control of rock bursts.
gas disaster controlhydraulic fracturingcrack propagationstress characteristicscoal seam permeability
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