Research On Evolution Mechanism And Application Of Coal Seam Stress During Hydraulic Fracturing

How to increase the permeability of coal seams in low permeability coal seams has become a bottleneck restricting coalbed methane extraction in coal mining areas.In recent years,the Chongqing Songzao Mining Area has applied hydraulic fracturing technology to the underground coal mine to enhance the permeability of coalbed methane.Hydraulic fracturing and infiltration technology has become a regional measure to increase coal seam permeability over a large area.Previous studies by many scholars on hydraulic fracturing mainly focused on the optimization of hydraulic fracturing process,and lacked corresponding theoretical support and practical experience for the stress transfer law of coal seam during hydraulic fracturing.To this end,relying on the key technology and application of large-scale oil and gas field and coalbed methane development national science and technology major project “Difficult to pump coal seam downhole infiltration”,Songzao Coal and Electricity Company was used as a field test point to carry out underground coal seam hydraulic fracturing experiment.Exploring the law of fracture extension and stress evolution during hydraulic fracturing,it is helpful to optimize the hydraulic fracturing process and enrich the hydraulic fracturing effect evaluation system.In this paper,theoretical analysis,numerical simulation,laboratory and field experiments are used to summarize the crack propagation criterion in coal seam hydraulic fracturing process,the crack extension law under different strength conditions,and the relationship between fracture extension and stress transfer in coal seam hydraulic fracturing process.According to the model,the fracture distribution in the hydraulic fracturing process is inversion,and the purpose is to enrich the evaluation mechanism of hydraulic fracturing effect.The main findings are as follows:1 Coal stress evolution theoryduringhydraulic fracturing.The distribution law of the original in-situ stress of coal seam is expounded.The original in-situ stress of Shiqian coal mine is measured by four original geostress test holes.The distribution law of the original geostress is obtained based on the analysis.The variation law of stress around the fracture hole is analyzed.Based on the elastoplastic mechanical model,the stress distribution around the borehole of the coal seam is calculated and the displacement of the plastic ring is calculated.The extension of the plastic ring is the main expansion of the hydraulic fracture of the coal seam.the reason.The influence law of coal seam stress field on fissures is summarized.The influence of coal seam stress field on fissures is analyzed from different angles.Based on this,the size of damage localized zone under different stress conditions is obtained.The numerical and statistical solutions are used to simplify the complicated factors.The length of the crack under conditions.2 Experimental study on stress evolution of coal during hydraulic fracturing.During the hydraulic fracturing process,the stress is transmitted by the coal sample as the medium,and the transfer process is affected by factors such as the original fracture and the density of the coal sample.During the confining pressure loading process and the hydraulic fracturing process,the coal body is subjected to different stresses.Both of them can pass the stress transfer of the coal sample and the stress transfer effect becomes weaker,but the stress increase during the hydraulic fracturing process is greater than the confining pressure loading.It is caused by the difference in stress.By arranging the stress sensors at different positions of the coal samples,the stress-time evolution mechanism of the hydraulic fracturing process in the coal body is summarized by using the measured stress data combined with the stress changes at different moments in the fracturing process.It can be seen from this experiment that it is feasible and effective to use the large-scale physical simulation experiment to simulate the field conditions to explore the stress transfer law of the coal fracturing process,which can provide scientific guidance for the on-site hydraulic fracturing project.3 Experimental study on stress evolution of fractures based on coal seam hydraulic fracturing.A stress evolution mechanism based on the hydraulic fracturing fracture distribution of coal bodies is established.The laboratory experiment is used to determine the macroscopic fracture distribution in the hydraulic fracturing process of coal body.Based on this,the relationship model between stress evolution and fracture extension in the hydraulic fracturing process of coal body is established by mathematical statistics.This model describes the hydraulic fracturing of coal body.The quantitative relationship between stress evolution and fracture extension combines the stress evolution of coal body with the fracture extension during the hydraulic fracturing process of coal body,and realizes the mathematical influence of using coal seam stress to determine the influence range of coal seam hydraulic fracturing.Hydraulic fracturing effect evaluation system.4 Engineering application of hydraulic fracturing stress evolution mechanism in coal.Design stress test field experiment,based on the established mathematical model,use the measured coal seam stress data to invert the fracture extension area in the hydraulic fracturing process and determine the hydraulic fracturing range.The results show that the hydraulic fracturing impact range is 40 m.The stress conditions during coal seam mining are determined by the stress unloading cycle of the coal seam after fracturing.The results show that the stress concentration of the coal seam caused by hydraulic fracturing has been completely relieved,which will not affect the later coal seam mining.The effectiveness of the fracture range determined by the fracture extension and stress model obtained in Chapter 4 was verified by microseismic test,transient electromagnetic test,moisture content and gas extraction test.Verification shows that this model can serve the on-site hydraulic fracturing evaluation.