| Deep geothermal energy is a clean and abundant renewable resource.China is extremely rich in geothermal energy resources.Vigorously developing unconventional energy such as geothermal energy is of great significance to ensure the security of national energy strategy and improve energy structure.The geothermal resources in China are mainly dry-hot rock resources.The reservoir is generally characterized by deep burial,ultra-low permeability and water shortage,which makes the exploration and development difficult.Hydraulic fracturing technology has become the key technology in the development of enhanced geothermal system.Multi-cluster perforation increases reservoir reconstruction volume and reduces construction cost.However,the field operation results show that the fracture does not reach the expected fracturing effect.Due to the short time of hydraulic fracturing test and the difficulty of field hydraulic fracture monitoring,it is difficult to monitor the fracture propagation path and its mechanical response,and it is impossible to fully grasp the dynamic propagation mechanism of multiple fractures and the inter-fracture stress interference mechanism.Soundless cracking demolition agent(SCDA)fracturing is similar to the special form of hydraulic fracturing of high viscosity fluid or ultra-low permeability rock,which has the characteristics of long action time and stable performance.SCDA fracturing test can obtain more information about fracture growth and stress distribution,which has important scientific value and engineering significance for revealing the law of multiple fractures propagation and the response mechanism of stress shadow effect.By adjusting SCDA fracturing construction parameters to optimize the fracture propagation trajectory,it will help guide the optimization of field fracturing construction schemes.In this paper,the multi-fractures propagation law,stress shadow effect and fracturing operation parameters optimization are studied by theoretical analysis,laboratory test and numerical simulation.The main contents research are as follows:(1)Based on the elastic theory and thick-wall cylinder model,the strain test of SCDA steel pipe is designed,the formula of the expansion stress development in hole is deduced,and its expansion law is analyzed.The theoretical model of stress distribution of porous fracturing under internal pressure is established,which explicitly characterizes the stress concentration and distribution in the inter-hole region,and indicates that the increase of borehole spacing can weaken the degree of stress concentration.(2)The experimental study of granite porous fracturing under different initial stress conditions and fracturing operation parameters is carried out by SCDA fracturing technology.By means of acoustic emission location,strain measurement and real fracture monitoring,the temporal and spatial evolution of acoustic emission events,fracture propagation path and stress evolution results in the process of fracture propagation are obtained,and the dynamic propagation law of multiple fractures and stress shadow effect are comprehensively revealed.(3)The calculation methods of fracture surface area,roughness and surface fracture fractal dimension are proposed to quantitatively evaluate the fracture morphology and fracture surface characteristics of fracturing samples.Based on the SCDA fracturing test results under different initial stress conditions,injection schemes,borehole directions and spacings,the optimization scheme of granite fracturing parameters is established.(4)The finite element numerical model of SCDA fracturing expansion is established by ABAQUS software,and the dynamic propagation law of multiple fractures in SCDA fracturing is studied.By analyzing the fracture propagation path and stress redistribution in the fracturing process,the multi-fractures dynamic propagation law and stress shadow effect are deeply studied,which provides a further explanation for the experimental phenomenon and theoretical analysis results. |
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