Experimental And Numerical Simulation On Hydraulic Crack Propagation Of Rock-like Material

For China’s unconventional oil and gas reservoirs,it is very difficult to extract oil and gas due to the tight rock formation,extremely low porosity and permeability,which seriously restricts the production of unconventional oil and gas in China.As a production technology process,hydraulic fracturing technology has become an important means to improve the utilization efficiency of China’s oil and gas resources.However,the existing research on the mechanism of hydraulic fracture propagation still lags behind actual engineering applications.Therefore,it is necessary to further study the behavior of hydraulic fracture propagation.The background of this paper is the engineering application of hydraulic fracturing technology.This article is based on theoretical analysis,model tests,numerical analysis.Aiming at two fracturing technologies,conventional hydraulic fracturing and circulating hydraulic fracturing.The initiation and expansion behavior of hydraulic fractures in the process of hydraulic fracturing were studied from a qualitative and quantitative perspective.The main work is as follows:(1)The conventional hydraulic fracturing laboratory model tests were carried out.The study found that only one hydraulic crack appeared when the model was destroyed.The crack propagation direction is perpendicular to the minimum principal stress,and the presence of preset cracks will reduce the rupture water pressure of the test block.Increasing the initial angle of the crack will tend to decrease the cracking pressure,and will also increase the perpendicularity of the crack initiation direction to the direction of the minimum principal stress.(2)Laboratory model tests of circulating hydraulic fracturing were carried out.The experiment found that multiple cracks appeared on the upper surface of the model,and the upper layer was peeled off.But there is only one main crack,the crack propagation direction is perpendicular to the minimum principal stress.At the same time,the initial crack will affect the propagation of the hydraulic crack.(3)Based on the theory of linear elastic fracture mechanics,the extended finite element numerical calculation model of hydraulic fracturing is established.By combining this model and based on the relationship between water pressure and crack tip energy release rate,a numerical method for inversion of material fracture energy is proposed.By comparing with the laboratory test results,the reliability of the method is proved.(4)Based on the conventional hydraulic fracturing numerical calculation model,the effects of initial crack angle and confining pressure stress on hydraulic fracture propagation behavior were studied.It is found that for conventional hydraulic fracturing technology,confining pressure stress and initial crack angle have obvious effects on the broken water pressure,tensile stress at the crack tip,cracking angle and propagation path.The degree of deflection of the hydraulic crack propagation path is positively related to the injected water pressure(5)Based on the linear elastic fracture mechanics and Paris fatigue crack growth formula,the numerical calculation model of fatigue hydraulic crack growth is established.The mechanical properties related to fatigue hydraulic crack propagation are studied,The study found that for cyclic hydraulic fracturing,the stress level has a significant effect on the degree of deflection of hydraulic fractures,and crack deflection angle is negatively correlated with stress level.Therefore,it is possible to produce a significant deflection effect of hydraulic fractures with a small broken pressure.