| In recent years,reserves and production of conventional oil and gas fields are decreasing.The exploitation of unconventional oil and gas reservoirs has gradually become a new trend of oil and gas production.Affected by the low permeability of unconventional oil and gas reservoirs,it is essential to adopt effective hydraulic fracturing before gas and oil production.Therefore,studying the influence of different factors on the complexity of hydraulic fracturing has become the focus of scholars.The main factors affecting the complexity of hydraulic fracturing are rock mechanical properties,Geostress,natural fractures and construction technology.The coupling influence of various factors on the complexity of hydraulic fracturing is very complicated.Rock is a typical brittle material.It is significantly important to study the influence of reservoir brittleness on the complexity of hydraulic fracturing.After fully considering the theoretical research,physical experiments and numerical simulation,numerical simulation models are established by rock failure process analysis software RFPA.Numerical simulation parameters are Based on the mechanic experience results of rock specimens taken from 3200 meters below the ground of a reservoir block in Shengli oilfield.The analysis is based on the numerical simulation results.Finally,orthogonal experiments are used to analyze the sensitivity of multiple factors.The main results of the study are as follows:(1)Based on the existing evaluation method of rock brittleness,the improved brittleness index B based on the complete stress-strain curve is put forward.The improved brittleness index B takes fully consideration of the peak stress rise rate,the post peak stress drop rate and the stress drop level for the first time.These three factors are important for the evaluation of brittleness.The results are verified by previous physical experiments.The improved brittleness index B can evaluate rock brittleness more comprehensively,and its applicability is better.(2)Based on the result of calibration of numerical simulation parameters,the relationship between the elastic modulus,the Poisson's ratio,ratio of uniaxial compressive strength to tensile strength,residual stress ratio and the confining stress with the brittleness of rock are studied by numerical simulation.According to the degree of crack development,the angle of fracture surface and the complete stress-strain diagram,Elastic modulus and ratio of uniaxial compressive strength to tensile strength are positively correlated with rock brittleness;Poisson's ratio,residual stress ratio,and Geostress are negatively correlated with rock brittleness.(3)Fully considering the development of natural joints in tight oil and gas reservoirs,two groups of natural joints are prefabricated in the numerical simulation model.The influence of different elastic modulus,Poisson's ratio,ratio of uniaxial compressive strength to tensile strength,residual stress level and Geostress on the complexity of hydraulic fracturing is studied.By calculating the complex fractured network index FCI of hydraulic fracturing,elastic modulus and ratio of uniaxial compressive strength to tensile strength are positively correlated with FCI;Poisson's ratio,residual stress ratio,and Geostress are negatively correlated with FCI.The results of numerical simulation show that the rock brittleness are stronger,the higher the complexity of hydraulic fracturing.(4)The sensitivity of different factors to the complexity of hydraulic fracturing is studied by orthogonal experiment.The ranking of sensitivity from strong to weak are: Geostress,elastic modulus,residual stress ratio,ratio of uniaxial compressive strength to tensile strength,Poisson's ratio.Geostress and elastic modulus have a significant influence on complexity of fracture.Residual stress ratio effect complexity of fracture.However,The effect of the sensitivity of ratio of uniaxial compressive strength to tensile strength and Poisson's ratio on complexity of fracture is weak. |
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