Fracture Initiation And Propagation Mechanism Of Shale Oil Reservoir In Jiyang Depression

China's shale oil geological resources are abundant,about 152.92×108t,and only the shale oil geological resources in Jiyang Depression is 98.44×108t.Driven by technological advances in shale gas exploration and development,shale oils in the United States have been effectively developed in areas such as Bakken and Eagleford.The exploration and development of shale oil reservoirs must draw on the experience of shale gas exploration and development,and use horizontal well network fracturing technology to effectively explore and develop.Practice of China's Jianghan Oilfield Qianjiang Depression,Henan Oilfield Biyang Depression and Shengli Oilfield Jiyang Depression and other shale oil have been also carried out,but the effect was not ideal.The production after fracturing of Shengli Oilfield Jiyang Depression Boye1 well and Boye 2 well are only 2.3-3.4m3/d.On the one hand,it is due to the poor physical and oil-bearing properties of shale oil reservoirs in China.On the other hand,shale oil reservoirs in Jiyang depression are deep buried,high in in-situ stress and more plastic in rock,resulting in difficult to form complex fractures.At the same time,it is more difficult to understand the law of the initiation and propagation of complex fractures in shale oil reservoirs.It leads to the lack of pertinence of fracturing technology and seriously restricts the exploration and development of shale oil reservoirs in china.In this paper,the shale oil reservoir in Jiyang depression of Shengli Oilfield is studied.On the basis of understanding the characteristics of shale oil reservoirs,three axis rock mechanics experiments are carried out to analyze rock stress and strain characteristics of shale oil reservoirs.A model for rock plastic initiation is established to analyze the initiation modes of rocks.The influence of slippery water and glue on the fracture propagation is analyzed by using the core to study the fracture propagation of shale oil reservoir.The finite element method is used to establish the numerical simulation model of hydraulic fracture expansion,and the influence of the fracture propagation law of the shallow and horizontal well shale oil reservoirs is carried out,and the influence law of the complex fracture propagation is studied.By using the digital core technology,the analysis methods of pore and micro cracks in the core are established to analyze the characteristics of micro cracks and the influence of liquid on the micro cracks of shale oil in fractured shale.The fractures initiation and propagation pattern of shale oil reservoirs in Jinyang depression is recognized preliminarily,and the main research work and achievements are as follows:(1)Based on the shale oil reservoirs in Shengli Oilfield Jiyang Depression,the development characteristics of natural fracture and the geostress characteristics of the shale oil reservoir were analysized by the mineral component test and the rock mechanics experiment.The results show that the rock of shale oil reservoir in Jiyang Depression has strong plasticity,natural fractures are more developed,and has high ground stress,and the ability to form fracture network.(2)The model of nonlinear constitutive and well stress field about shale reservoir are established by analyzing the rock stress-strain analysis and rock failure characteristics.Based on the analysis of the actual stress-strain curve strength,the rock nonlinear constitutive model is established.Combining the initiation criterion of each stage,the elastic-plastic stratum initiation model is established and the sensitivity of the initiation pressure is analyzed.The results show that the fracture of the shale oil reservoir rock in Jiyang depression is dominated by plastic shear initiation,the compressibility of the rock is poor depending on the initiation mode and pressure.(3)The analysis method of the formation,extension and morphological changes during rock fracture and extension are established by combining the real triaxial simulation test system and CT scanning and testing machine.The results of fracturing experiments with slippery water and linear rubber crack propagation under different stress difference conditions show that complex fratures can be formed by using slippery water of which the stress ratio is less than 1.3;the fractures formed by the fracturing of the gel are more than that of the single and the horizontal stress difference has little effect on the fracture propagation.(4)A finite element model is established based on the Biot seepage theory of fluid mechanics by using of the finite element technique.A new algorithm of material distribution is introduced to establish the mathematical model of fracturing and expansive expansion.The numerical simulation of crack initiation and expansion of two-dimensional and three-dimensional shale is carried out the results show that when the stress ratio is less than 1.3,the complex cracks can be produced.When the viscosity of the fracturing fluid is less than 10mPa.s,the complex fractures can be produced.The complexity of the cracks increases rapidly while the increasing of the natural fracture density.(5)The shale CT scanning test was carried out to establish analysis method of the shale core numerical model and the fracture-matrix porosity dual-medium two-phase seepage rock physical property.On the basis of this,the CT scan experiment before and after shale fracturing was carried out,and the microcracks in the core before and after fracturing and the effect on the core flow capacity were analyzed comprehensively.The results show that after the fracturing,the rock can produce microfractures with a certain angle,which can improve the connectivity by effectively penetrating the horizontal fissure.The transformation of rock fracture by acid solution is more obvious while alkali solution has little influence on rock pore and micro fractures.(6)Based on the research results and reservoir characteristics,a technique for improving the complexity of fractures in shale oil reservoirs in the Jiyang Depression was proposed,and field applications have yielded good results,indicating that the research results have been adapted to the shale oil reservoirs in the Jiyang Depression.The results of this study have deepened the understanding about the law of fracture initiation and propagation and provides experimental and theoretical support for the fracturing of the shale oil reservoir in Jiyang depression.At the same time,Shale oil reservoir exploration and development provides a theoretical reference.