| In recent years,unconventional oil and gas resources have become the major part of China’s reserve growth.Among them,continental shale oil has abundant recoverable resources in large basins such as Junggar,Ordos,Bohai Bay and Songliao,which have huge development potential.Unlike North American marine shale oils,our organic-rich shales are mainly formed in continental lake depositional environments,with a late formation of basins and a relatively low degree of thermal evolution experienced.These shales have a number of unique characteristics,including a narrower planar distribution of oil layers,more vertical sequences,relatively thin thicknesses of individual layers,more developed natural fractures,high clay mineral content,lower brittleness,and a complex stress distribution.Due to the complex mineral composition and pore structure of continental shale oil reservoirs and extremely poor property conditions,conventional drilling methods are unable to realize large-scale commercial exploitation,and it is necessary to utilize horizontal wells with segmented multi-cluster volumetric fracturing technology in order to realize the economic development of medium-high maturity shale oil.With the promotion of volumetric fracturing factory,the stress interference between hydraulic multi-fractures becomes more and more prominent,and it is difficult to control and estimate the volume of the fracture network formed by intersecting with natural fractures,which affects the accurate evaluation of the production capacity of the stimulation wells as well as the optimization of the fracturing construction parameters.Therefore,this paper combines literature research,physical experiments,theoretical analysis,numerical simulation and other methods to conduct an exhaustive study on the geological and engineering factors which affect the expansion of hydraulic fractures,as well as the evolution mechanism of the fracture network formed after interacting with the natural fractures,by working from the macroscopic to the microscopic levels,as follows:(1)The law of synchronous propagation of multi-fractures was studied,the distribution of the superimposed stress field and the intensity of stress interference among sequential fracturing,zipper fracturing and improved zipper fracturing were compared one by one,the influence of segment distance and well distance on the induced stress field of different fracturing processes as well as on the length and deflection angle of fracture propagation were analyzed,and the internal reasons for the asymmetric competitive propagation of multi-fractures were revealed.(2)An experimental study on the interaction between hydraulic fractures and preformed natural fractures was carried out using the Ordos Basin Chang 7 outcrop shale.The influence of the slip characteristics of the natural fractures penetrating through the rock samples on the results of hydraulic fracture penetration was analyzed by varying the angle of hydraulic fracture approximation,the distance from the injection point to the natural fractures,the cement strength of the natural fractures,the confining pressure,and the rate of displacement,and the numerical intervals of the slip distances and slip rates that affect the results of the fracture intersections in the experimental environment were proposed.(3)The intersecting propagation behaviors of hydraulic fractures with friction-type and cemented-type natural fractures were investigated,respectively,revealing the differences of the interaction laws between hydraulic fractures and them,and proposing that under the conditions of the same geometric characteristics and the same spatial distributions of natural fractures,hydraulic fracturing forms a more complex network of seams in friction-type natural fracture formations than in cemented-type natural fracture formations.(4)Developed a two-dimensional discrete natural fracture network model,combining a global embedded cohesive elements integration method,established a coupled two-dimensional hydraulic fracturing numerical model of discrete natural fracture networks.The influence of horizontal stress difference,fractal dimension,natural fracture orientation,natural fracture length and injection volume on the interaction between hydraulic fractures and natural fractures was analyzed.Identified the main reasons for the formation of multi-well segmented fracturing networks in fractured continental shale reservoirs.(5)A trapezoidal damage factor with triaxial effect was introduced,and a numerical model to simulate different damage characteristics of shale was established.A numerical model to characterize the heterogeneity of fine-scale minerals in rocks is proposed,and the anisotropic characteristics of rocks with different mineral contents are simulated by using a program to assign finite element meshes,so as to introduce the influencing factors of the hydraulic fracture propagation law from the macroscopic analysis to the fine-scale domain.(6)A numerical model of multi-cluster hydraulic fracturing under the condition of mineral heterogeneous reservoir was established,and the influence mechanisms of mineral heterogeneity,reservoir leachability,fluid injection method,number of shot holes and shot hole diameter on the fracture network evolution were parametrically analyzed,which revealed the reasons why most numerical simulation results of the hydraulic fracturing cohesive zone model do not agree with the results of the microseismic monitoring.The above research results have revealed the competition propagation law of multiple fractures in horizontal wells with segmented multi-cluster fracturing in continental shale oil reservoirs,the interaction mechanism between hydraulic fractures and natural fractures,and the evolution law of fracture network with closely cut volume,which provide theoretical basis and technical support for the efficient development of continental shale oil reservoirs and the optimization of fracturing schemes. |
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