Mechanism Of Fractures Propagation Of Multi-stage Fracturing In Heterogeneous Shale Reservoir

China has rich shale gas reserves,which is roughly the same as the United States.However,compared with the North American shale reservoirs distributed mainly in the plain area,China shale reservoirs are mainly located in the mountains,and most experienced complex geological tectonic movements,resulting in poor natural surface conditions and natural fractures development.Meanwhile,some different sublayers may be drilled through by the long horizontal section,leading to the large difference of physical parameters in the long horizontal section,which makes there are obvious heterogeneity characteristics of reservoir properties in the long horizontal section.For the sake of convenience,conventional fracturing design generally treats the shale reservoir in the same horizontal well as the homogeneous reservoir,using the same cluster perforation and fracturing parameters.Therefore,the current fracturing design is quite different from the actual situation,which will impact the effect of reservoir stimulation to some extent.Considering the shale heterogeneity in the long horizontal section,the mechanism of hydraulic fracture propagation is still not very clear,which cannot guide the design of cluster perforation and multi-stage fracturing,so it is necessary to conduct in-depth research.Based on the research background of multi-stage fracturing of horizontal wells in heterogeneous shale reservoirs,the evaluation of shale reservoir heterogeneity and the mechanism of hydraulic fracture propagation in heterogeneous shale reservoirs and naturally fractured shale reservoir are carried out by methods of laboratory experiments,theoretical analysis,and numerical simulation.The main research works and results are as follows:(1)Based on logging data and fracturing data,two evaluation methods of shale reservoir heterogeneity were established.Based on the experiment of rock mechanics and mineral composition,it is revealed that the shale mechanical parameters have the characteristics of lateral heterogeneity in the same sublayer and vertical heterogeneity in different sublayers.Based on logging data and considering multi-factor coupling characteristics,an evaluation method of shale reservoir heterogeneity was established by using empirical mode decomposition technique(EMDT)and analytic hierarchy process(AHP).In addition,considering the limitations of the limited characterization range of logging data and multi-factor coupling,a new evaluation method of the far-field shale reservoirs heterogeneity was proposed from fracturing data.The results show that the two evaluation methods can quantitatively evaluate the shale reservoirs heterogeneity and provide a reference for the characterization of reservoir heterogeneity in the numerical simulation.These two evaluation methods have their advantages and disadvantages,and the evaluation method based on well logging data can be used for adjacent well formation,while the method based on fracturing data can be used for evaluation of far-field formation,which can provide guidance for stage and clustering optimization and fracturing design of the adjacent wells.(2)Based on the characteristics of the shale reservoirs heterogeneity,multi-cluster fractures propagation in the heterogeneous shale reservoirs were studied.Based on the characteristics of the shale elastic modulus following the Weibull distribution,a numerical model of multi-cluster fracture propagation in the heterogeneous shale reservoirs was established.The effects of horizontal in-situ stress,tensile strength,injection displacement,and fluid viscosity on the multi-cluster fracture propagation in the strong heterogeneous shale reservoir were analyzed,which reveals the influence law of stress interference between fractures on hydraulic fracture propagation in heterogeneous shale reservoir,and the clustering scheme of different heterogeneous reservoirs is optimized.The results show that: the hydraulic fracture shows asymmetric propagation in the heterogeneous reservoir.The smaller the minimum horizontal in-situ stress and the tensile strength are,the larger the total length and total area of the hydraulic fractures are.the stronger the reservoir heterogeneity is,the more obvious the effect of stress interference between the fractures on restraining the hydraulic fracture propagation is,so it should be considered to reduce the perforation cluster and increase cluster spacing in the multi-stage fracturing design for the strong heterogeneous reservoir.(3)According to the characteristics of natural fractures in shale reservoirs,a new numerical model was established by using the mesh node splitting methods to study the hydraulic fractures propagation in naturally fractured shale reservoirs.A new method of embedding cohesive element with zero thickness into finite element mesh was established by using the mesh node splitting methods,which was integrated into ABAQUS platform by using Python language,which makes up for the deficiency that the cohesive element of ABAQUS platform cannot effectively simulate the random propagation behavior of hydraulic fractures.Finally,a new model of hydraulic fractures random propagation in naturally fractured shale reservoir was established by using the new method.The effect of the horizontal in-situ stress difference,reservoir heterogeneity,natural fractures cementation strength,and friction coefficient,injection displacement,and fluid viscosity on hydraulic fracture propagation were analyzed.The research results show that under the lower horizontal in-situ stress difference conditions,the smaller the cementation strength and friction coefficient of natural fractures are,the larger the total length and area of hydraulic fractures are.Under the higher horizontal insitu stress difference conditions,the stronger the shale reservoir heterogeneity is,the easier it is to open the high angle natural fracture intersecting with the hydraulic fracture.Based on the research,the shale reservoir heterogeneity is evaluated,and the propagation law of hydraulic fracture in the heterogeneous shale reservoir and naturally fractured shale reservoir is clarified.The research results can provide a reference for the optimal fracturing design of heterogeneous shale reservoirs and have positive significances for improving shale gas development in China.