Study On Complex Fracture Propagation Law Of Volume Fracturing In Horizontal Well

Tight glutenite reservoir is an unconventional oil and gas resource with extensive distribution and considerable oil and gas reserves.In recent years,its effective development mode has become the research focus in the petroleum industry and it has been widespread concern worldwide.The low porosity and permeability characteristics make it necessary to use the hydraulic fracturing or other stimulation measures to stimulate reservoir,so as to enhance the production of oil and gas wells.Due to the insufficient understanding of the rock characteristics and the fracturing fracture propagation morphology in tight glutenite reservoir,part of oil and gas wells have problems such as poor productivity after fracturing and short production period after fracturing.Based on the test results and stress characteristics of rock mechanics parameters of tight glutenite reservoir,volume fracturing fracture complexity evaluation index in horizontal well and the numerical model of two-dimensional hydraulic fracturing fracture initiation and propagation are established.The fracturing fracture propagation morphology of this type of reservoir and the important factors affecting the stimulated reservoir effect are studied,the main works are as follows:The rock mechanics parameters test was carried out on the tight glutenite reservoir in Daqing.The mechanical properties of the tight glutenite were analyzed and the related rock mechanics parameters were obtained.The experimental results show that the tight glutenite reservoir is brittle and easy to form complex fractures;the tight glutenite shows strong anisotropy;the micro-fractures present in tight glutenite are the main reason for the large difference between the dynamic and static elastic parameters;the lower the tensile strength of tight glutenite is,the easier the hydraulic fracture connected natural fractures can be.The petrophysical characteristics of the tight glutenite reservoir are analyzed,the direction of the in-situ stress is determined,and the calculation formula of the maximum and minimum horizontal principal stress is modified according to the in-situ stress results calculated by the fracturing software and the mathematical formula.The porosity of the tight glutenite reservoir is mainly less than 2.7%,and the permeability is mainly less than 0.1×10^-3μm²;the in-situ stress contrast of the tight glutenite reservoir is 8-12 MPa.By considering the brittleness,horizontal stress difference coefficient and the fracture toughness with confining conditions in tight glutenite reservoirs,the fracture complexity evaluation index of the volume fracturing is established to evaluate and classify the fracturing fracture complexity of tight glutenite reservoirs.According to the pilot production data,the accuracy of the calculation results of fracture complexity evaluation index is verified.The results show that the tight glutenite reservoir can produce complex fracture when the fracture complexity evaluation index is greater than 6;the tight glutenite reservoir can form a single symmetric fracture when the fracture complexity evaluation index is less than or equal to 6.The method of batch embedding cohesive element is used to simulate the complex fracture propagation morphology of volume fracturing in horizontal well.The accuracy of the cohesive model is verified by comparing the calculation results of the cohesive model and the KGD model.Based on this,the complex fracture propagation morphology at different stages,fracturing fluid flow direction and main influencing factors of fracture complexity were studied.The results show that some branch fractures will close with different degrees in the process of complex fracture propagation,and it is difficult to produce effective sand-filling fractures.When branch fractures are generated,part of the fracturing fluid will flow into the branch fractures.When the branch fracture propagates to a certain extent,the fracturing fluid will no longer flows into the branch fractures but flows along the propagation direction of the main fracture.This is the main reason of partial branch fractures close.When the simultaneous fracturing with multi-fractures,the fracture spacing of 40 m can effectively reduce the degree of interference between fractures,and reservoir can be fully stimulated;large injection rate and low in-situ stress contrast are important factors to enhance the fracture complexity.The simulated ground pressure variation curve coincides with the ground pressure curve measured in the field of the complex fracture propagation,it indicates that the established complex fracture propagation model conforms to the complex fracture propagation law in tight glutenite reservoirs.A two-dimensional hydraulic fracturing fluid-solid coupling model is established,and the discrete forms of the extended finite element and the solution equation are given.The extended finite element method（XFEM）is used to simulate fracture initiation and propagation for simultaneous fracturing,sequential fracturing,and multi-cluster staged fracturing in horizontal well.The numerical model is compared with the KGD model to verify the accuracy of the XFEM model.The main factors affecting the stimulated reservoir area with different fracturing methods are analyzed.It can be concluded that the interference between fractures is one of the important factors affecting the stimulated reservoir effect;when two and three fractures are simultaneously fractured,the optimal fracture spacing is 30m;when four fractures are simultaneously fracturing,the optimal fracture spacing is 40 m;the optimal fractures number of sequential fracturing is six,and the optimal fracture spacing of sequential fracturing is 50 m;the optimal stage spacing of multi-cluster staged fracturing is 50m;in the case of low in-situ stress contrast,increasing injection rate makes the stimulated reservoir area expand by 32.1%;when the fluid viscosity is high,improving the injection rate to make the stimulated reservoir area increase by 70.6%.High viscosity fluid,large injection rate and low stress contrast can effectively improve the stimulated reservoir area.Comparing the ground pressure curves of simulation and field measurement of the single fracture propagation,it is found that the established single fracture numerical model conforms to the single symmetric fractures propagation law in tight glutenite reservoirs.The numerical model,research methods and results established in this paper provide some reference for the selection of fracturing construction parameters in tight glutenite reservoir.