Study On Fractures’ Initiation And Propagation Law And Optimization Of Fractured Reservoirs In Songliao Basin Based On Finite-discrete Element Method

The northern part of Songliao basin is a large Meso Cenozoic continental sedimentary basin with extensive unconventional oil and gas resources.With the progress of exploration and development,it is found that the deep unconventional reservoir in Songliao basin has dense lithology,complex fracture development characteristics and significant differences in rock mechanics,which needs reasonable reservoir reconstruction to realize the efficient development of oil and gas.Complex geological fracture characteristics and high-strength rock mechanical properties have become the research difficulties in unconventional reservoir development.Based on the above problems,this paper starts with the study of the fracture characteristics of the reservoir,and the typical natural fracture characteristics of the study block are obtained through field outcrop observation,coring observation and logging interpretation,and the rock mechanics test results are obtained through laboratory tests.Then,the characterization method of particle flow control parameters with rock mechanics results as constraints is constructed,and the numerical simulation and prediction research of coring scale constitutive relationship and fracture mechanism under arbitrary meso fracture development characteristics are carried out.Based on the micromechanical parameters,the particle flow code program of near wellbore fracturing model is developed,the random fracture initiation simulation of open hole wells under different stress and fracture States is realized,and the random fracture initiation law of near wellbore fracturing fractures under open hole wells and perforation completion modes is revealed.Based on the advantages of discrete element and finite element method,the finite discrete element fracture propagation model of fractured reservoir is constructed.The discrete element constitutive relationship parameters and macro fracture characteristics are introduced into the model to study the fracture propagation law under typical fracture characteristics.On this basis,through the combination of orthogonal test and numerical simulation,the engineering parameters under the characteristics of specific volcanic fractured reservoir are further analyzed and optimized.The main research works are as follows:(1)Different fracture characteristics and attributes have different response characteristics.According to the test results of coring and rock mechanics,based on the particle flow discrete element theory,the constitutive relationship parameters of fractured rock mass are established;On this basis,rock mass models with different fracture morphological characteristics and fracture mechanical properties are constructed,and the effects of different fracture structural planes and their mechanical parameters on meso cracks and macro mechanics of rock fracture are studied.(2)According to the initiation characteristics of near wellbore fracturing,considering the arbitrariness of open hole wall fracturing and the intervention limitation of preset cracks in the finite element method,based on the discrete element method,the near wellbore fracturing model and code program under the action of fluid-solid coupling are developed,and the fracturing laws of open hole and perforated wells are analyzed.The results show that the number of fracture initiation after fracturing in open hole wells is significantly affected by the stress difference.When the stress difference is greater than 5MPa,the borehole starts to crack and expand in the form of double wings;When the stress difference is less than 5MPa,the borehole is mainly cracked by multiple fractures,and the number of cracks increases with the decrease of stress difference.In addition,aiming at the perforated wellbore,the fracture initiation law of perforated completion wellbore is obtained.(3)Through field outcrop observation and characteristic description,the typical fracture characteristics of volcanic reservoir are obtained.Based on the idea of finite discrete element,a globally embedded cohesive element program is developed,and the fracturing calculation model of fractured reservoir is established.Through the mutual verification with the indoor fracturing test,the reliability of the simulation method to study the fractured reservoir is tested.On this basis,the fracture propagation law under the action of different natural fracture morphology and cementation strength ratio is analyzed.Among the parallel natural fractures reservoirs,hydraulic fractures mainly expand along the natural fractures under the action of medium-low cementation strength ratio;In the random intersectional fractures reservoirs,the hydraulic fractures expand along the natural fractures in the direction of the maximum principal stress under the action of medium to high cementation strength ratio;and the connected fractures perpendicular to the direction of the maximum principal stress will be activated after fracturing under the action of low cementation strength ratio to form a complex fractures network.In the lattice fractures reservoirs,the hydraulic fracture mainly expands along the latticed fractures under the medium-low cementation strength,but directly penetrates the natural fractures and extends along the main stress direction under the action of high strength.Due to the differences of natural fracture morphology and fracture cementation strength,there are significant differences in fracture geometry,pumping pressure and fracture length after fracturing.(4)According to the geological fracture characteristics of specific volcanic rocks and considering two groups of natural fractures with development characteristics,the fracturing optimization calculation model is established.The injection displacement,perforation length and included angle are analyzed and studied by the combination of numerical simulation and orthogonal test.Taking the cumulative fracture length and maximum fracture width as reference objects,the fracturing effect is analyzed,and the rationality of the evaluation method is tested by daily gas production.The results show that the pump displacement has a significant influence on the cumulative crack length and has a certain influence on the maximum crack width.The perforation length and included angle have no obvious influence on the fracturing effect of field scale.Only when the displacement is not less than 0.01m~3/s/m can multi-branch fractures fracturing expansion be realized in high-strength volcanic reservoir.The research results have certain guiding significance for the efficient development of on-site volcanic rocks.