Research On Influencing Factors Of Shale Fracture Toughness And Induced Fracture Front Morphology

Affected by shale reservoir's characteristics,it is a common method to "break" target layer and form the fracture network by volume fracturing,it aims to improve the condition of formation seepage and to achieve the purpose of effective development.Fracture toughness is an important parameter of fracture design and also affects fracture's extension and morphology of crack front.Due to the formation influence and shale structure characteristics,the crack front will appear bifurcation or reorientation in addition to go along the direction of maximum principal stress.It is the combination of these crack front shapes,which makes crack propagation more complicated and favorable for forming the fracture network.In this paper,the core sample was taken from some area of Longmaxi formation and Wufeng formation in Sichuan basin.Based on analyzing components and structural characteristics and triaxial mechanical properties of shale,the fracture toughness of shale was tested by the Brazil disc method,RFPA was used to analyze the influence of bedding on fracture toughness and crack front shape when testing the fracture toughness of shale by Brazil disc method,at the same time,the effect of stress on morphology of induced crack front was also analyzed by two dimension model.There were some conclusions as follows:(1)Analyzed from Brazil disc test,the average value of fracture toughness of longmaxi forrmation shale is 1.0056Mpa·m0.5,the one of wufeng formation shale is 0.9631Mpa·m0 5.Observation of expansion of cracks in the process of experiment,the crack front mainly expands along the direction of prefabricated crack,but the crack front also appears reorientation or bifurcation influenced by bedding or micro-cracks.Conferred from propagation of crack in Brazil disc specimens,structural weakness planes such as shale bedding and micro-cracks are important factors of complicated fracture.(2)Analyzed from Brazil disc numerical simulation,the angle between loading direction and shale bedding,bedding density and mechanical properties of bedding of shale will affect fracture toughness.With the angle increases,the fracture toughness's value of shale decrease first and then increases and then decrease again;with bedding density increases,fracture toughness's value of shale decreased basically;with mechanical properties of bedding increases,fracture toughness's value of shale increases.(3)Bedding has an effect on shale's crack front shape.When the angle between loading direction and shale bedding is less than 40°,the crack front appears reorientation along the bedding direction when it extends to bedding;when the angle is between 50° and 70°,effect of bedding becomes weak,the crack front may appear reorientation and bifurcation,crack front shape becomes more complicated;when the angle is more than 70°,the crack front easily crosses through bedding and no longer expand along the direction of bedding;with bedding desity increases,the crack reorientation becomes easier along the direction of bedding;with the mechanical properties of bedding increases,the crack front may also appear bifurcation except appear reorientation towards bedding direction.(4)The ratio of maximum principal stress and minimum principal stress,the angle between induced crack and direction of maximum principal stress,induced crack's length and the angle between bedding and direction of maximum principal stress were used to study the effect of stress on morphology of induced crack front.With the angle between induced crack and direction of maximum principal stress increases near the wellbore,the crack front appears reorientation,and the turning angle increases gradually;when the ratio of maximum principal stress and minimum principal stress is small,crack front appears bifurcation easily and the bifurcation angle is small,when the ratio of maximum principal stress and minimum principal stress is up to 1.4,cracks have little bifurcation.With the ratio of maximum principal stress and minimum principal stress increases near the wellbore,turning angle becomes bigger.With induced crack's length increases,the influence of wellbore on crack front shape reduces.Influenced by bedding,when the angle between bedding and direction of maximum principal stress is more than 70°,bedding has influence on crack arrest.