Characterization Of Rock Local Brittleness And Study On Fracture Propagation Mechanism Of Hydraulic Fracturing In Dense Rock Mass

The influence of micro mineral composition difference of dense rock mass on macro brittleness is taken as the starting point,a quantitative characterization and evaluation method of local brittleness index based on different mineral content is established in this paper.And the expansion form of local brittle region of rock mass hydraulic fracturing is described by laboratory experiments.The hydraulic fracture propagation law model under the local brittleness evaluation method is established,and the influence of the local brittleness area of rock mass on the hydraulic fracture propagation law in the process of hydraulic fracturing is analyzed and judged combined with numerical simulation and indoor experiment.The specific research work and research results of the subject include the following aspects:Firstly,the mechanical properties and micro mineral composition of the dense rock mass of Shahejie formation are measured through experimental analysis.Combined with stepwise regression analysis method,the quantitative characterization relationship between micro mineral composition and macro mechanical properties of dense rock mass of Shahejie formation is established.Combined with the elastic parameter method,the characterization and evaluation method of rock local brittleness is established.It is found that the influence of the properties and content of minerals in the rock on the local brittleness of the rock is obvious through regression analysis.Among them,the more content of quartz and analcite,the greater the rock brittleness index.And the more calcite,dolomite,siderite and clay minerals,the smaller the rock brittleness index.The local brittleness characterization method of rock mass can discretize the distribution of reservoir brittleness.It provides a basis for the study of hydraulic fracture propagation law under local brittle conditions.Secondly,under different hydraulic fracturing conditions,the distribution of natural fractures is observed.And the propagation law of hydraulic fracture is analyzed and summarized.The results show that the different preset local brittle regions have obvious influences on the propagation pattern of hydraulic fracture.The existence of local brittle region changes the hydraulic fracture from tensile evolution and expansion mode to tensile shear mixed evolution and expansion mode,induces the extension direction of hydraulic fracture and increases the complexity of hydraulic fracture propagation.When the angle between the local brittle region and the horizontal minimum principal in-situ stress direction is small,the hydraulic fracture is easy to deflect under the influence of the induced stress field,extends perpendicular to the local brittle region and runs through the local brittle region.With the increase of the included angle of the local brittle region,the hydraulic fracture is not easy to deflect.There is not only the situation of turning directly and expanding close to the natural fracture,but also the situation of ignoring the induced stress field and expanding along the direction of the maximum horizontal main in-situ stress.At this time,the randomness of hydraulic fracture propagation direction is strong.Thirdly,the fracutre interactive propagation model of hydraulic fracture affected by induced stress field under the condition of local weak brittle region distribution is established.And the variation law of the included angle of local brittle region of rock mass on the induced steering of hydraulic fracture propagation is analyzed through numerical simulation and laboratory experiments.With the increase of the included angle of the local weak brittle region,the influence of the change of the overall brittleness of the rock on the deflection of hydraulic fractures is significantly reduced,and the propagation mode of hydraulic fractures finally changes to a single induced propagation mode.At this time,it is not easy to form the fracture network structure.Finally,through the numerical simulation method,the hydraulic fracture propagation law under the conditions of different in-situ stress difference,different brittleness distribution,different local brittleness area angle and different local brittleness area thickness is analyzed,and the laboratory experiment of hydraulic fracture propagation of multi-layer artificial square core is carried out.The results show that the local brittleness distribution of rock increases the complexity of hydraulic fracture propagation.When the brittleness of the local brittle region is too high or too low,the hydraulic fractures will deflect at a large angle.When the brittleness of the local brittle region is close to the overall brittleness of the rock,the deflection angles of the hydraulic fractures decrease obviously.And hydraulic cracks are easier to pass through the low brittle region from the high brittle region.Therefore,when there is a large difference in local brittle distribution,the reservoir is more likely to form fracture network distribution,which provides a basis for the multidirectional expansion of fractures.In conclusion,starting from the quantitative characterization of local brittleness of rock mass,this paper studies the general law of hydraulic fracture propagation of tight rock mass under different local brittleness regions.The topic and content of this subject open up a new idea and method for the study of hydraulic fracturing and fracture pattern of tight rock mass.Accurate analysis of reservoir properties makes it easier to control the propagation form of hydraulic fractures in the process of reservoir fracturing.It lays a scientific foundation for the evaluation of tight rock reservoir reconstruction effect.