Study Of Rock Mass Nonlinearity Damage Evolutionary On Hydraulic Fracturing

The precise description of hydraulic fracturing crack formation and distribution is an important factor for optimizing fracture operation parameters and improving the effect of fracture. The study is aimed at mainly single fracture mechanics and crack propagation based on the hydraulic cracks now. However, The massive practical applications show that the hydraulic fracturing forms not single fracture,but zone of local damage owing to many tiny cracks growth,expansion and concourse.It is difficult to simulate or describe accurately and reasonably underground reservoir actual break process. In order to solve this problem, the deeper research and exploration is needed for hydraulic fracture actual shape, fracture mechanism from a new theory view. Damage characteristics and micro cracks forming process of hydraulic fracture rock are studied based on damage mechanics in this paper.; The study of stress distribution around wellbore rock is carried on based on damage mechanics;The study of rock nonlinear damage mechanics and nonlinear damage evolution in the process of hydraulic fracturing are carried on, the research method of the description of hydraulic fracturing rock mass destruction and micro crack formation is built up, and the new study and calculation method is provided for making up for a deficiency of fracture mechanics research hydraulic fracturing. Main work and research results are as follows:Firstly, double medium rock seepage effects is considered in this paper, wellbore area is divided into breach zone,the damage zone and the elastic zone, the model and calculation of rock stress distribution brech radius and damage radius are built up based on damage theory,Which lays a foundation for hydraulic fracturing rock damage mechanics model and nonlinear damage evolution.Secondly, it is considered that hydraulic fracturing loads rock by the way of dynamic and static combinations, dynamic stress intensity factor calculation method is put forward, the model of micro crack formation and rock stress distribution of nonlinear damage degradation under the hydraulic fracturing dynamic loading are built up.Moreover,Taking no account of the energy loss, according to the principle of energy conservation, the constitutive model of rock damage degradation in the process of hydraulic fracturing is built up. Rock damage variable, rock damage strain and the relationship between the elastic modulus and protected are calculated and analyzed. And it is assumed that in the process of loading release energy prompted new micro cracks evolution, micro crack is formed by the furcation-growth- furcation model, the constitutive model of micro cracks growing number and length bifurcation damage evolution at any fracturing stage is built up. The calculation results tally closely with previous experimental results.The model of evolution and growth distribution shape of fracture on any fracturing stage is established,the accessory microfracture of"branched"shape aroud main fracture is found as evolution and growth of fracture of rock mass is obtained .the boundary primary bifurcation angle on branched and initial microfracture and the rule on bifurcation angle with bifurcation series is established. The model of fracture open degree is builted, and calculating the open degree of rectangle,ellipse and wedge microfracture and the model is simulated and verified through finite element method , the result coincides well.The model of growth density and growth length of microfracture of rock mass on hydraulic fracturing are built; The bifurcation amount,bifurcation density and interval of microfracture are obtained; the change rule of growth length of microfracture with pump pressure and initial microfracture.At last, the model on permeability tensor on rock mass on hydraulic fracturing is built based on morphological model,density model and length model, simulating the deliverability of an oil well of Xinmin block of Jilin oilfield using finite element software.the result show that the permeability tensor based on damage theory calculate is more coincide the practice.In this paper hydraulic fracturing rock damage and micro cracks formation are studied by innovative use of damage mechanics theory, a new research direction is opened up for hydraulic fracturing rock damage characteristic and fracture distribution status. This paper from the rock micro-to-macro point of view shows that rock macroscopy major fracture extendibility and expandability are the physical connotation of rock microscopic damage deterioration and micro cracks evolution and development. It is concluded that the major fracture extendibility and expandability is the macroscopy token of micro cracks microscopic damage evolution. why the single fracture extendibility and expandability in the hydraulic fracturing process studied by use of fracture mechanics exist errors compared with practical engineering is explained in this paper. The more mature and reliable scientific evidence is provided for more profoundly understanding the rock fracture mechanism in the hydraulic fracturing process, the vigorous theoretical guidance and technical support are provided for further improving hydraulic fracturing design, which has great academic value and practical significance.