Research On The Stability Of Natural Fractures Under Complex Stress Conditions

With the increasing global demand for energy and the gradual depletion of conventional oil and gas reservoirs,the development of density oil,shale oil or shale gas and coal bed methane resources has attracted more and more attention.Although the pore and permeability conditions of unconventional reservoirs are generally poor,the natural fractures in those reservoirs are often well developed.The hydraulic fracturing technology used in this kind of reservoir is also quite different from the conventional hydraulic fracturing technology.In the process of stimulating the reservoir with many natural fractures,artificial fractures may form complex fracture network under the gradual induction of natural fractures during its propagation process,which greatly enhance the communication between well bore and formation.Thus,natural fracture is an important condition for the efficient development of unconventional reservoirs,the study of natural fracture's stability is very important.According to the different fracture scale,the effect of large-scale natural crack to the surrounding stress field is firstly studied by means of ABAQUS stimulation software,the stimulation results show that large-scale natural cracks have certain degree of impacts on the values and direction of the original stress field.Transverse or nontransverse crack as well as the angles of crack will have different influence to the stress field.Starting by the theory of fracture mechanics,this thesis designed and manufactured the original experimental apparatus which can achieve the loading of non-uniform load on one head face of test sample in triaxial compression test,combined with ultrasonic flaw identification method we studied the conditions of multi-angle weak structural plane in the equivalent and separated load environment.The experiment results show that: the 0° structural plane under two load modes are difficult to crush.The 30° structural plane can be fractured,and the max stress in equivalent load and independent load is 30 MPa and 38.7MPa respectively.The 60° structural plane is the most easy to crush,in equivalent load the final stress is 24 MPa while in the independent load the amount is rises to 30.5MPa.Ultimately we establish two-dimensional models based on extended finite element method(XFEM)to stimulate the breakage and propagation of crack with different shapes.The results obtained are in agreement with the experimental results.The stimulation results indicate that the stability of crack will be affected by the crack angle and length as well as the stress loading mode and rate.Under uniform and non-uniform loading conditions,cracks at 45 and 60 degrees are easy to be activated,the crack propagation distance is the longest and the propagation rate is the fastest;the larger the stress loading rate is,the easier the crack is activated;the larger the initial crack length is,the lower the fracture initiation pressure is.Comparing the two loading modes,the crack initiation is easier under uniform loading,but the crack propagation rate is faster under non-uniform loading.