Numerical Simulation Research On Mechanism Of Induced Stress Perturbation In The Process Of Hydraulic Fracturing

Hydraulic fracturing technology can play an important role in the reconstruction of unconventional oil and gas resources. The difficulty in mining of unconventional oil and gas resources is rising gradually. The simple single crack created by traditional hydraulic fracturing process can not meet the requirements of production, and it's also difficult to acquire obvious stimulation effect. Volume fracturing technology represented by "Multi-cluster Staged Fracturing", "Simultaneous Fracturing", "Alternative Fracturing" generates naturally. The hydraulic fractures overlap on each other in the process of volume fracturing, and the fracture spacing decrease meanwhile. There will be perturbations between adjacent hydraulic fractures, The natural fractures can also produce an effect on hydraulic fractures.Based on the induced stress perturbation between multiple fractures, by using RFPA2D-Flow software which is based on the flow-stress-damage(FSD) coupling model, the mechanism of induced stress perturbation between multiple parallel fractures and the stress interference rule of simultaneous fracturing in jointed rock masses is simulated and investigated.A comparison of the theoretical solution and numerical results of the induced stress both parallel and normal to the fracture is made firstly, which revealing the influence law of induced stress on initial stress field. The influence mechanism of induced stress to hydraulic fracture geometry and natural fracture is analyzed further.Secondly, the stress distribution of two prefabricated fractures and two synchronously extended fractures is discussed, it is found that the small fracture spacing, long fracture length and small principal stress ratio are conducive to the appearance of stress-reversal region and complex hydraulic fracture network. The geometry of three synchronously extended fractures is analyzed afterwards, and the results show that in the case of small fracture spacing and big principal stress ratio, the middle fracture propagates more difficultly or even stop propagating soon after initiation. The small principal stress ratio is advantageous to the appearance of branching cracks parallel to the direction of original minimum principal stress direction, thus the possibility of complex fractures in the reservoir formation by the subsequent hydraulic fracturing is greater. Under appropriate fracture spacing and in-situ stress condition, the middle fracture can overcome the "extension inhibition period" and step into the "secondary extension period".Finally, a numerical simulation is carried out to study the mechanism of induced stress perturbation in the same fracturing stage of adjacent horizontal wells. Then the impact mechanism of natural fracture to hydraulic fracture is studied, and the simultaneous fracturing in jointed rock masses is simulated at the same time. The calculated results indicate that when fractures extend towards each other, the induced stress overlaps, the interference to the original stress field is stronger than a single fracture. If the horizontal principal stress difference is small, fractures in the same stage of adjacent horizontal wells more like to attach each other. The small principal stress difference, small approaching angle and high brittleness filling material are conducive to the appearance of crack direction change; As to unfilled natural fractures in en-echelon arrangement, if the angle between nature fracture and the direction of maximum principal stress is between 30°nd 60°, the initiation and instability pressure is moderate, which can effectively reduce the construction difficulty. meanwhile the range of angles are advantageous to the appearance of branching cracks. Simultaneous fracturing can obtain an efficient reconstruction to the reservoir, which has a good application prospect.