Research On The Effects Of Induced Stress On Natural Cracks Opening For Multi-stage Fracturing

The main technique for unconventional reservoir fracturing is horizontal wellmulti-stage fracturing. There are lots of natural cracks in the unconventional reservoir. Inorder to improve the effect of fracturing, the cracks must be on the state of opening. In theprocess of multi-stage fracturing, the induced stress produced by previous cracks make itdifficult for natural fractures to be opening. The designed fracturing fluid pressure that notconsidering induced stress can’t make the natural cracks open, which will decrease fracturingeffect. In this paper, the following research work was done aiming at the problem.The horizontal well stress distribution calculation model in fracturing was established.The effect region of fracturing fluid pressure on in-situ stress was20time borehole diameter.When the distance away from borehole axis was less than20times borehole diameter, wherewas called near wellbore area, the in-situ stress was affected by fracturing fluid pressure.When the distance away from borehole axis was greater than20times borehole diameter,where was called far wellbore area, the in-situ stress was affected by original in-situ stress,and the stress distribution wasn’t affected by fracturing fluid pressure.According to plane strain issue in elastic mechanics theory, the induced stresscalculation model for single fracture was established. And the induced stress calculationmodel for multiple fractures in horizontal multi-stage fracturing was established bymechanical superposition principle. By calculating, the distribution of induced stress wasobtained.The stress distribution calculating model for near wellbore area and far wellbore areaconsidering induced stress were established. The force summing state of natural cracks indifferent zones was analyzed. The mechanics conditions for judging natural cracks tensiledamage and shear failure were obtained. The effects of natural fracture parameters on forcesumming state were obtained by calculating.Taking Ping267Well in Jilin Oilfield as an example, by calculating, the open mode ofnatural cracks in the zone was tensile failure. In the process of fracturing, the second andthird sections were not opened, and the reason for which was that the fracturing fluid pressurewas low, and the natural cracks was not opened. By calculating, the fracturing fluid pressureof the second and third section for forming complex cracks was15.65MPa and14.93MParespectively, which was2.41MPa and1.77MPa higher than the designed pressurerespectively. The induced stress produced in fracturing makes it hard for natural cracks to open. Inorder to make natural cracks open, in the process of fracturing design, the fracturing fluidpressure must be higher than the one not considering induced stress. A new understanding ofnatural fracture opening laws in multi-stage was obtained by the research of the paper, whichcan provide the theory basis for accurately designing fracturing fluid pressure to formcomplex fractures.