Study On Multi-fracture Propagation And Deflection Law Of Horizontal Wells In Tight Sandstone

The multi-stage fracturing of horizontal wells can form a stable main fracture and fracture network system in tight rock layers with extremely low permeability and good homogeneity.It has successfully solved the low oil and gas production in tight reservoirs and the mine strong pressure caused by hardly fracturing of dense and hard roofs in coal mines.However,in engineering site,most horizontal wells are not effective in perforation and fracture propagation,and some fractures have not propagated in the expected direction,but deflecting or even turn to the other direction.The essence of fracture deflection is that the opening hydraulic fracture will produce an induced stress field in the vicinity,which forms a new superimposed stress field with the in-situ.The new stress field will change the value and direction of the original stress.It causes the subsequent fracture expansion deflecting or turning,which affects the expected expansion path of the fracture,and poses new problems for horizontal well fracturing.Therefore,this paper analyze the stress distribution around the single-open fracture and the mechanism of mutual interference of multi-fracture stress,through the ways of theoretical model analysis,large-scale true triaxial slotting-fracturing physical experiment,and FLCA3 D numerical simulation,revealing multi-fracture from the evolution mechanism of initiation to deflection,the effect of different fracturing parameters on the deflection of multiple fractures is studied,and the following understandings are mainly obtained:(1)Based on mechanical theories of rock mechanics,elastic mechanics,stress superposition,etc.,the stress of horizontal wells under different azimuth angles is analyzed,and the theoretical model of stress distribution near single-open fracture and multi-fracture stress interference is constructed,which reveals the mechanism of the redistribution of the surrounding stress field generated by single-open fracture and mutual interference between fractures during multi-fracture propagation.(2)The multi-fracture propagation and mutual interference law under different fracturing parameters were obtained.The laboratory experiment was used to analyze the influence of different fracture spacing,displacement,and main stress difference on the multi-fracture propagation morphology.Through profile observation and pressure curve characteristics,it was found that the typical fluctuation peak of the subsequent pressure curve after multi-fracture initiation is an obvious feature of the mutual interference of fracture;the smaller the crack spacing,the larger the crack-induced stress influence area,which increases the degree of stress interference and crack deflection.The stress shadow effect is very sensitive to the fracture spacing.Under high main stress difference,it is difficult for the induced stress field to change the size of the original principal stress and reduce the angle of fracture deflecting.The subsequent pressure curve fluctuates more smoothly,and the fracture is more likely to form a transverse fracture parallel to the direction of maximum principal stress.(3)The path evolution law of multi-fracture from initiation to the final deflection process and the mechanism of suppression of intermediate cracks were revealed.The maximum and tip stress of crack propagation were analyzed by FLAC3 D numerical simulation,indicating that the fractures will grow through different evolution stage: the initial cracks,principle stress expanding,preliminary deflection,secondary principal stress propagation and deflection,severe inter-fracture interference to final turning direction.The small spacing and low stress difference are easy to make the middle cracks in the high stress concentration field by the inhibition effect on both sides,leading to the four cracks extend unevenly.Large displacement is easy to form longitudinal fractures,increasing the deflection angle and degree of multi-fracture.The middle fractures significantly inhibited deviate with large angles and tend to eventually stop expanding,while the fractures on both sides extend longer distance;The simulation calculation results are basically consistent with the laboratory physical fracturing experiment results.