Abstract
The conventional hydraulic fracturing fails in the target oil development zone (remaining oil or gas, closed reservoir, etc.) which is not located in the azimuth of maximum horizontal in situ stress of available wellbores. The technology of directional propagation of hydraulic fracture guided by vertical multi-radial boreholes is innovatively developed. The effects of in situ stress, wellbore internal pressure and fracturing fluid percolation effect on geostress field distribution are taken into account, a mechanical model of two radial boreholes (basic research unit) is established, and the distribution and change rule of the maximum principal stress on the various parameters have been studied. The results show that as the radial borehole azimuth increases, the preferential rock tensile fracturing in the axial plane of radial boreholes becomes increasingly difficult. When the radial borehole azimuth increases to a certain extent, the maximum principal stress no longer appears in the azimuth of the radial boreholes, but will go to other orientations outside the axial plane of radial boreholes and the maximum horizontal stress orientation. Therefore, by reducing the ratio between the distance of the radial boreholes and increasing the diameter of the radial boreholes can enhance the guiding strength. In the axial plane of the radical boreholes, particularly in the radial hole wall, position closer to the radial boreholes is more prone to rock tensile destruction. Even in the case of large radial borehole azimuth, rock still preferentially ruptures in this position. The more the position is perpendicularly far from the axis of the wellbore, the lesser it will be affected by wellbore, and the lesser the tensile stress of each point. Meanwhile, at a certain depth, due to the decrease in the impact of the wellbore and the impact of the two radial boreholes increases accordingly, at the further position from the wellbore axis, the tensile fracture is the most prone to occur and it will be closer to the axial plane of the two radial boreholes. The study provides theoretical support for the technology of directional propagation of hydraulic fracture promoted by radial borehole, which is helpful for planning well-completion parameters in technology of hydraulic fracturing promoted by radial borehole.
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Abbreviations
- σ H :
-
Maximum horizontal stress
- σ h :
-
Minimum horizontal stress
- σ v :
-
Overburden stress
- z :
-
The axis of wellbore
- z 1 :
-
The axis of radial borehole 1
- z 2 :
-
The axis of radial borehole 2
- R :
-
Radiuses of the wellbore
- R 1 :
-
Radiuses of radial borehole 1
- R 2 :
-
Radiuses of radial borehole 2
- L :
-
The distance between the axis of two radial boreholes 1 and 2
- β :
-
The azimuth angle of the radial borehole
- θ :
-
The azimuth angle of research point ‘M’
- σ xx , σ yy and σ zz :
-
The component of normal stress in (x, y, z)coordinate systems
- τ xy , τ yz and τ zx :
-
The component of shear stress in the rectangular coordinate system
- σ r , σ θ and σ z :
-
The component of normal stress in the cylindrical coordinate system
- τ rθ , τ θz and τ rz :
-
The component of shear stress in the cylindrical coordinate system
- ν :
-
Poisson’s ratio of the rock
- r :
-
The distance between any point in the formation and the axis of the wellbore
- P w :
-
Well internal pressure
- c :
-
The correction factor (0.9 < c < 1)
- δ :
-
Penetrability coefficient
- P p :
-
Pore pressure
- Φ :
-
Rock porosity
- a :
-
Biot porous elastic coefficient, α = 1 − C r/C b
- C r :
-
The compression rate of the rock skeleton
- C b :
-
The compression rate of the rock volume
- σ 1, σ 2 and σ 3 :
-
Three principal stresses
- σ max :
-
The maximum tensile stress
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Acknowledgements
The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51404288) and express their gratitude to the Fundamental Research Funds for the Central Universities (Grant No. 15CX02012A).
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Guo, T., Liu, B., Qu, Z. et al. Study on Initiation Mechanisms of Hydraulic Fracture Guided by Vertical Multi-radial Boreholes. Rock Mech Rock Eng 50, 1767–1785 (2017). https://doi.org/10.1007/s00603-017-1205-3
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DOI: https://doi.org/10.1007/s00603-017-1205-3