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Hydrofracturation (Figure) allow to measure directly both the orientation of the horizontal components od stress (SH et Sh) and the magnitude of this stress (SH - Sh). This technique is based on the rock fracture in presence of fluids using a fluid pressure larger than the lithostatic pressure.




Figure 6.8. The hydrofracturation technique. A section of the borehole is isolated in-between inflating packers, and a liquid is injected in the isolated portion up to fracture (Pfract). After fracturation, pressure decreases and stabilizes at a Pic value.



Can you represent the different stages of hydrofracturation in a Mohr diagram ?


These techniques give a measurement of the local state of stress. However the local state of stress is perturbated by the borehole itself, or by the pressure induced by the packers. In certains cases, the measured stress values can be corrected, for example by modeling the stress trajectories aroung the borehole.


The fractures are viewed, either directly via their print-marks onto packers placed alon the borehole, or indirectly by imaging them (Figure).



Figure. Printmarks (recorded on a packer) of the microfractures resulting from hydrofrac-turation, and observed after extraction from the borehole.






Figure ; Images obtained through tele-cameras introduced in a borehole (1050 deep) before (left-hand side) and after (righ-hand side) hydraulic fracturation. In-between the white arrows, the imaged fracture has a ESE azimuth.



This technique is the most precise for the determination of the orientations and magnitudes of Sh and SH.




Exercice : Traduire cette légende et expliquer la succession des 3 figures suivantes qui illustrent comment la contrainte se concentre autour dun trou de forage.



Date: 2014-12-28; view: 741

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