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Report No.

In situ stress evaluation using cores taken from MIZ-1 borehole

Kato, Harumi*

In situ stress measurements (AE/DRA method, ASR method, DSCA method) were carried out using cores taken from MIZ-1 borehole in order to evaluate in situ state of stress from surface down to 1000m. AE/DRA method was carried out using cores taken from the depths of 291.0mabh, 455.0 mabh and 494.0 mabh. The in situ stress variation with depth obtained from AE/DRA method was different from the results of hydraulic fracturing test at Shobasama Site. The direction of principal stresses determined by AE/DRA method showed extreme dispersions, and systematic variation of the direction was not found. ASR method was carried out using cores taken from the depths of 462.7 mabh, 462.8 mabh, 502.5 mabh and 502.6 mabh. Quantitative evaluation of in situ stress was difficult because the anelastic strain recovery of cores were less than ten micro strains. DSCA method was carried out using cores taken from the depths of 298.86 mabh, 455.78 mabh and 494.09 mabh for evaluating anisotropic property and three dimensional micro crack property of Toki granite. The intrinsic linear compressibility of Toki granite showed moderate degree of anisotropy ranging between 20% and 27%. The ratio of principal stress magnitude estimated from ($$Delta$$ $$beta$$/$$beta$$$$^{I}$$) revealed that ($$sigma 1$$/$$sigma 3$$) varied between 1.5 to 2.7 and ($$sigma 2$$/$$sigma 3$$) varied between 1.2 to 2.0 although considerable dispersions were found. The direction of the maximum principal value of ($$Delta$$ $$beta$$/$$beta$$$$^{I}$$)$$_{I}$$, ($$Delta$$ $$beta$$$$^{I}$$) and ($$eta$$$$_{OI}$$) were NNW orientation in two cores taken from shallower depths (298.86 mabh and 455.78 mabh), and WNW orientation in the core taken from the deepest depth (494.09 mabh).



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