NIRE Annual Report		1998

Objectives

Behavior of underground fractures is greatly controlled by in-situ stress state. Hence stress measurements are useful to design an artificial fracture system in the HDR, and to characterize conventional geothermal reservoirs where fractures are dominated.

Methods using a drilled rock core are considered to be convenient, especially for stress measurements at great depth. We have tried to apply the differential strain curve analysis (DSCA), which uses a drilled core specimen, on a geothermal field. DSCA was applied to estimate a stress state at Soultz-sous-Forets, test site of the European hot dry rock, in the upper Rhine graven. Four sets of cores had been chosen from different depths, 1477 m, 1589 m, 1844 m and 2191 m, in well EPS1.

Results

Fig.1 shows the directions of principal stresses of the shallower specimens, at 1477m and 1589 m, on stereo net. Shallower specimens show good agreement. The direction of the maximum principal stress, ₁ is N-E and it deviates about 30º from vertical. The intermediate principal stress, ₂ is NE-SW and it deviates 70º from vertical. The minimum principal stress, ₃ is NNW-SSE and it deviates 75º from vertical. The direction of ₁ generally agrees with an estimated direction from geological construction, i. e. high sv. The principal stress ratio of ₁, ₂ and ₃ using DSCA is 1:0.90:0.65 at 1477 m and 1589 m. Although deeper specimens show varied results, vertical stress tends to be high.

Table 1 shows calculated DSCA results in the horizontal plane. The directions of the maximum horizontal stress, _H and the minimum horizontal stress, sh are varied. Although N62E is a frequent direction of _H, N110E also occur.

The ratio of _H:_h of DSCA result at shallower depth is 1:0.78. This is lower than that of hydraulic fracturing method at the same depth in GPK1. The ratio of _H:_hat 2191m depth is 1:0.70. This is lower than that of hydraulic fracturing method but higher the ASR method.

From these results, the direction and magnitudes of principal stresses estimated from DSCA are thought to be not contradict with previous researches.

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