In-situ stress measurements were carried out under a 290m overburden in the Kamaishi mine in Iwate Prefecture of Japan. The geology of the measurement site consists of granite. The Hydraulic Fracturing (HF) technique and the Compact Conical- ended Borehole Overcoring (CCBO) technique were applied in the same horizontal borehole. The three-dimensional stress state as measured by CCBO is explained below. The directions of the maximum principal stress (σ1) and the intermediate principal stress (σ2) are almost horizontal. The direction of σ1 is perpendicular to the borehole axis, and the direction of σ2 is parallel to the borehole axis. The direction of the minimum principal stress (σ3) is approximately vertical. In this case, σ1/σ3 ratio is about 5 which is considered to be beyond the scope of the application of the HF technique. To observe the behavior of re-opening and shut-in under a large principal stress ratio, we tried to apply the HF test under these conditions. While, the re-opening pressure could not be evaluated, the shut-in pressure was observed. The results confirmed that σ3 estimated by HF is substantially equal to the vertical stress evaluated by CCBO.

1. Introduction

After the 2011 Tohoku-oki earthquake, in-situ rock stress was measured repeatedly at a drift in the Kamaishi mine at a depth of 290m [1]. Stress was measured conducted by the Compact Conical-ended Borehole Overcoring (CCBO) technique [2–4]. The magnitude of the rock stress measured one year after the earthquake was more than twice the values measured at nearby locations in the same drift before the earthquake. To confirm the reliability of the values measured after the earthquake, we tried to check the vertical stress using the Hydraulic Fracturing (HF) method. In this case, the σ1/σ3 ratio in the plane perpendicular to the borehole axis is about 5, which was considered to be beyond the scope of the HF application. If HF tests are performed under this stress condition, they should not be used to measure the reopening pressure of artificial fractures. However, since we were interested in the relationship between pressure and flow rate in the re-opening test, we performed this test anyway. While the reopening pressure was not used, the shut-in pressure was compared to the stress component measured by the CCBO. As a result, we found that the rock stress measured by the CCBO test at five years after the earthquake was highly reliable.

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