A New Strategy and Its Verification for In-situ Measurements of the Maximum and Minimum Stresses at Deep Depths by Hydraulic Fracturing

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Hydraulic fracturing is suitable in principle for in-situ stress measurements at deep depths. However, as has been pointed out recently, its current testing systems and procedure involve essential problems which have to be solved before putting the principle into practice. The most serious problem is associated with the hydraulic compliance of fracturing systems. For measuring the reopening pressure, it is necessary to use the fracturing system with sufficiently small compliance. If not, there is no way to estimate the maximum horizontal stress from the reopening pressure assuming a vertical borehole, even though the maximum horizontal stress is the greatest concern in the stress measurement. This limitation makes it difficult to apply hydraulic fracturing for the stress measurement, because the compliance of the conventional system is so large. Taking into account of these facts, we have proposed recently a new method which will allow us to measure both of the maximum and minimum stresses by in-situ tests of hydraulic fracturing at a depth greater than a few km. We have proceeded to test and verify the method step by step so far through laboratory and field experiments using a borehole of 80 m in depth drilled from the ground surface at Matsushiro. Finally, we succeeded to apply the method to an in-situ test of hydraulic fracturing at a depth of 811 m in a vertical borehole drilled from the ground surface at Tazawako.