For the effective measurement of the reopening pressure in hydraulic fracturing, it is necessary to use the testing equipment with sufficiently small compliance. This limitation makes it difficult to apply the hydraulic fracturing for the measurement of the maximum stress, because the compliance of conventional equipments is generally so large. Taking account of this situation, 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 call the method the Baby Borehole Hydrofracturing or BABHY for short. In order to put the new method into practice, we developed the BABHY sonde and finally we succeeded in applying it to an in-situ test at a depth of 811 m in a vertical borehole drilled from the ground surface.
Hydraulic fracturing is suitable in principle for in-situ stress measurements at deep depths. However, as has been pointed out recently [1-3], 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 about 30 m in depth drilled from the floor of the Kamioka mine and a borehole of 80 m in depth drilled from the ground surface at Matsushiro. Last year, 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.
2. STRATEGY FOR THE MAXIMUM STRESS MEASUREMENT
2.1. Effects of system compliance on reopening pressure
Hydraulic fracturing in a vertical borehole induces fractures that will be vertical and normal to the minimum horizontal stress Sh (parallel to the maximum horizontal stress SH), if there are no natural fractures. The induced fractures close with venting and open with re-pressurization. Then there appear two kinds of critical borehole pressure, i.e. the reopening pressure Pr and the shut-in pressure Ps, which characterize the variation of borehole pressure during the test. The conventional theory states that those two pressures are related to the two stress components of SH and Sh as follows [e.g., 5]:
(mathematical equation available in full paper)