Rock stresses measurement with high stiffness hydraulic fracturing system in ILC kitakami site

The hydraulic fracturing testing has the highest track record for techniques used in sub 1000 meter stress measurement. The predominant advantage of this method is that elastic constants such as Young's modulus and Poisson's ratio are not required for determining the value and direction of initial stress. The factors to determine the stress state are the water pressures of re-opening and shut-in of fractures around the borehole wall and its directions. With conventional methods, however, the accuracy of the derived stress states from the observation equations was often adversely affected by the lack of consideration of the remaining fluid pressure in fractures and insufficient stiffness of the pressurization system. We have developed a new hydraulic fracturing system based on a new test method concept overcoming these defects, and applied to a 300 meter deep borehole in granite. The most important improvement of the new system is an employment of a syringe pump that enables an accurate flow control of fluid with less than 1ml/min up to 30MPa. The new test system indicated pressure variations of the pressurized interval zone immediately after fluid injection, and the measured data was confirmed extremely accurate. Thus the rock stresses in ILC Kitakami site were determined quite accurately with our high stiffness hydraulic fracturing system.