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

水力压裂原则上适用于深部地应力测量。然而，正如最近指出的那样，它目前的测试制度和程序涉及一些基本问题，在将该原则付诸实施之前必须解决这些问题。最严重的问题与压裂系统的水力顺应性有关。为了测量重开压力，必须使用柔度足够小的压裂系统。如果不是，即使最大水平应力是应力测量中最关心的问题，也无法根据假设的垂直井眼的重新打开压力来估计最大水平应力。这种局限性使得水力压裂在应力测量中的应用变得困难，因为常规系统的柔度很大。考虑到这些事实，我们最近提出了一种新的方法，使我们能够通过在深度超过几公里的现场水力压裂试验来测量最大和最小应力。到目前为止，我们已经通过实验室和现场实验，利用松下地面钻出的80m深的钻孔，逐步检验和验证了该方法。最后，我们成功地将该方法应用于Tazawa ko地面垂直钻孔中811m深度的水力压裂现场试验。

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.