Pressure Data in 3-D Hydrofrac Stress Measurement

在地壳深部构造应力的原位测量中，水压致裂法是最有前途的方法。本文阐明了构造应力估算技术中所用压力数据的物理背景，提出了几种由水压致裂过程中的压力-时间记录确定压力数据的新方法。首先，我们介绍了一个新的标准，从井筒表面受到流体压力的裂纹萌生，并成功地定量解释了依赖于井筒直径和注入流量的破裂压力，破裂压力可以是有用的压力数据带来的信息的应力分量平行于裂纹表面。其次，根据断裂力学理论分析了裂缝闭合过程，发现裂缝闭合过程包括三个阶段：裂缝停止扩展到裂缝尖端闭合，裂缝尖端闭合后到裂缝完全闭合，裂缝完全闭合后到停止试验。结果表明，关井后井筒压力下降速率的倒数与关井初期和关井末期的流体压力呈线性关系。它还表明，远场最小压应力可以确定这些特性的基础上。最后，基于断裂力学和流体动力学理论，分析了再加压过程中裂纹的再张开行为。发现所谓的再张开压力与裂纹真正张开时的压力并不一致。所谓的重新打开压力高度依赖于注射流速，并且接近垂直作用于裂纹的压应力的大小。

For in situ measurement of tectonic stress in the deep earth's crust, hydraulic fracturing technique is most promising. In the present project, physical background of the pressure data which are used in the technique to evaluate the tectonic stresses are clarified and several new methods for determining the pressure data from pressure-time records during hydraulic fracturing are proposed. Firstly, we introduced a new criterion for initiation of a crack from a wellbore surface subjected to a fluid pressure and succeeded to explain quantitatively the dependency of breakdown pressure on the wellbore diameter and also on the injection flow rate to show that the breakdown pressure can be useful pressure data bringing the information on the stress component acting parallel to the crack surface. Secondly, we analyzed, based on the fracture mechanics, closure process of the crack after shut in to find that the closure process consists of three stages : from cessation of crack growth until crack tip closure, from just after crack tip closure until complete crack closure, and from just after complete crack closure until the test is stopped. It is revealed that the inverse of the wellbore pressure decrease rate after shut-in is linear with respect to the fluid pressure in the first and the last stages. It is also shown that the far field minimum compressive stress can be determined on the basis of these characteristics. Finally, we also analyzed the reopening behavior of the crack during repressurization cycles, based on fracture mechanics and fluid dynamics. It is found that the so-called reopening pressure does not coincide the pressure at which the crack really opens. The so-called reopening pressure is highly dependent on the injection flow rate and approaches the magnitude of the compressive stress acting normally to the crack.