Pressure Data in 3-D Hydrofrac Stress Measurement

对于地壳深部构造应力的原位测量，水力压裂技术是最有前途的。在本项目中，阐明了该技术中用于构造应力评价的压力数据的物理背景，并提出了几种从水力压裂压力-时间记录中确定压力数据的新方法。首先，我们引入了在流体压力作用下井筒表面起裂的新准则，并成功地定量解释了破裂压力与井筒直径和注入流量的关系，表明破裂压力可以作为有用的压力数据，提供平行于裂缝表面的应力分量的信息。其次，基于断裂力学分析了裂纹闭合后的闭合过程，发现闭合过程包括三个阶段：从裂纹停止扩展到裂纹尖端闭合，从裂纹尖端刚刚闭合到裂纹完全闭合，从裂纹完全闭合刚刚闭合到试验停止。结果表明，关井后井筒压力下降速率的反比与第一级和最后一级的流体压力呈线性关系。还表明，根据这些特征可以确定远场最小压应力。最后，基于断裂力学和流体力学，我们还分析了加压循环过程中裂纹的重开行为。研究发现，所谓的重新打开压力与裂缝真正打开时的压力并不一致。所谓的重新打开压力高度依赖于注入流量，并接近正常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.