瞬态压力分析方法在小型压裂地应力测试中的应用

小型压裂地应力测试是最可靠的深部岩体原位地应力状态测量方法,裂缝闭合压力等于最小水平主应力,故确定裂缝闭合压力是小型压裂测试分析的关键。小型压裂法分析基础为一维流体模型,考虑在一个竖直井旁产生交叉的垂直裂缝,该裂缝具有有限或无限的导流能力,这就自然引入了瞬态压力分析(PTA)的概念。尽管PTA方法已经广泛应用于石油/天然气行业,但在岩石力学工程的小型压裂测试分析中是新引入的概念,少有相关研究。首先回顾了瞬态压力分析的基本原理及其在小型压裂地应力测试中的应用;其次用偏微分方程描述小型压裂地应力测试过程中的压降过程,不同的流体状态代表张开状态的裂缝、正在闭合的裂缝以及闭合后的裂缝;这些不同的流体状态可以在压力导数曲线中表现出来,并利用这些特性识别裂缝的闭合压力;最后结合某深埋隧道小型压裂地应力测试,采用PTA方法分析确定了最小水平主应力,并与其他分析方法作了对比,为隧道工程小型压裂地应力测试分析提供一定的参考。

Application of Pressure Transient Analysis Method in Mini-Frac Test

Mini-Frac test is the most reliable in-situ testing protocol for measuring the in-situ stress condition at deep. A critical part of a Mini-Frac testing analysis is the analyzing of the fracture closure pressure which, in theory, equals to the minimum in-situ stress. The foundation of all Mini-Frac analysis is the one-dimensional fluid flow model considering a vertical well intersected by a vertical fracture with finite or infinite fracture conductivity, which naturally lead to the concept of pressure transient analysis (PTA). Although the PTA method has been widely used in the oil/gas industry for more than three decades, the application of PTA in the Mini-Frac test in rock mechanics engineering is new and few publications are available. Based on literature review, we introduce the concept of pressure transient analysis and its application in the Mini-Frac stress measurements. With the support of the partial differential equation (PDE) simulating the process of pressure falloff stage during a Mini-Frac test, we present different flow regimes describing open, closing, and closed fracture systems. These flow regimes are manifested on the pressure derivative plots which are used to identify the fracture closure pressure. In the last section of this paper, we share a real Mini-Frac test example and discuss the analysis results and compare it with other analysis methods.