Abstract: 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.

Key words: rock mass at deep, Mini-Frac test, fracture closure pressure, PTA, flow regimes