Experimental Determination of Poroelastic Properties of Utah FORGE Rocks

In this paper we present the results of a laboratory experimental study to measure the poroelastic properties of Utah FORGE reservoir rock. The target formations are granite and granodioritic rocks of low permeability so that the determination of their poroelastic properties is particularly challenging. In this work, we present the measurements of the Biot’s effective coefficient α and the Skempton’s B. The measurements on α are made by using strain gauges by following two different approaches: (1) α =1-K/Ks, K is the drained bulk modulus and Ks the grain bulk modulus; (2) α =-K/H, in which H is the so-called poroelastic expansion coefficient. These approaches yield two sets of data which can be compared. The Skempton’s B measurement is based on the equation α=dPp/dPc(undrained condition), i.e., the ratio between pore pressure change and confining pressure change under an undrained condition, and distilled water is used as the pore fluid. The test results show that both the Biot’s coefficient and Skempton’s B are all stress dependent and decrease with the increase of the effective stress. Furthermore, we observe that the Biot’s coefficient falls in the range of [0.5, 1) rather than [0, 1) for many different types of rock, including the rock samples in this research.