Abstract

The characterization of the poroelastic response of a rock is key to understanding the impact of pore pressure change during depletion/injection on the coupled deformation of the rock matrix and the pore fluids. This paper evaluates the existing experimental methods of Biot's coefficient (a) determination and proposes an alternative way to directly measure the poroelastic constant. The method involves using a high precision system to continuously monitor the P-wave arrival time in real time during small perturbations in stress. The use of P-wave velocity or arrival time is evaluated as an effective gauge to capture the rock response to stress changes. Using a high precision oscilloscope with a resolution of 1 nanosecond to measure P-wave arrival time enables the measurement of α with much higher accuracy and over small stress perturbations without inducing hysteresis, especially in rocks with soft or compressible constituents. Numerous samples representing distinct rock types with varying clay, quartz, carbonate and TOC content were tested using this method. The measured α was found to vary between 0.3 and 1.0 illustrating the variability of α. Finally, the poroelastic response characterized above is tied back to the rock fabric and the rock's mineral components by comparing the poroelastic constants against mineralogical data and other rock properties to evaluate their impact.

Introduction

The coupled rock matrix and pore deformation, also known as the poroelastic effect, is vital to many applications of geomechanics encountered in the lifecycle of a reservoir. A comprehensive geomechanical analysis requires the knowledge of elastic properties (Young's modulus, Poisson's ratio, Biot's coefficient (α)); rock strength properties (cohesion, UCS, friction angle, maximum compressive strength); and the current stresses in the formations of interest. Knowledge of α and the effective stress acting on the rock plays a key role in understanding the horizontal stress contrast between formations which affects the accuracy of the fracture geometry prediction (Gu et al., 2016). Characterizing α is also critical in understanding the coupled rock deformation resulting from activity that causes the current stresses to change, which includes stimulation through hydraulic fracturing, wellbore stability and fluid injection for enhanced oil recovery or carbon sequestration (Ghassemi, 2009; Ma and Zoback, 2016; Rutqvist, 2012).

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