Laboratory Investigation and Characterization of the Poroelastic Response of Ultra-Tight Formations

Abstract

The poroelastic response of organic-rich formations like shales to stress and pressure changes is yet to be fully understood. The characterization of the poroelastic response of a rock is imperative to understanding the coupled deformation of the rock matrix and pore fluids. In this paper, a new technique is proposed for the laboratory measurement of the poroelastic coefficient utilizing real-time ultrasonic velocity and the comparison with volumetric strain as a function of stress. The high-resolution oscilloscope enables capturing the rock response more accurately over small stress changes. The compressional P-wave first arrival time is monitored during changes in confining stress and pore pressure and is used to obtain the effective stress coefficient (ESC). In theory, the effective stress coefficient is calculated based on accurate grain and bulk modulus measurements. It is rather difficult to capture the small changes in volumetric strain as a function of stress in ultralow permeability and porosity rocks. The difficulties are associated in the first order with the limitations in the in-vessel instrumentation, accuracy of radial and axial displacement measurements along with the required resolution to obtain representative measurements. In this paper, the effective stress coefficients measured using the automated P-wave real-time first arrival are compared against the effective stress coefficients measured using volumetric strain over a broader pressure range. Results clearly indicate the P-wave velocity to correlate well with the volumetric strain and can be utilized to estimate the poroelastic coefficient. Moreover, utilizing the compressional velocity enables measuring the ESC over small stress perturbations because of its higher sensitivity and better representativeness. The poroelastic coefficient was measured for numerous samples of distinct rock types from various conventional and unconventional plays and was found to vary between 0.2 and 1.0.