CO<sub>2</sub>地中貯留条件下における水およびCO<sub>2</sub>を包有するBerea砂岩の多孔質弾性パラメータ

Abstract

Inversion techniques utilizing changes in elastic wave velocities, and tilt data of the ground surface, associated with migration of CO2, respectively, are promising options for short- and long-term monitoring in geologic sequestration of CO2. Poroelastic parameters of reservoir rocks (e.g., sandstone) should be well understood to increase reliability of these techniques, because these techniques are based on the poroelastic theory. In this context, the present study, that extends our recent study on Terzaghi's effective stress (σeff) dependencies in poroelastic parameters of sandstones, experimentally explores influences of fractional CO2 saturation (Sx(x=CO2)), pore pressure (Pp), and temperature (T) on poroelastic parameters of Berea sandstone containing water and CO2 under undrained condition (i.e., Skempton's coefficient B, Young's modulus, Poisson's ratio, bulk and shear moduli), to finally provide predictive equations for all poroelastic parameters at σeff of ≥2 MPa, Sx(x=CO2) of 0-1, Pp of 10-30 MPa, and T of 40-60 °C. Experimental results, and a theoretical calculation of bulk modulus of water-CO2 two-phase pore fluid show that, for the undrained poroelastic parameters, CO2 saturation, pore pressure, and temperature dependencies are, respectively, an exponential decrease, linear increase, and linear decrease, in response to the similar dependencies of the bulk modulus of the pore fluid, at the above specific conditions. This finding, in combination with the well-known Terzaghi's effective stress dependency, provides the predictive equations, which quantitatively demonstrates variability of poroelastic parameters. Additionally, an application of the predictive equations for CO2 monitoring in a reservoir consisting of a Berea-sandstone-like rock strongly recommends that fractional CO2 saturation should be smaller than approximately 0.2 (i.e., common residual saturation level) by making a clear CO2 injection strategy, and pore pressure should be continuously monitored, to quantitatively monitor migration of CO2 by elastic wave velocity measurements, with maximizing safety by the capillary trapping mechanism.