CO₂地中貯留条件下における水およびCO₂を包有するBerea砂岩の多孔質弾性パラメータ

抄録

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 (σ_eff) dependencies in poroelastic parameters of sandstones, experimentally explores influences of fractional CO₂ saturation (S_x(x=CO₂)), pore pressure (P_p), 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 σ_eff of ≥2 MPa, S_x(x=CO₂) of 0-1, P_p 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.