砂岩の多孔質弾性パラメータにおけるTerzaghiの有効応力依存性

Abstract

A long-term monitoring of carbon dioxide (CO2) in a reservoir at depth is required for the geological storage of CO2. For this requirement, an inversion technique utilizing tilt data of the ground surface associated with migration of CO2 may be one of promising techniques. Poroelastic parameters of reservoir rocks (e.g., sandstone) should be well understood to increase reliability of the monitoring technique, because the inversion technique is based on the poroelastic theory. Better understanding of poroelastic parameters also contributes to geomechanical reservoir characterizations. For this purpose, focusing on a water-saturated part within a sandstone reservoir, a set of five kinds of laboratory tests has been conducted on Kimachi sandstone and Berea sandstone, saturated with water, to determine poroelastic parameters at various combinations of confining pressure (7-40 MPa) and pore pressure (5-25 MPa), namely various Terzaghi's effective stresses (2-35 MPa). Skempton's coefficient B and undrained bulk modulus have been determined by the B-test, in which volumetric strain and pore pressure changes with confining pressure. Drained bulk modulus and parameter H (inverse number of poroelastic expansion coefficient) have been determined by the P-test and the H-test, in which volumetric strain changes with confining pressure and pore pressure, respectively. Young's modulus and Poisson's ratio have been determined by both drained and undrained triaxial compression tests. Confining pressure and pore pressure dependencies for the poroelastic parameters have been examined separately, revealing that both dependencies may be integrated by Terzaghi's effective stress dependency. That is, every poroealstic parameter (y) having Terzaghi's effective stress dependency may be described by a function of Terzaghi's effective stress (σeff), y=a+b(1-e-σX(X=eff/10)), where a and b are constants. Additionally, it has been revealed that Biot-Willis (effective stress) coefficient of sandstone with swelling clays can exceed unity at relatively high Terzaghi's effective stresses.