Poroelastic Parameters of sandstones with and without swelling clay minerals

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. Since the inversion technique is based on the poroelastic theory, poroelastic parameters of reservoir rocks (e.g., sandstone) should be well understood to increase reliability of the monitoring technique. For this purpose, a set of five kinds of laboratory tests has been conducted on Kimachi sandstone containing swelling clay, and Berea sandstone containing non-swelling clay 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 are determined by B-test, in which volumetric strain and pore pressure changes with confining pressure. Drained bulk modulus and poroelastic parameter H are determined by P-test and H-test, in which volumetric strain changes with confining pressure and pore pressure, respectively. Young's modulus and Poisson's ratio are determined by both drained and undrained triaxial compression tests. Confining pressure and pore pressure dependencies for the poroelastic parameters of two sandstones are evaluated separately, revealing that both dependencies may be integrated by Terzaghi's effective stress dependency. That is, every poroealstic parameter (y) is described based on the same function of the effective stress (σeff), y = a + b (1 - e -σeff/10), where a and b are constants. Some of the poroelastic parameters increase and the others decrease with the effective stress, where the effective stress dependency is much more significant at effective stresses of < 10 MPa. The remarkable difference between the results of the two sandstones is inverse effective stress dependencies in Biot-Willis coefficient, which may have been caused by the different kinds of clay (swelling and non-swelling clays).