Laboratory measurements of ultrasonic P- and S- waves velocities and strain have been performed on a dolostone under dry and CO2 saturated conditions at different stress levels. The current work attempts to assess the effects of the CO2''s varying phase state on the static and dynamic bulk moduli. Experiments were conducted under a constant differential pressure of 15 MPa (difference between the confining and the pore pressure), so the seismic effects recorded would be reflective of changes in the pore fluid properties. Results show that P- and S- waves are sensitive to changes in the pore space and CO2 phase changes (gas to liquid and gas to supercritical fluid) are evident from signal variations. Stress-strain curves of the CO2 saturated sample indicate an increase of the strain with pore pressure. This is likely attributed to the closure of isolated pores (that means pore not connected to the inter-crystalline pore space available for fluid flow) with increasing confining stress. The static bulk modulus of the saturated sample decreases with pore pressure. The dynamic bulk modulus also decreases with pore pressure but recovers when CO2 turns from gas to liquid or to supercritical fluid in the rock pore spaces.
Generally, there will be a difference between static and dynamic bulk moduli and this is usually ascribed to the difference in strain amplitude. Another potential cause for the difference between the two moduli is the presence of cracks because the difference appears to diminish at high confining pressure [1, 2]. Today, increasing interest for CO2 sequestration will require further understanding of the effects of pore fluid on elastic properties. This is quite critical as free CO2 at depths intended for CO2 sequestration can be in gas, liquid or supercritical phase states.
