Acoustic response during brine-CO2 relative permeability testing of Bluff and Entrada sandstones

We present experimental results of brine and supercritical CO2 (scCO2) relative permeability experiments with active ultrasonic monitoring at reservoir conditions in the Bluff and Entrada Sandstones (NM, USA) from outcrop, which are being considered as possible geologic carbon storage targets, at reservoir conditions. The Bluff and Entrada Sandstones have distinct differences in grain size and cementation composition and amounts. We measured their relative permeability properties curves on outcrop rock using a steady-state method at possible reservoir conditions of elevated temperature and high pressure, i.e., at 85 to 89°C and, 22.8 to 24.1 MPa pore pressure in a CoreLabs Acoustic Core holder. To better understand scCO2 plume migration and residual trapping, both drainage and imbibition relative permeability were measured with increasing and decreasing fractional flows. At every steady saturation reached, we estimated the relative permeability measurement and measured the compressional wave velocity (Vp), and two shear wave velocities (Vs and Vs*) at the 500 kHz using a multichannel pulser-receiver. We found that Vp decreased and Vs and Vs* increased slightly as brine saturation decreased. This is consistent with the literature and further supports using seismic monitoring of the CO2 plume. Most tests revealed hysteresis in Vs and Vs* indicating possibly significant lithology dependent pore-scale processes leading to isolation of portions of the rock. This suggests that, at the laboratory scale, acoustic monitoring may reveal residual trapping mechanisms, transient wetting behavior, and other fluid-framework coupling to shear waves tied to different pore structures and surface chemistry related to the primary and diagenetic fabrics of the rocks.