| dc.description.abstract | CO2 storage is considered as one of the most promising solutions to overcome global increase in temperature. In order to ensure the safety of the sequestrated CO2, several geophysical monitoring methods are required, among them electrical resistivity and seismic velocity which depends on porosity, texture, mineralogy and fluid saturation. However, seismic data seldom allow us to estimate saturation accurately in geological reservoirs. On the other hand, resistivity is very sensitive to porosity and help to calculate fluid saturation. A combined use of both electrical resistivity and seismic velocity enables two complementary measurements: Seismic velocity measurement, which has better resolution efficiency in fluid mapping in geological reservoirs compared to electrical resistivity while the latter has a better precision in terms of quantifying relative saturation levels of immiscible fluids. The main challenge is to combine these two data sources to monitor CO2 storage where water saturation varies due to CO2 injection. Using an advanced experimental setup, a series of laboratory experiments have been carried out to monitor P-wave velocity and resistivity simultaneously in selected porous sandstones during liquid CO2 injection. The sandstones studied are medium to fine grained Red Wildmoor (RW) and medium grained Berea. For the Red Wildmoor, two core plugs: one drilled perpendicular and the other drilled parallel to the layering were used. There experiments were conducted by simulating the reservoir conditions at depth of about 1000 m. A constant pore pressure of 10 MPa and confining pressure of 25 MPa were maintained throughout the flooding of liquid CO2. Prior to CO2 flooding, the sandstone core plugs were saturated with CO2 and brine. Multidirectional acoustic velocity and resistivity measurements were then taken during drainage and imbibition processes. The laboratory results were compared with Gassmann s model based on CO2 saturation estimated by Resistivity Index (RI) assuming that the samples were brine saturated prior to CO2 injection. The results show that resistivity increased throughout the injection process and the P-wave velocity decreased drastically after the start of CO2 injection. It is observed that the layering of core plugs influenced the fluid distribution pattern and saturation level. The observed velocities are in good agreement with predicted velocities using the Gassmann Fluid Substitution Model with the exception of the Berea sandstone. By comparing the velocity- saturation relation estimated by Gassmann and RI models, P-wave velocities becomes less sensitive after injecting 2 PV CO2 for vertical Red Wildmoor, 1 PV for horizontal Red Wildmoor and 0.4 PV for Berea while resistivity kept increasing with increase in CO2 saturation. The study shows that electrical resistivity measurements can efficiently track the development of CO2 front during injection and effectively complement the difficulty of P- wave velocity on quantifying the stored CO2. | eng |