Seismic velocity change in sandstone during CO₂ injection

Abstract

This paper presents analytical and experimental studies of the effects of supercritical CO2 injection on the seismic velocity of sandstone initially saturated with saline water. The analytical model is based on poroelasticity theory, particularly the application of the Biot-Gassmann substitution theory in the modeling of the acoustic velocity of porous rocks containing two-phase immiscible fluids. The experimental study used a high pressure and high temperature triaxial cell to clarify the seismic response of samples of Berea sandstone to supercritical CO2 injection under deep saline aquifer conditions. Measured ultrasonic wave velocity changes during CO2 injection in the sandstone sample showed the effects of pore fluid distribution in the seismic velocity of porous rocks. CO2 injection was shown to decrease the P-wave velocity with increasing CO2 saturation whereas the S-wave velocity was almost constant. The results confirm that the Biot-Gassmann theory can be used to model the changes in the acoustic P-wave velocity of sandstone containing different mixtures of supercritical CO2 and saline water provided the distribution of the two fluids in the sandstone pore space is accounted for in the calculation of the pore fluid bulk modulus.

Integrated thermo-poro-mechanical characterization for CO2 Sequestration at deep aquifer conditions

“…The compressional velocity changed from an initial 10991 ft/sec to 10325 ft/sec which is a 6.44% change. This is similar to results obtained by [17], [18] and [27]. Greater decreases in velocities have been observed in carbonates but at lower effective stresses [16].…”

“…𝐾 𝐹 = ∑ 𝑆 𝑖 * 𝐾 𝑖 (7) It was also shown in [17] that the Biot-Gassmann-Brie formulation was able to best describe the experimental observations.…”

“…Prior uncertainty in petrophysical parameter estimations have a significant effect on geophysical monitorability and there is a need for accurate benchmark measurements and reservoir characterization [14]. Injecting CO2 under supercritical conditions which can be sustained in the deep aquifers has inherent injectivity benefits [17]. The major challenge for laboratory experimental characterization for CCUS is performing the test at in-situ representative pressure and temperatures while accounting for the solubility between the supercritical CO2 (SC-CO2) and water and the properties of the SC-CO2 [12].…”

“…The major challenge for laboratory experimental characterization for CCUS is performing the test at in-situ representative pressure and temperatures while accounting for the solubility between the supercritical CO2 (SC-CO2) and water and the properties of the SC-CO2 [12]. There have been multiple studies aimed at characterizing the fluid substitution phenomenon in the lab space [12,14,15,16,17,18,19,20,21]. To replicate CO2 injection into the deep aquifers, most approaches involved injecting dry CO2 gas or CO2-brine solutions into brine saturated rock samples at varied conditions of temperature, pressure, and usually low effective stresses.…”

“…Reference [17] illustrated another expression for the undrained bulk modulus for a fluid saturated rock as:…”