In Situ Sonic and Laboratory Utrasonic Velocity Measurements: Analysis and Modeling for CO2 Sequestration in a Saline Aquifer

Robust quantitative verification and monitoring of CO₂ sequestration in geological formations based on surface seismic methods poses significant challenges. In our study we developed an analysis and modeling workflow that enables synergistic incorporation of laboratory ultrasonic velocity measurements into both characterization of a saline aquifer, targeted for pilot CO₂ injection, and fluid replacement modeling, In this study, ultrasonic measurements, well logs, thin sections and the results of Gassmann fluid replacement modeling have been integrated and analyzed and effects of varying levels of effective pressure on ultrasonic velocities of various petrophysical facies have been laboratory simulated with the aim of developing a quantitative estimates of a presumed CO₂ leakage (increase in effective pressure and decrease of CO₂ saturations). Ultrasonic velocity measurements were carried out through the use of the Ultrasonic Velocity Measurement System manufactured by GCTS Testing Systems. The set of core plugs samples used in this study are from the deep saline aquifer of Arbuckle, Sumner County, Kansas, from a newly drilled well (Wellington KGS 1-32). We compared ultrasonic velocities calculated from first arrival-time picks of P- and S-wave propagation through core plugs samples were compared with P- and S-wave velocities calculated from in situ sonic and dipole sonic well logs. Gassman fluid replacement modeling was then performed based on sonic and dipole sonic velocities and compared with results from theoretical modeling assuming different porosities. The ultrasonic velocity measurements made in this study facilitate a better understanding of modeled time lapse responses due to a number of CO₂ saturations and leakage scenarios. From the integration and analysis of the measurements and data used in this study there are implications that using time lapse-seismic monitoring and verification for CO₂ geosequestration is a feasible option for the Arbuckle saline aquifer. The use of Gassmann modeling for the different petrophysical properties is permitted through understanding of the rock data and elastic moduli, which are derived from the dry rock frame. This study provides baseline data for modeling fluid replacement effects, and also notes the change in seismic amplitude and velocity response as a function of a change in effective stress. This response makes the use of time lapse-seismic monitoring of CO₂¬ injection (in terms of leakage or saturation) a feasible option. Fluid replacement modeling for the two main petrophysical facies for the Wellington field was completed. The significance of this work lies in the integration of this modeling with the ongoing lithofacies mapping efforts of the Arbuckle and Mississippian groups. This integration will enable the study of the viability of time-lapse monitoring of CO₂ geosequestration in the Arbuckle Group. This study provided significant insight into the feasibility of achieving improved rock formation characterization and fluid replacement modeling for pilot carbon geological sequestration in a previously far less known saline aquifer than in cases of depleted hydrocarbon reservoir settings.