Processing of full waveform sonic data for shear wave velocity at the Ketzin CO2 storage site

Abstract: The accumulation of carbon dioxide gas (CO2) in the atmosphere is considered be the main cause of global warming effects. These emissions can be reduced substantially by capturing and storing the CO2. The CO2SINK project started in April 2004 in the northeast German Basin (NEGB) at the town of Ketzin near Berlin, Germany. Uppsala University is one of the main participants in the seismic part of the CO2SINK project. Full waveform sonic data were acquired in the Ktzi-201 injection well at the Ketzin CO2 storage site. The mode of logging was monopole logging. The target was the Stuttgart Formation, a saline sandstone aquifer at the depth of 500-700m. A total of 1210 shots were conducted and data were recorded on 13 channels. Receiver spacing was 6 inches (15.24 cm). The focus of the CO2SINK project was to develop the basis for the CCS technique by injecting CO2 into a saline aquifer and monitoring of the injected CO2 in the aquifer as a pilot study for future geological storage of CO2 in Europe. The objective of this study is to calculate P-wave & S-wave velocities from full waveform sonic data recorded in Ktzi-201 injection well. In hard formations, shear wave velocities can be determined directly from full waveform sonic data recorded in monopole logging. However, in slow formations like Stuttgart Formation as in the Ketzin CO2SINK project, shear wave arrivals are absent in full waveform sonic data recorded in monopole logging. In this case, shear wave velocities can be determined from Stoneley wave velocities provided that one knows the P-wave velocity in the borehole fluid. P-wave velocities were calculated by picking the P-wave arrivals on full waveform sonic data. Due to the absence of shear wave arrivals, the shear wave velocities were estimated from the larger amplitude Stoneley waves. The estimated S-wave velocities from Stoneley waves were less than the fluid wave velocity in the borehole, confirming the mode of logging was monopole and the formation is a slow formation. The reliability of shear wave velocities estimated from Stoneley waves also depends on five other parameters such as formation permeability, borehole fluid property, tool diameter, borehole radius etc.