Geological Structure and Time-Lapse Studies of CO2 Injection at the Ketzin Pilot Site, Germany

Abstract: 3D seismic time-lapse surveys (“4D seismics”) are an essential tool for large scale reservoir characterization. The target reservoir of the Ketzin pilot project for CO2 storage is a saline aquifer of the heterogeneous Stuttgart Formation (Upper Triassic) in the Northeast German Basin. The focus of this project is on testing and further developing monitoring CO2 storage technologies. For time-lapse seismic monitoring, three seismic surface sources were characterized with respect to S/N (signal to noise) ratios, signal penetration, and frequency content by analysis of raw shot gathers and stacked sections along two lines at the Ketzin site. Differences in reflectivity between these 2D lines reflect the differences in the nature of the sources tested and how they influence the signal bandwidth (resolution) and signal energy. All three sources image the target horizon. The weight drop source was recommended as the primary source for 3D surveys based mainly on logistics and cost. Results of processing, including equalization of a 4D (3D time-lapse) data set from the Ketzin site and cross-correlation, indicate that the injected CO2 can be monitored. The highly irregular amplitude response on the time-lapse data can be attributed to the reservoir heterogeneity. Time-lapse seismic processing, petrophysical measurements on core samples and geophysical logging of CO2 saturation levels allow for an estimate of the total amount of CO2 visible in the seismic data to be made. In spite of some uncertainty, the close agreement between the injected and observed amount is encouraging for quantitative monitoring of a CO2 storage site using seismic methods. By integrating seismic modeling and multiphase fluid flow simulations, the impact of the reservoir temperature on the 4D seismic data from Ketzin was estimated. The modeled time-lapse seismic differences for two temperature scenarios present in the reservoir are minor regarding the qualitative analysis. However, the influence of temperature on the volumetric estimation of the CO2 using the 4D seismic data is significant. Future issues to be considered include expanding the temperature range (34-38°C in this study) to be investigated and the resulting effects on the seismic response, as well as the role of the reservoir heterogeneity. In addition, it would be important to investigate the impact of temperature on the CO2 storage at other sites with favorable P-T conditions in the reservoir. Further seismic modeling using results of petrophysical experiments for estimating the effect of the CO2 injection at the Ketzin site on the AVA/AVO response on time lapse seismic data was performed. Two effects were considered: the CO2-saturation- and the pore-pressure-related effects. The results indicate that it is worth investigating if it is possible to discriminate between these effects on future 3D repeat surveys at the Ketzin site.