The present study is part of the CLEAN project (CO2 Large-scale EGR in the Altmark Natural-gas field), the scientific programm accompanying the EGR ('enhanced gas recovery') project accomplished by GDF SUEZ E&P Deutschland GmbH (GDF SUEZ) and Vattenfall Europe in the Altmark, Germany. It examines the CO2-induced changes in the seismic wave field as well as their measurability.
In 2D-wavefield simulations it could be shown that P-wave velocities of -1.5% and the relative changes in the S-wave velocity and density are necessary to cause amplitude changes above the threshold of 5%. This value is typical for time lapse marine seismic and was taken as threshold for the MSP/VSP-experiments. Data from the test at the KTB and results with noisy data show that this threshold is realistic. In a second step the pre-post injection difference sections were imaged to the depth section with standard Kirchhoff migration and Fresnel-Volume migration. It was tested, if a localization of the origin of these amplitude changes is possible. These migrations showed that multiples and primary reflections from interfaces below the reservoir that travelled through the CO2-saturated reservoir range also have changed amplitudes. They produce migration artefacts that can not be distinguished from a layered CO2 distribution.
In order to achieve information to guarantee an optimized setup for the field experiment, different propagation scenarios and source-receiver offsets were tested and the changes in the seismic wavefield as well as in the migrated section were analyzed. The results show that large offsets of up to 5.5km are necessary to image the whole region of the expected CO2 expansion.
Time shifts in the wavefield were detected with coda wave interferometry. With the chosen approach it is possible to detect even small mean velocity changes. However, besides the small expected changes in the velocities the small spatial expansion (especially in vertical directions) is problematic. Furthermore a small sampling rate and receivers directly above the target zone are necessary, at best above and below the zone where the velocity changes occur. This makes the method interesting for the detection of near-well leakages. The tests show that even small amounts of CO2 would be detectable.
Furthermore, the size of the CO2-injection induced changes in the physical parameters of the reservoir rocks were estimated. They suggest that the changes of -1.5% P-wave velocity, the results from the wavefield simulations, would need large pressure (∆p≈+10MPa) and/or temperature changes in addition to a complete replacement of the porefluid. These estimations are based on data of core samples from the Peckensen reservoir, a neighbouring block of the region under consideration. These results were compared with the results from reservoir simulations, another subproject of CLEAN. They suggest that changes of the pore pressure and the temperature in this dimension are unrealistic.
Finally, an optimized survey plan was carried out for the MSP/VSP experiments in the Altmark. However, as the prospects for a successful direct monitoring of the CO2 in the reservoir are low, a surface seismic with a larger spatial coverage and the focus on the monitoring of possible leakages is advisable.
Falls Ihr Browser eine Datei nicht öffnen kann, die Datei zuerst herunterladen und dann öffnen.