Zero-Offset VSP Monitoring of CO₂Storage: Impedance Inversion and Wedge Modelling at the Ketzin Pilot Site

Abstract

At the CO 2 storage pilot site near the town of Ketzin (35 km west of Berlin, Germany) the sandstone reservoir at 630 m-650 m depth is thin and heterogeneous. The time-lapse analysis of zero-offset VSP measurements shows that CO 2 -induced amplitude changes can be observed on near-well corridor stacks. Further, we investigate whether CO 2 -induced amplitude changes in the monitoring data can be used to derive geometrical and petrophysical parameters governing the migration of CO 2 within a brine saturated sandstone aquifer. 2D seismic-elastic modelling is done to test the processing workflow and to perform a wedge modelling study for estimation of the vertical expansion of the CO 2 plume. When using the NRMS error as a measure for the similarity between the modelled and recorded repeat traces, the best match is achieved for a plume thickness of 6-7 m within the reservoir sandstone of 8 m thickness. With band limited impedance inversion a velocity reduction at the top of the reservoir of 30%, influenced by casing reverberations as well as CO 2 injection, is found. The relation of seismic amplitude to CO 2 saturated layer thickness and CO 2 -induced changes in P-wave velocities are important parameters for the quantification of the injected CO 2 volume.

Vertical seismic profiling using a daisy‐chained deployment of fibre‐optic cables in four wells simultaneously – Case study at the Ketzin carbon dioxide storage site

“…; Götz et al . ). The zero‐offset vertical seismic profiling (VSP) was recorded with 3C geophones lowered into the well and clamped inside the casing at 80 depth levels, from 325 to 720 m below ground level (m b.g.l.).…”

“…This general character of the recorded VSP data applies to all four wells, and has also been observed during previous VSP surveys recorded with conventional borehole geophones in the Ktzi202 well (Götz et al . ), which are described in more detail in Section ‘Zero‐offset at the Ktzi202 well – DAS–VSP compared with geophone VSP next’. In case of the wells Ktzi200 and Ktzi203 (Fig.…”

“…At the Ketzin carbon dioxide (CO 2 ) storage site, seismic downhole measurements were also performed with conventional three-component borehole geophones (Daley et al 2013;Götz et al 2014). The zero-offset vertical seismic profiling (VSP) was recorded with 3C geophones lowered into the well and clamped inside the casing at 80 depth levels, from 325 to 720 m below ground level (m b.g.l.).…”

“…), offset and zero‐offset vertical seismic profiling (VSP) (Götz ; Götz et al . ) and cross‐hole seismic surveys (Zhang et al . ) were acquired.…”

“…1; 61 kt of CO 2 injected, Ivandic et al 2015) and 2015 (2 years after injection closure). Besides the 3D seismic, 2D (pseudo 3D) seismic (Ivandic et al 2012), moving source profiling (MSP, downhole acquisition of dense shot profiles on the surface using sparse downhole receivers; Yang et al 2010), offset and zero-offset vertical seismic profiling (VSP) (Götz 2014;Götz et al 2014) and cross-hole seismic surveys (Zhang et al 2012) were acquired.…”