Processing four-dimensional vertical seismic profiling in the CO2CRC Otway Stage 4 Project

Abstract

Long-term containment assurance is critical for regulatory approval and public acceptance of geological carbon dioxide (CO₂) storage, placing strict requirements on the repeatability and robustness of time-lapse seismic monitoring over multi-year operational periods. Multi-well vertical seismic profiling (VSP) using distributed acoustic sensing (DAS) offers a compact, repeatable monitoring option; however, long-term deployments can be compromised by non-geological changes, such as altered surface optical routing, interrogator artifacts, or fiber integrity loss. The CO2CRC Otway Stage 4 Project provides a relevant test case because non-repeatable acquisition conditions occurred between the baseline and monitor surveys. This study presents the processing and initial evaluation of a four-dimensional (4D) VSP dataset acquired in February 2025, following the injection of approximately 10 kt of CO₂-rich gas into the Paaratte Formation (~1,500 m depth) via the CRC-3 injector. Processing was adapted from the previous Otway DAS-VSP monitoring workflow, with cross-vintage comparability improved through well-specific channel alignment, including correction for time-varying channel shifts. The workflow further included cross-equalization and Wiener matching to reduce residual source–signature differences and improve repeatability, wavefield separation, and in-house Kirchhoff migration. Time-lapse analysis of migrated volumes and root mean square difference attributes computed over the injection interval shows coherent anomalies at CRC-4 and CRC-5 that are geometrically consistent with the Stage 4 injection interval, although no usable 4D data were available from the injector well CRC-3. CRC-6 exhibits no coherent anomaly. An anomaly near CRC-7 is spatially separated from the injector and is likely due to the continued evolution of CO₂ plumes from previous Otway injections. These results demonstrate that multi-well 4D DAS-VSP can provide robust qualitative detection and delineation of plume-related seismic anomalies under substantial acquisition non-repeatability, strengthening conformance monitoring workflows for long-term CO₂ storage.

Keywords

Four-dimensional vertical seismic profiling / Distributed acoustic sensing / Carbon dioxide storage

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