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Quantifying the Effect of Injected CO2 Using Full Waveform Inversion of Vertical Seismic Profile Data: CO2CRC Otway Project Example

Monitoring of the injected CO2 is an important part of CO2 geosequestration process. Geophysical methods, in particular surface seismic investigations and vertical seismic profiles (VSPs), are commonly used for the purpose of tracking the injected CO2. Currently, surface seismic methods are applied as a default tool for monitoring due to uniform and wide illumination of the subsurface. In case of surface seismic, transition from the seismic image to the changes in rock properties introduced by the injection is possible using quantitative interpretation (QI) techniques, e.g. AVO inversion. However, surface seismic has its drawbacks, i.e. relative complexity and high cost of permanent receiver deployment and its significant surface footprint (which is of particular importance in environmentally sensitive and busy areas). VSP can partially address these issues. In case of VSP geometry, the receivers are located in a well and only the sources remain on the ground. Using VSP for monitoring is, however, complicated by the fact that conventional VSP tool occupies the space inside the wellbore and halts any other operations. VSP also has non-uniform illumination of the target, which complicates the application of standard QI techniques. Some of the recent technologies solve these problems of VSP acquisition. Distributed acoustic sensing (DAS) can be used instead of conventional receivers. It utilizes fibre optic cables as sensors, instead of commonly used geophones or hydrophones. As opposed to the traditional geophone arrays, DAS can cover the whole well. Permanent installation of fibre cables behind the well casing is possible, it improves the repeatability of the acquired data and vacates the wellbore space. For QI of VSP data, full-waveform inversion (FWI) can be used. FWI is a technology that allows one to assess the changes in medium properties by using the acquired field data directly with minimum pre-processing, which significantly reduces the number of unknowns in the processing/interpretation workflow. FWI can be applied to VSP data without any restrictions, transmitted waves recorded by VSP geometries assist FWI significantly. With these new technologies, VSP becomes a promising monitoring tool. Our tests on field data show that time-lapse FWI is possible for offset VSP geometry with multicomponent geophone receivers. Preliminary research also shows that FWI is applicable to field DAS data after its conversion to particle velocity. In this study, we use a full-earth model of the CO2CRC Otway site (Victoria, Australia) in order to perform a feasibility study of FWI on DAS VSP data for the purpose of detecting and quantifying a time-lapse anomaly introduced by a