Time‐lapse full waveform inversion of vertical seismic profile data: Workflow and application to the CO2CRC Otway project

Abstract

Vertical seismic profile (VSP) is one of the technologies for monitoring hydrocarbon production and CO2 geosequestration. However, quantitative interpretation of time‐lapse VSP is challenging due to its irregular distribution of source‐receiver offsets. One way to overcome this challenge is to use full waveform inversion (FWI), which does not require regular offsets. We present a workflow of elastic FWI applied to offset vertical seismic profile data for the purpose of identification and estimation of time‐lapse changes introduced by injection of 15,000 t of CO2‐rich gas mixture at 1.5 km depth. Application of this workflow to both synthetic and field data shows that elastic FWI is able to detect and quantify the time‐lapse anomaly in P wave velocity with the magnitude of 100–150 m/s.

Elastic full-waveform inversion of vertical seismic profile data acquired with distributed acoustic sensors

“…Our FWI workflow is similar to the workflow for the inversion of geophone VSP data (Egorov et al, 2017). We use a time-domain open-source software package IFOS2D (Köhn, 2011).…”

“…The results obtained with this workflow will also be used, in conjunction with the log data (available in the depth interval from 920 to 1650 m), to construct baseline models for the monitoring of CO2 injection in the Stage 3 of the Otway Project. The workflow for application of FWI to time-lapse borehole seismic data for monitoring CO2 sequestration is outlined by Egorov et al (2017).…”

“…FWI aims to estimate spatial distribution of physical properties using the seismic data ). FWI has been successfully applied to field VSP datasets acquired with geophones (Owusu et al, 2016;Charara et al, 1996), and has demonstrated its potential for estimating the time-lapse changes in the elastic properties from VSP data (Egorov et al, 2017;Liang et al, 2013).…”

Analysis of signal to noise and directivity characteristics of DAS VSP at near and far offsets — A CO2CRC Otway Project data example

“…Moreover, converting DAS data to particle velocity allows one to apply existing processing and imaging routines to the converted DAS data. An example of such a technique is full-waveform inversion (Virieux and Operto, 2009;Egorov et al, 2017). DAS measures strain, or its rate of change, in the optical fiber (Parker et al, 2014).…”

“…Seismic imaging of the evolution of the injected CO 2 plumes has been carried out to provide assurance that the gas is safely stored in the subsurface and to validate our understanding of the behavior of CO 2 . Stage 1 of the project, completed in 2010, utilized conventional surface 4D seismic in conjunction with 4D vertical seismic profiling (VSP) acquired with geophones to conduct primarily assurance monitoring of 65,000 t of a CO 2 /CH 4 gas mixture injection into a depleted gas reservoir at 2 km depth (Jenkins et al, 2012;Gurevich et al, 2014). The Stage 2C study successfully imaged the evolution of a small (5000-15,000 t) plume at 1.5 km depth using a buried geophone array and 4D VSP (Pevzner et al, 2017).…”