Repeatability of Land Time-lapse Seismic Surveys – Otway Project 2D Test Line Case Study

Abstract

Full range of 2D and 3D borehole and surface seismic methods are utilised for time-lapse surveys for monitoring of CO2 sequestration at Otway Basin pilot project site. Conducting land time-lapse surveys is challenging task because of typical variability of ground conditions, source-receiver coupling, ambient noise which results in poor repeatability of land seismic data. We analysed the major factors influencing repeatability of land seismic data. We also analysed both synthetic and field data for possible limitations of the surface reflection seismic method when it comes to repeatability. We show that changes in near surface conditions will produce kinematic differences but also different ground roll patterns. This will in general require slightly different parameters for processing of two successive surveys if they are acquired for vastly different soil conditions. We then conduct extensive numerical and field tests to show that the S/N variability as function of the source strength relative to the background noise level is crucial. Source type is less important for time lapse surveys as long as S/N ratio is high. Our tests included impact (weight drop) and vibrating (IVI Mini-Buggy) sources.

Seismic Monitoring of CO2 Geosequestration in Otway Basin, Australia

“…In this presentation we discuss critical processing steps that enabled successful cross-equalisation of different vintages of seismic data taken during last 10 years over Waarre C reservoir. The success of the processing and data cross-equalisation we measure by using the methodology developed by Pevzner et al (2009). Our results suggest that time-lapse seismic is also valuable tool for onshore CO2 monitoring, even in case of depleted gas reservoir.…”

Seismic Monitoring Feasibility Study of CO2 Injection into Saline Aquifer – A Case Study from Otway Basin, Australia

“…However from the field tests conducted (Pevzner et al, 2009) we have no reason to believe that the actual NRMS will be higher than 80-100%, and which should be still detectable by time-lapse seismic. Still with field data we can expect to have significant contribution of source generated noise (in the low frequency range) and possibly even lower non-repeatability than predicted here.…”

“…To be detectable, a time-lapse seismic signal caused by CO2 injection has to be greater than all other changes between baseline and monitor surveys. The repeatability analysis of the real field data acquired along 2D test line located inside of planned 3D survey identified that a low energy of the seismic source relative to the background noise will yield poor repeatability (Pevzner et al, 2009). To investigate Paaratte CO 2 injection scenario, we conducted synthetic sensitivity analysis where we analysed the effect of signal-to-noise (S/N) ratio on time-lapse signal predicted by rock-physics model.…”

Time-lapse seismic monitoring of CO2 injection into a depleted gas reservoir—Naylor Field, Australia

“…To detect changes in the elasticity of the reservoir rock caused by CO 2 injection at around 2 km depth, a high degree of survey repeatability is needed, which also presumes a high signal-to-noise ratio (as discussed in more detail by Pevzner et al, 2009) would be necessary for monitoring of CO 2 sequestration at Otway. Furthermore, considering the small reservoir extent and the relatively small quantity of CO 2 to be injected (around 66,000 t), a high-resolution seismic survey had to be implemented at this site.…”