Monitoring subsurface changes by tracking direct-wave amplitudes and traveltimes in continuous distributed acoustic sensor VSP data

Abstract

Instrumenting wells with distributed acoustic sensors (DAS) and illuminating them with passive or active seismic sources allows precise tracking of temporal variations of direct-wave traveltimes and amplitudes, which can be used to monitor variations in formation stiffness and density. This approach has been tested by tracking direct-wave amplitudes and traveltimes as part of a CCS project where a 15 kt supercritical CO2 injection was monitored with continuous offset VSPs using nine permanently mounted surface orbital vibrators (SOVs) acting as seismic sources and several wells instrumented with DAS cables cemented behind the casing. The results show a significant (from 15 to 30%) increase of strain amplitudes within the CO2 injection interval, and travetime shifts of 0.3 to 0.4 ms below this interval, consistent with full-wave 1.5D numerical simulations and theoretical predictions. The results give independent estimates of the CO2 plume thickness and P-wave velocity reduction within it.

Monitoring Injected CO2 Using Earthquake Waves Measured by Downhole Fibre-Optic Sensors: CO2CRC Otway Stage 3 Case Study

“…These observations can be clearly interpreted as an amplitude anomaly caused by the presence of injected CO 2 . The strength (and shape) of the amplitude trough varied between events but is broadly consistent with the results obtained from active seismic analysis and forward modelling [ 25 ]. More specifically, the data presented in Figure 9 show an average decrease of the DAS impedance by about 25%, which is similar but slightly larger than with active sources (~22%) [ 25 ]; this difference is likely within the uncertainty of the method.…”

“…The strength (and shape) of the amplitude trough varied between events but is broadly consistent with the results obtained from active seismic analysis and forward modelling [ 25 ]. More specifically, the data presented in Figure 9 show an average decrease of the DAS impedance by about 25%, which is similar but slightly larger than with active sources (~22%) [ 25 ]; this difference is likely within the uncertainty of the method. However, the method based on earthquake waves has a coarser vertical resolution, probably due to variability of the earthquake wave signals and raypaths in deep crust.…”

“…The Otway project is ideal for such a test, as it operates nine permanent sources of seismic energy known as surface orbital vibrators (SOVs). Analysis of SOV/DAS data over 2 years confirms the stability of this system [ 7 , 25 ]. Longer-term stability is yet to be explored.…”

“…In active seismic, sources are approximately stable over time; thus, measured strain (or strain rate) over time is directly proportional to the changes in the DAS impedance [ 25 ]. However, this approach is not directly applicable to earthquakes because strain in an earthquake wave depends first and foremost on the size of the earthquake, which varies between earthquakes and is a priori unknown.…”

“…Kazei, et al [ 24 ] expanded the theory for high-contrast medium using synthetic active VSP data. Pevzner, et al [ 25 ] demonstrated that this approach can detect the presence of CO 2 in a monitoring well by using active seismic source data from the CO2CRC Otway Stage 3 experiment, where 15 kt of supercritical CO 2 -rich mixture was injected into a saline aquifer 1.5 km beneath the surface. During the experiment, the DAS array also recorded many regional earthquakes.…”