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

Abstract

Monitoring changes of formation properties along the well bore associated with the presence of carbon dioxide can be important for both tracking the plume inside of the primary containment and detecting leakage into the zone located above the reservoir. This can be achieved with time lapse wireline logging, but this approach requires well intervention and is not always possible. If the well is permanently instrumented with an optical fibre, it can be used as a distributed seismic receiver array to detect gas behind the casing by monitoring changes in amplitude of the seismic waves generated by active or passive seismic sources. Previous research showed the efficacy of this technique using continuous seismic sources. The Stage 3 Otway Project presented an opportunity to test this technique using passive seismic recording, as downhole fibre-optic arrays recorded numerous regional earthquakes over the period of nearly 2 years before, during, and after CO2 injection. Analysis of P-wave amplitudes extracted from these downhole gathers shows a consistent amplitude anomaly at the injection level, visible in all events that occurred after the start of injection. This indicates that the anomaly is caused by changes in elastic properties in the reservoir caused by CO2 saturation. However, extracted amplitudes show significant variability between earthquakes even without subsurface changes; thus, multiple events are required to distinguish the time-lapse anomaly from time-lapse noise. Ubiquity of these events even in a tectonically quiet region (such as Australia) makes this technique a viable and cost-effective option for downhole monitoring.

Multiwell Fiber Optic Sensing Reveals Effects of CO2 Flow on Triggered Seismicity

“…By assuming the values, σ C ∼ 30 mN=m and θ C ∼ 30°( see subsection 6.5 in Cook, 2014) and r C ∼ 10 μm, the minimum pressure inside the plume should be ∼5 kPa. This pressure may be sufficient to trigger the microseismic events on a critical fault adjacent to the Splay Fault 2, because it exceeds the strength of earth tides as well as the strongest ambient seismic noise signals at the site (Glubokovskikh et al, 2021;Shashkin et al, 2022) that have reactivated the seismogenic faults with lower critical pressure. In addition, Rothert and Shapiro (2007) showed that fluid-induced seismicity may be triggered by fluid pressure just above 1 kPa.…”

“…We estimate the earthquakes magnitudes in CRC-3 and CRC-7 using a stacked P-wave wavelet in the interval 600-800 m true vertical depths. We chose this interval, because it features (1) relatively uniform formation stiffness and its effect on the amplitudes (Pevzner, Gurevich, et al, 2020;Shashkin et al, 2022); (2) most reliable travel-time picks with neither polarity flips nor interference of various wave modes;…”

A critical review of distributed fiber optic sensing applied to geologic carbon dioxide storage

“…DASs are installed in deep monitoring wells (CRC3-CRC7) to continuously collect seismic data, allowing for the inversion of CO 2 distributions based on DAS information. 56,57 Additionally, nine surface orbital vibrators (SOV1-SOV9) stimulate seismic signals daily for 2.5 h, recording scans performed through seismic detectors buried 3 m below the source. The data are automatically collected and processed on-site, achieving the goal of intelligent processing, with results available within 2 days.…”