Search

We demonstrate rapid convergence ( < 60 min) of passive seismic images down to reservoir depths (~2.0 km) at the CO2CRC Otway CO 2 geological storage research facility, Australia, using ambient seismic noise (ASN) recorded continuously with a buried geophone array. Our passive seismic images are created by applying seismic data processing and interferometry techniques, and show we can recover both surface and body waves from the ambient noise data. Using a recording time interval in which body waves dominate the ASN, we generate passive seismic images that correlate well with the major reflectors imaged by conventional activesource 3-D seismic data at the site. We present a mathematical model for image convergence, where the variance converges inversely proportional to recording time, and show for the first time an excellent agreement between a mathematical model and the observed convergence rate of interferometric images made from ASN.

4D seismic is widely used to remotely monitor fluid movement in subsurface reservoirs. This technique is especially effective offshore where high survey repeatability can be achieved. It comes as no surprise that the first 4D seismic that successfully monitored the CO2 sequestration process was recorded offshore in the Sleipner field, North Sea. In the case of land projects, poor repeatability of the land seismic data due to low S/N ratio often obscures the time-lapse seismic signal. Hence for a successful on shore monitoring program improving seismic repeatability is essential. Stage 2 of the CO2CRC Otway project involves an injection of a small amount (around 15,000 tonnes) of CO2/CH4 gas mixture into a saline aquifer at a depth of approximately 1.5 km. Previous studies at this site showed that seismic repeatability is relatively low due to variations in weather conditions, near surface geology and farming activities. In order to improve time-lapse seismic monitoring capabilities, a permanent receiver array can be utilised to improve signal to noise ratio and hence repeatability.A small-scale trial of such an array was conducted at the Otway site in June 2012. A set of 25 geophones was installed in 3 m deep boreholes in parallel to the same number of surface geophones. In addition, four geophones were placed into boreholes of 1–12 m depth. In order to assess the gain in the signal-to-noise ratio and repeatability, both active and passive seismic surveys were carried out. The surveys were conducted in relatively poor weather conditions, with rain, strong wind and thunderstorms. With such an amplified background noise level, we found that the noise level for buried geophones is on average 20 dB lower compared to the surface geophones. The levels of repeatability for borehole geophones estimated around direct wave, reflected wave and ground roll are twice as high as for the surface geophones. Both borehole and surface geophones produce the best repeatability in the 3

The next stage of CO2CRC Otway project involves injection of a small amount (around 15,000 tonnes) of CO2/CH4 gas mixture into saline aquifer (Paaratte formation) at a depth of ~1.5 km. The seismic time-lapse signal will depend largely on the formation properties and the injection scenario, but is likely to be relatively weak. In order to improve time-lapse seismic monitoring capabilities by decreasing the noise level, a buried receiver arrays can be used. A small-scale trial of such array was conducted at Otway site in June 2012. A set of 25 geophones was installed in 3 m deep boreholes in parallel to the same number of surface geophones. In addition, four geophones were placed into boreholes of 1 to 12 m depth. In order to assess the gain in signal-to-noise ratio and repeatability, both active and passive seismic surveys were carried out. The surveys were conducted in relatively poor weather conditions, with rain, strong wind and thunderstorms increasing the noise level. We found that noise level for buried geophones is on average 20 dB lower compared to the surface ones. Furthermore, the combination of active and passive experiments has allowed us to perform a detailed classification of various noise sources.

A key objective of Stage 2 of the CO2CRC Otway Project is to explore the ability of geophysical methods to detect and monitor injection of greenhouse gas into a saline formation. For this purpose, injection of some 10,000 – 30,000 t of gas mixture (80/20% CO2/CH4 by mole fraction in the supercritical state) into the Paaratte formation, a saline aquifer located at a depth of about 1,400 m is planned. Before such an injection experiment is undertaken, we assess the feasibility of geophysical monitoring using computer modeling. To examine the detectability of the plume we need to estimate the time-lapse signal and time-lapse noise. The time lapse signal is modeled using flow simulations, fluid substitution and seismic forward modeling. The level of time lapse noise is estimated from existing time-lapse 3D data in the area. The results confirm that the plume stemming from the injection of 10,000 t of CO2 / CH4 will probably be detectable, and that an increase of the injection volume to 30,000 t will give a significantly higher confidence of the plume detection.