Signal processing of hydrographs for the long term monitoring and verification of freshwater aquifers in the vicinity of a CCS project

Project Summary

The CO2CRC Otway project has been collecting water level data from three groundwater wells screened in the freshwater Dilwyn Aquifer since 2006.

The objective of this project was to investigate the usefulness of monitoring these wells for small leak detection, were it to occur, and to identify better locations for groundwater monitoring wells, were they to be purpose drilled. This was done through the application of spectral analysis techniques.

The water level dataset represents a continuous time-series prior to the injection of CO₂ and continuing post-injection. Spectral analysis of this dataset showed clear evidence of earth tides affecting the recorded water levels in the three wells. The spectra derived from this analysis were used to estimate aquifer properties, including specific storage and porosity. The values determined in this way compared reasonably well with published estimates from standard pumping tests and are significantly easier and less expensive to obtain.

A 2D TOUGH2 reservoir simulation model was built to investigate the spatial extent of the pressure pulse and chemical plume that would be generated were a slow, buoyancy driven, CO₂ leakage to occur from a slightly over-pressured storage reservoir. This model was originally intended to be based on the Dilwyn Formation. However, the initial simulations suggested that the current monitoring well locations in the Dilwyn Formation were too far away from the injection site to detect either pressure or chemical composition changes due to the presence of leaking CO₂.

In this model, CO₂ was allowed to move via buoyancy from the storage reservoir into an overlying monitoring aquifer. With consideration for the minimum leakage criteria of carbon storage for climate abatement, a minimum detection limit (MDL) that a shallow groundwater monitoring system would have to achieve was defined, and found to be virtually undetectable within the time frame of between 1-10 years. Further, it was found that, at the MDL, there was no detectible difference between the pressure pulse in advance of the CO₂ plume itself.

Key conclusion:

• The leakage rates in the scenarios developed here are primarily driven by buoyancy and are consequently so low as to be virtually undetectable in the short term. Therefore, for groundwater wells to provide effective short-term (less than 10 years) leakage indication or groundwater protection they would need to be located within a few metres of the site where CO₂ enters the aquifer.