The Otway Stage 3 Project team is gearing up for the start of the CO2 injection phase at the end of this month. From November to April 2021, fifteen thousand tonnes of CO2-rich gas from the Buttress well will be injected into the subsurface beneath the Otway National Research Facility in a phased operation. Then, over the next 18 months, a number of innovative and cost-effective techniques will be trialled to monitor the location, movement and, ultimately, the stabilisation of the CO2 plume.
One of the innovative techniques being explored is active pressure tomography, a technique which shows great potential in the development of lower cost monitoring solutions suitable to the offshore environment.
In a world first, CO2CRC will demonstrate pressure tomography on a CO2 plume to test the range and sensitivity of the technique to imaging a plume’s distribution. With downhole pressure gauges acquiring data continuously, the pressure data obtained from each survey performed will be analysed to produce an image of the CO2 plume in the subsurface. CO2CRC’s research partner, CSIRO, developed the fundamental equations for calculating how pressure signals can be used to determine the distribution of the injected gaseous CO2. One of the key benefits for industry is that the acquisition of pressure data is already done routinely with durable low-cost instruments in onshore and offshore environments. It therefore provides a novel, low-risk monitoring option.
Active pressure tomography involves the injection of fluid (in this case brine) into a subsurface well, creating a pressure signal which moves through the formation. Under the Otway Stage 3 Project, the pressure signal will be tracked at the nearby recently installed CRC 4,5,6 and 7 monitoring wells. The pressure signal gives information about the formation thickness, the reservoir fluids (brine, CO2) and the permeability and porosity of the storage formation. Through sequential injections at each well and continuous monitoring, a ‘tomographic image’ can be built up of the formation properties.
A baseline must first be established through a set of water injections that provide an understanding of the properties of the formation and how the physical and chemical properties of the rock interact with fluids (e.g. porosity, permeability). Understanding this prior to CO2 injection means that further water injections after CO2 injection will pick up the differences in these properties which can be used to map the location of the CO2 plume. The quality of imaging obtained depends on the signal-to-noise ratio of the pressure measurements, the geometrical arrangement and number of wells, and the extent to which variations in permeability affect how the pressure pulse travels through the formation.
To create an accurate baseline prior to the CO2 injection, two sets of water injections have been completed. Instrumentation installed in the monitoring wells is continuously acquiring highly sensitive surface and subsurface pressure data to further improve the baseline information.
The second monitoring solution CO2CRC is investigating as part of the Stage 3 Project is downhole seismic monitoring using a configuration of well-based distributed acoustic sensors (DAS) and permanently deployed above ground seismic sources (known as surface orbital vibrators or SOV’s). This provides an alternative or complementary monitoring method to traditional seismic surveys that use mobile above ground sources, like trucks onshore or ships offshore, to create the seismic signal. The novelty of this approach is that downhole seismic monitoring combined with stationary sources provides an on-demand, permanent monitoring solution, enabling continuous plume data acquisition, transmission, and analysis. Data processing times can be cut from months to days and the need for long regulatory permitting processes associated with traditional seismic surveys is dramatically reduced.
For commercial-scale CCS projects, successful application of these innovative CO2 monitoring techniques is likely to translate into lower costs, higher quality of information and a smaller environmental footprint. Proving and refining the readiness of these technologies and techniques for direct application in industry is fundamental to CO2CRC’s purpose.
To learn more about CO2CRC’s Otway Stage 3 Project click here.