Otway Stage 3To develop a monitoring and validation capability that was cost-effective, high resolution, on-demand and non-invasive.
The Otway Stage 3 Project
The Otway Stage 3 Project was conceived in 2012 with the primary objective to develop a monitoring and validation capability that was cost-effective, high resolution, on-demand and non-invasive.
This objective was realised and demonstrated through the application of the monitoring modalities of downhole seismic and pressure tomography, with other techniques such as pressure inversion, earth tides analysis and passive seismic acquisition also demonstrating value.
Through field validation at the Otway International Test Centre (OITC), these various techniques were demonstrated:
- Demonstrated accuracy through a clear agreement with the conventional monitoring technique of 4D seismic and unmatched turnaround times with new downhole seismic images available every two days and pressure tomography results available within two weeks of any survey.
- Demonstrated higher than expected sensitivity with downhole seismic detecting 300 tonnes of injected gas and pressure tomography detecting the plume at the first survey with ~5,000 tonnes of gas injected.
- Demonstrated a capability of remote operation with downhole seismic operating continuously and independently throughout the operational period (including pre injection) for 18 months and pressure tomography operations requiring only the intervention of a single site operator to perform the surveys. The entire operation was conducted during covid lockdowns with minimal site access allowed.
- Confirmed the possibility to scale up to an industrial application – through a more detailed understanding of the technologies gained during the field test, workflows have been developed to customise the technologies to industrial sites. These techniques are risk focused to provide an early warning for industrial CCS projects but are not applicable in all cases and will need to be confirmed as suitable for the site, the project objectives and the regulatory and operator requirements.
During the demonstration of these techniques many additional lessons learned and findings were made such as the ability to remotely acquire a 4D seismic survey, that pressure tomography was able to be performed without the need of stopping the gas injection, that automated processing of seismic data was possible, that earth tides proved to be a viable means to passively identify a plume approaching a monitoring well, that high definition fibre optics can be safely run and installed outside of casing without compromising well integrity, that suspending pressure gauges and fibre optics inside a well provide an accurate and cost effective means of acquiring monitoring data – and many more.
Stage 3 Scientific Outcomes and Industry Applications
The Otway Stage 3 Project achieved final investment decision in May 2019 and after 18 months of drilling and completions work, construction, commissioning and site characterisation the injection operations commenced on the 2nd December, 2020. As planned, with three separate intervals of injection and continuous monitoring activities performed, the program concluded at the end of April 2021, having safely sequestered 15,050 tonnes of buttress gas in the Paaratte saline aquifer, 1600m below ground.
With this, the CO2CRC Otway Project Stage 3 achieved its objectives: it has demonstrated the feasibility of risk-motivated monitoring methods that are spatially specific, return results quickly, and are unobtrusive at the surface. This was achieved through the pandemic and border closures, which prompted the full development of effective remote operations and data processing, especially for seismic data.
This summary is not an exhaustive list of techniques trailed and demonstrated for the Stage 3 Project, but it does describe some of the most promising – downhole seismic monitoring, pressure tomography and earth tides monitoring – as effective and low cost monitoring options for commercial CCS projects.
Downhole seismic monitoring
The downhole seismic technique, a time-lapse VSP approach using permanent sources and in-well fibre optics – the SOV/DAS system – was utilized to track the propagation of the Stage 3 plume on a day-by-day basis. The Stage 3 project was the first successful deployment of any comparable system and provided continuous, on-demand monitoring of the plume.
Critical to the success of the technique was the successful deployment of the fibres cemented securely outside of the casing. The ability to perforate the injection and monitoring wells without damaging this fibre was demonstrated on all the wells using an oriented gun system. Nevertheless, a back up fibre was deployed inside of the casing on a deviated monitoring well and suspended from the wellhead. This style of deployment not only facilitated deployment and retrieval, but also provided an analogue for the repurposing of legacy wells in operating or depleted fields.
First gas was detected after only 2 days of injection with less than 300 tonnes of gas injected. The system was sensitive enough to detect changes in the legacy Stage 2C plume as the new Stage 3 plume intersected and merged with it, and ultimately identified movement in the far east edge of the plume.
Figure 1 – Processed SOV/DAS images of the plume at stated times
A key outcome of the field test was for the first time to completely automate the in well seismic data acquisition and processing and to date, more than a petabyte of data has been acquired and analysed.
Prior to this, seismic data generated at the OITC was manually acquired in its raw format on hard disc (typically hundreds of terabytes) and transported (via postal mail or hand carry) to offsite processing facilities.
This processing then required months of time to produce the subsurface images. In contrast, the SOV/DAS system automated on-site processing of the data which meant that only 1GB /day was sent offsite (via commercial NBN) for final processing and quality checks with final images produced every 2nd day.
Figure 2 – Data processing steps showing raw data size to final migrated image files
Figure 3 – Evolution of the CO2 plume during various stages of the injection
Our confidence in the results of the SOV/DAS plume have been justified with the overlay of the 4D surface seismic results which show clear agreement between the two methods.
The SOV/DAS system was tuned to the Stage 3 injection operation to validate the technologies and system application. The analysis of the data has provided key insights into the applicability of the technique for industrial applications and general approaches for composition, data logistics and analysis approaches can be generalized and applied to different projects after the necessary customization steps.
The downhole seismic system can be scaled up spatially, but this would involve more wells or deeper wells to widen the illumination and retain the density of transects – providing a greater definition of the plume with less interpolation required. Alternatively, well counts can be kept constant, at the price of sparser transect information and more dependence on interpolation and smoothing. For geometrical reasons – based on monitoring well trajectories, downhole seismic is best suited to locating the plume more precisely along transects close to monitoring wells so locating these monitoring wells in line with key site and operational risks, is important.
4D DAS VSP was successfully deployed to verify SOV/DAS and pressure tomography (PT) findings. 4D VSP is a relatively standard tool in reservoir monitoring, however the use of multiple deviated wells allowed us to understand the level of uncertainties arising from the acquisition geometry.
Due to COVID restrictions we had to modify the acquisition strategy and align with the necessity of predominantly remote operation with most of the professional crew (observers and geophysicists) operating the acquisition equipment remotely.
This was only possible as the site was designed specifically for unmanned operation (in DAS/SOV) mode with all the required data logistics and computing facilities in place. This is (while unplanned) one of the most critical learning experiences directly applicable to most (if not all) seismic monitoring operations.
Stage 3 successfully demonstrated plume detection and location using pressure tomography over three separate monitoring surveys.
These were conducted at fixed points in the injection operation which were preselected specifically to test the technique at various plume extents, however, the surveys could have easily been applied at any point on an “on-demand” basis.
The core of the modality is pressure monitoring and for Stage 3, downhole pressure gauges were used in monitoring wells in an innovation suspension style deployment, which facilitates efficient retraction and redeployment in the event of reliability issues and provides an analogue for the re-purposing of legacy wells in existing fields.
Each survey clearly showed the growth and migration of the plume at each stage and corroborated well with the final 4D surface seismic surveys used as the benchmark for comparison. Each survey was able to be operated with minimal operator involvement and the results showing the plume image were available within weeks of each survey completion.
Subsequently, further work has been completed in understanding areas not previously investigated. Firstly, we rigorously analysed the uncertainty in the saturation inversions, providing realistic confidence estimates based on the combined experimental, post-processing and model uncertainty. We compare the results to those from seismic imaging, namely offset-VSP and 4D VSP. Generally, good agreement is found between the different monitoring modalities.
After the uncertainty quantification, efforts were made to explore the industrial scaling of pressure tomography. We use the controlling non-dimensional scaling factors to discuss conceptually the scale up of the system and well configurations that are possible. Using the available data, it’s possible to demonstrate reduced well (a single cross-well pair) monitoring at the Otway site, and key considerations therein. This can then be scaled up to simulate larger injections – e.g., 1Mtpa CO2 injection simulations – at the Otway site, and a monitoring approach to capture the large-scale migration.
Figure 4 – Overlay of PTI inverted saturation results with seismic results – rows from top to bottom are PTI surveys 4, 5.
Earth Tides Monitoring
Of the feasibility trials for possible monitoring techniques included in the Stage 3 technical objectives, earth tides monitoring made it through the various stage gates to final field implementation.
The acquisition and analysis of changes in the earth tides response at the reservoir level, has delivered low cost, opportunistic data to understand plume migration in the subsurface.
The Stage 3 investigations developed numerical models to provide useful nomograms linking the change in response in earth tide measurements with the size and distance from the plume centre. While still approximate, such tools provide useful, early and cost-effective information to help understand plume migration in the subsurface and provide an early warning in the event of the plume nearing an “at risk” area.
Figure 5 – Earth tide nomograms linking plume size and distance from the monitoring well
Earth tides are the cyclic pressure perturbations imposed on the subsurface through the movement of the moon across the earth’s surface. They can be easily and accurately measured using downhole gauges and can be modelled effectively to remove them as “noise” from conventional subsurface pressure data.
However, a detailed analysis has shown a strong reaction in these induced pressure cycles from the passage of the Stage 3 plume. There were strong responses of the earth tides’ amplitudes and phases to the creation and propagation of the plume. Simple analytical estimates of the size of these effects are possible and give useful indications of the size and proximity of the plume. Full numerical models, coupling geomechanical and fluid flow effects, predict pressure variations that are similar to the observations.
In conclusion, the Otway Stage 3 Project was conceived in 2012 as means to test and validate innovative monitoring techniques that provided operators with cost effective and reliable information that would be acceptable to both the regulators and the communities in which CCS operations were conducted. In April 2021, after years of planning, modelling and design, a significant site upgrade and 15,000 tonnes of CO2 injected into the Paaratte saline aquifer, the project has come to a conclusion and has achieved all its objectives.
The Stage 3 project has evolved a range of novel monitoring methods from a bench top design to a full field demonstration, all informed by our member’s stated need for simpler, less costly, less invasive yet more frequent monitoring.
There have been significant developments of techniques to make this possible, especially implementing “remote seismic” and the complex novel methods to invert pressure data to produce images.
While the methods have been demonstrated across relatively small spatial distances and with multiple monitoring wells in an onshore environment, the analysis of the experiment shows how to evaluate the scale-up to different situations that will be of industrial relevance.
Importantly, the field demonstration at the OITC has matured many of the underlying monitoring technologies – such as suspended pressure gauges and the fibres installed behind casing – to a degree that operators can now adopt them directly into their own operations with confidence in their reliability and the information they will provide.
The development of the primary monitoring and validation methods of pressure tomography, pressure inversion and downhole seismic, expands the range of options that are available to project proponents, and makes it possible to optimize a monitoring plan to be more specific to defined risks.
The opportunistic data from earth tides and passive seismic provide further indicative information on the condition of the subsurface that can inform operators and support their decision making process in real time.
The versatility of fibre sensors, both for strain and temperature, and possibly for pressure, has been demonstrated and constitutes a strong argument for instrumenting wells with fibre. The precision and stability of modern pressure sensors also gives new monitoring opportunities.
Permanent sensors at the reservoir depth have long-term advantages which may outweigh or offset the costs of monitoring wells. Existing wells may be repurposed with these sensors, either for reservoir monitoring, or to provide information on well integrity. Wells into non-hydrocarbon bearing formations may benefit from cheaper slimhole techniques from mineral exploitation.
As the scale and complexity of CCUS projects increases, the findings of this project can now be expanded and developed to meet particular project needs. CO2CRC remains committed to working with our members and the industry in general, to customise these methods and find ways to apply them in specific industrial applications, in offshore environments, remotely with little support and operator guidance, to support the acceleration of the CCS industry and the adoption of low carbon emission technologies.
RPT 22-6427: Pevzner, R, et al, Seismic monitoring of a small CO2 injection using a multi-well DAS array – Final report for Stage 3 Otway Project (2022)
RPT 22-6442: Jackson, S, Gunning, J, Ennis-King, J, Dance, T and Jenkins, C, CO2 storage monitoring with pressure tomography in the Otway Stage 3 project: Final report (2022)
All the above reports are avaible to CO2CRC’s Members.
To discuss Membership of CO2CRC, please contact the Strategic Partnerships Manager, Roy Anderson on email@example.com