The Otway Stage 3 Project uses five new injection and monitoring wells in addition to existing wells onsite.
The injection zone is Para Sequence 1 (PS1) in the Paaratte formation and the injection well (CRC-3) is down dip in the reservoir to provide buoyancy for CO2 migration. The location of the wells (one injection well, CRC-3, and 4 monitoring wells, CRC-4 to CRC-7 as well as existing wells, CRC-1 and CRC-2) is shown in Figure 1.
The Otway Stage 3 Project will evaluate the following monitoring methodologies:
- Pressure inversion and tomography, based on downhole pressure sensors.
- Downhole seismic monitoring (VSP vertical seismic profiling) using well-based (DAS distributed acoustic sensors) and permanently deployed seismic sources known as (SOV surface orbital vibrators).
Surface seismic surveys will be conducted prior, during and after the injection as benchmarks to validate the results of the new technologies being tested.
The demonstration of downhole seismic monitoring involves using an array of permanent SOV to create a seismic signal which will be received by the DAS in the monitoring wells and detecting the plume as it grows and migrates during injection.
The development of ancillary monitoring methods such as using the analysis of earth tides and passive seismic will enable future geo-mechanical research to be conducted
Using high resolution pressure gauges, two distinct modalities of pressure monitoring will be investigated.
Pressure inversion locates a pressure source by way of triangulation from pressure measurements in the monitoring wells during and post CO2 injection. The acquired pressure data will be inverted to identify the compressible pressure boundary as the CO2 enters and migrates through the formation.
Pressure tomography denotes a cluster of techniques that rely on interpreting the pressure changes resulting from perturbing the reservoir with water injections at one monitoring well and monitoring pressure at other wells. In a world first, pressure tomography will be demonstrated on a CO2 plume to test the range and sensitivity of the technique to image a plume’s distribution. With downhole pressure gauges set to acquire data continuously, the pressure data obtained for each survey performed will be inverted to produce an image of the CO2 plume in the subsurface.
The quality of tomographic imaging will depend on the signal-to-noise ratio of the pressure measurements, the geometrical arrangement and number of wells, and extent to which variations in permeability affect pressure propagation.
To meet the requirements of the proposed monitoring techniques, the following well design and equipment specifications were selected.
- All wells will be cased and perforated at PS1 level.
- Down hole pressure gauges in each well.
- DAS external to the casing, and inside the well (on tubing for CRC-2 and CRC-3 wells and on wireline for CRC-4, CRC-5 and CRC-6 as a backup).
- All monitoring wells will also be used as water injectors.
- All wells designed and constructed to oil and gas specifications with a maximum surface working pressure of 3,000 psi.
- DAS external to the casing, and inside selected wells (on tubing for CRC-2 and CRC-3 wells and on wireline for CRC-4).
Downhole seismic monitoring
In 2017, as part of the appraisal program, a pair of (DAS) fibres were deployed behind casing in CRC-3. The performance was tested with both (SOV) and conventional vibroseis-generated seismic sources. The seismic data acquired was excellent in both cases. In the upcoming injection and post injection phases, the data obtained from each well for each of the SOVs will provide single-offset (VSP seismic profiles) that can be used for continuous monitoring.
Algorithms are under development for processing the data on-site in near real time, mitigating the need for large volumes of data to be transported to remote computers and significantly reducing processing time.. The analysis to date indicates that signal-to-noise ratio is more than adequate, so the emphasis is on the geometry of ensuring there are enough transects across the predicted plume in Figure 3.
Jenkins, C 2019. Otway Stage 3 – how did we get here? Oral presentation given at the CO2CRC CCUS Symposium 2019.
Jenkins, C, Dance, T, Ennis-King, J, Glubokovskikh, S, Gurevich, B, La Force, T, Marshall, S, Paterson, L, Pevzner, R, Tenthorey, E and Watson, M, 2016. Validating subsurface monitoring as an alternative option to surface M&V. In: GHGT-13, Lausanne, 114-18 November 2015.
Jenkins, C, 2016. Developments and Opportunities at the CO2CRC Otway Project. Oral presentation given at the International Collaboration in CCS, British Geological Survey, 1-3 March 2015.