The worldwide intent to urgently mitigate climate change underscores the significance of CCS. However, to date, there is a major deficit in ready, or under development, commercially viable, CO₂ geological storage (CGS) resources. 

Realising the much-espoused potential of CCS requires more effective use of known CGS resources, and pursuit of sub-optimal CGS prospects. This is of particular importance for maximising CGS hubs and providing proximal CGS resources for hard-to-abate sectors. Further, technical solutions are required to address the uncertainty in the prediction of behaviour and long-term fate of the injected CO₂, to hasten resource development, safely approach reservoir technical limits, and enable effective site closure. CO2CRC, in collaboration with international partners, is actively addressing this challenge through the Otway Stage 4 Program.

Otway 4 aims to develop and demonstrate a suite of reservoir management strategies and solutions, that result in cost-effective CO₂ storage optimisation techniques, and to mature these for commercial readiness. New infrastructure is currently being installed at CO2CRC’s Otway International Test Centre to achieve this, including new bores and the CRC-8 monitoring well. From 2024, a series of CO₂ injection and monitoring operations will take place, to provide essential high-resolution data to achieve the Otway Stage 4’s storage optimisation objectives. 

This storage optimisation field R&D program comprises four key elements:

1.       Characterisation of Fine-Scale Heterogeneity and Fault Impact: Gain a deep understanding of how fine-scale heterogeneity and faults influence CO₂ distribution. The GeoCquest Field Validation (GFV) Project and Otway Shallow Fault Project are vital for this element.

2.       Innovative Performance Monitoring Techniques: Develop and test innovative monitoring techniques tailored to evaluate injection and storage performance. The innovative Distributed Strain Sensing (DSS) technique and the Seismic Monitoring Project, applied to all Otway Stage 4 injections, are the key monitoring techniques for this element. 

3.       Enhanced Injection Techniques: Develop and test novel injection techniques designed to improve CO₂ storage sweep efficiency and enhance trapping. Activities include the Microbubble Injection Project and the Surfactant Project. 

4.       Advanced Reservoir Modelling: Advance the capability to comprehensively model CO₂ behaviour and reservoir response to thermal, hydrological, geomechanical and geochemical changes. This includes the integration of the new monitoring data to conform models and the development of the capability to simulate the effects of the newly devised injection techniques.

This presentation forms the basis for several Otway Stage 4 presentations in the symposium, which delve into laboratory and modelling work to date, as well as forward R&D plans that will stem from the CO₂ injection operations commencing next year.

The GeoCquest Field Validation (GFV) project will for the first time monitor the migration of a CO₂ plume through an observation well where gas saturation will be measured using high-resolution pulse neutron logging. CO₂ plume migration and trapping will be predicted pre-injection and post-injection using the observational data. The project is part of CO2CRC’s Stage 4 Program and involves researchers from The University of Melbourne and Stanford University (USA).

The prediction of CO₂ migration has been shown to be notoriously difficult particularly in heterogeneous reservoirs due to the uncertainty in rock type distribution and sub-metre scale heterogeneity. The latter is not accounted for in conventional grid cells which are often at a scale of 10s of meter in horizontal and several meters in vertical direction. A number of different geo-models have been developed for the Paaratte Sequence 2 at the IOTC. These models are based on different modelling approaches (object-based, sequential indicator simulation), differ in grid cell sizes and partly include composite rock types and their properties accounting for small-scale heterogeneity. The different geo-models are the framework for the simulation of CO₂ migration and trapping leading to large differences in the CO2 migration behaviour. We expect models with small grid cell sizes and composite rock types will predict CO₂ migration more accurately, however, the model domain requires to be relatively small due to the large number of grid cells. Ultimately, the comparison to the GFV field experimental results will indicate which type of model is most suitable for modelling the migration of CO₂ in heterogeneous reservoirs.     

Carbon Capture and Storage (CCS) plays a vital role in climate change mitigation, yet enhancing CO₂ injectivity into geological formations remains challenging. This presentation examines the potential of surfactants in addressing this issue. It starts with the initial hypothesis that surfactants can lower interfacial tension and modify wettability between CO₂, brine, and rock, thereby affecting CO₂ injectivity. Both experimental evidence and numerical simulations lend credence to these assumptions.

The research adopts a multi-scale and multi-methodological framework. We evaluate various surfactants for their capacity to reduce interfacial tension and modify wettability. The work extends to multi-scale flow experiments, embracing advanced techniques ranging from pore-scale analyses using synchrotron light source X-ray scanners and microfluidic chips to core-scale CO₂ relative permeability assessments. Our findings suggest that surfactants can potentially enhance CO₂ saturation and relative permeability, with some experiments showing a twofold increase in relative permeability compared to the baseline.

Towards the end, the presentation will touch on initial plans to explore the feasibility of surfactant-aided CO₂ injectivity at the Otway International Test Centre (OITC).

The Otway Shallow Fault Project is a field demonstration that aims to better understand CO₂ migration in faults, by injecting a small amount of CO₂ into a shallow fault under highly controlled conditions to image the CO₂ migration behaviour and reservoir response. This experiment enables critical capabilities in the accurate representation of fluid flow in shallow faults to be developed and transferred to deeper faults.

The previous appraisal work answered key questions regarding the rock-fluid mechanics, subsurface structure through seismic and rock coring and groundwater quality. Numerical simulations of potential CO₂ were completed to assess the pathways and behaviour of the plume during and after the injection. Two bore holes, Brumbys-1 and Brumbys-2 were drilled in the previous phase to underpin the appraisal phase.

In preparation for the operation phase, where CO₂ injection operation occurs, two new boreholes Brumbys-3 and 4 are drilled. The wells are completed with fibre optics and pressure gauges outside the casing. A gravel pack is installed at the targeted injection interval of ~65 – 75 m depth in the injection well. Both wells are armed with fibre optics cables to record Distributed Acoustic (DAS), Temperature (DTS) and Strain (DSS) sensing. 

The experiment would provide an opportunity to demonstrate high-resolution, semi-continuous, combined reverse Vertical Seismic Profile (VSP) and Distributed Strain Sensing (DSS) monitoring of CO2 movement near and along a fault. In preparation for the injection phase in Q1 of 2024, a water pump test was completed to understand the injectivity of the injection well (Brumbys-3), pressure and hydraulic communication between the wells and to update the reservoir model with the collected data.

The behaviour of a small amount of gaseous CO2 is different from the larger plume in the dense phase. Significant efforts were dedicated to modelling the likely behaviour of the CO₂ plume around the Brumbys fault. The previous forward-modelled scenarios are updated with the outcome of the water injection test. Parallels are also drawn from experimental sand box models of the injection scenario to demonstrate the likely behaviour of the CO₂ during and after injection. The findings of the updated models are key in adjusting the planned monitoring solutions to enable effective imagining and detection of the injected CO₂.

A key question for the Otway Fault Project is what happens to the CO2 when it reaches the upper clay layer after migrating up Brumbys Fault? The Hesse Clay sits as a 2m thick blanket over the Port Campbell Limestone and Brumbys Fault. Permeability tests on clay core suggests the clay has very low permeability. Further, groundwater monitoring wells Brumbys 1 and Brumbys 2 are artesian (i.e. the ground water level in the wells is above the ground surface) during the high rainfall wintertime, suggesting a level of confinement. 

Shallow groundwater monitoring over a 4-year period has provided further insight into the permeability of the clay. Evidence from groundwater level response times after rainfall events and barometric response functions suggest the clay is semi-permeable. This suggests that if the controlled release experiment is conducted during dry conditions, it is likely that the CO2 gas will find leakage pathways through the clay and reach the surface above the fault zone. Surface soil flux monitoring will be employed during the experiment to assess this anticipated CO2 migration behaviour.  

Objectives and Scopes
For CO₂ geologic storage projects, faults present potential leakage risks. To better understand fluid flow behavior in and around faults, a shallow CO₂ controlled release field project will be conducted at the CO2CRC Otway International Test Centre by injecting into the Brumbys Fault. In order to de-risk the field project, in this study we conducted several sand tank experiments to model and validate field-scale simulation results prior to project commencement. 

Methods, Procedures, and Process
CO₂ will be injected at 60m below ground near the fault zone of the shallow Brumbys Fault. To replicate the field geologic pattern in the lab setting, we mixed different grain sizes of glass beads so that their capillary entry pressure ratios are equivalent to the permeability ratios measured for the various layers. We then manually packed the sand tank domain according to a 2D cross-section from the field-scale simulation domain shrunk to the size of the tank. Finally, we conducted buoyancy-driven fluid flow experiments at ambient conditions using different analog fluids and compared the results with field-scale simulations. 

Results, Observations, Conclusions
According to the results of the air/water experiments, once air (representing gaseous CO₂) enters the high-permeability fault, it rapidly migrates upward due to high buoyancy forces and accumulates underneath the capping clay layer. Therefore, the highest CO₂ saturation is expected to occur near the top of the fault, consistent with the field-scale simulation results. Even when the injection point is moved farther away from the fault, there is still a high chance that the injected gas would enter into the fault zone. Further experiments with a different analog fluid pair (Soltrol 220/glycerol-water mixture) have demonstrated that the fault leakage behavior is likely to be similar even when occurring at depth where CO₂ is a supercritical fluid. 

Significance/Novelty
Fault leakage is just one area of concern for CO₂ storage projects, and sand tank experiments provide a low-cost, visual, and dynamic way to validate field-scale simulation results before the projects start, hence boosting confidence and reducing risk. The sand tank experimental methods can also be applied to study various other aspects of CO₂ storage such as how heterogeneities, dip angles, and injection scenarios affect plume spread and stabilization, helping to further de-risk CO₂ geologic storage. 

The Shallow Fault Project aims to investigate the movement of CO₂ injection along and/or through the fault plane in the Port Campbell limestone reservoir. Significant efforts were dedicated to modelling the likely behaviour of the CO₂ plume around the Brumbys fault. In preparation for the injection phase in Q1 of 2024, a water pump test was completed to understand the injectivity of the injection well (Brumbys-3), pressure and hydraulic communication between the wells and to update the reservoir model with the collected data.

The pump test, completed in August 2023, provides valuable data on reservoir performance and injectivity.  Two wells, Brumbys-1 and Brumbys-3 were utilised for the injection and all four bores collected pressure, temperature and strain data during the test. Brumbys-1 is in communication with the formation through the screened interval at 98-110m, where it intersects the Brumbys fault. The screened interval of Brumbys-3 is open at 69-79m where the intended 10 tonne injection is to happen.

Visual checks through downhole camera showed that majority of the screened interval is covered by debris, hence potentially impacting the injectivity during the CO₂ injection operation. Well cleaning was performed prior and in-between the water test intervals and its impact was assessed against the injectivity performance.

The water injection adopted variable rates (50 litres/min to 150 litres/min) at Brumbys-1 where pressure analysis techniques are employed to evaluate the suitability of the reservoir for CO₂ injection. The results indicate that the injection well, Brumbys-3, exhibits suitable injectivity, suggesting its potential for successful CO₂ injection. 

As part of the study, efforts are made to update the previous understanding of the reservoir. By integrating data from the pump test, pressure analysis, and monitoring well, the reservoir model is updated. This model aids in predicting the behaviour of the injected CO₂ and assessing its potential implications. Notably, this project specifically investigates the movement of CO₂ injection along and/or through the fault plane.