• Energy Research

Carbon capture and storage (CCS) is vital to reduce CO 2 emissions to the atmosphere, potentially providing 20% of the needed reductions in global emissions. Research and demonstration projects are important to increase scientific understanding of CCS, and making processes and results widely available helps to reduce public concerns, which may otherwise block this technology. The Otway Project has provided verification of the underlying science of CO 2 storage in a depleted gas field, and shows that the support of all stakeholders can be earned and retained. Quantitative verification of long-term storage has been demonstrated. A direct measurement of storage efficiency has been made, confirming that CO 2 storage in depleted gas fields can be safe and effective, and that these structures could store globally significant amounts of CO 2 .

AbstractResidual and dissolution trapping are important mechanisms for secure geological storage of carbon dioxide. When appraising a potential site, it is desirable to have accurate field-scale estimates of the proportion of trapping by these mechanisms. For this purpose a short single-well test has been conceived that could be implemented before large- scale injection. To test this concept in the field, a residual saturation and dissolution test sequence was conducted at the CO2CRC Otway site during 2011. The test involved injection of 150 tonnes of pure carbon dioxide followed by 454 tonnes of formation water to drive the carbon dioxide to residual saturation. A variety of methods for measuring saturation were applied to the injection zone so the results could be compared. Here we provide an overview of the field-test sequence and the measurement methods.

Abstract Monitoring and Verification (M&V) was reviewed in this journal in 2015 as part of the Special Issue to mark the tenth anniversary of the IPCC report on CCS. This article provides an update, focusing on identifying areas where there has been technical progress. Activity in CCS has continued since 2015, but the shift towards commercial utilization has altered the context for M&V. Published field experimentation, and verification with monitoring methods, has not progressed as much as was hoped. While much high-quality theoretical work has continued, especially in the area of the design of monitoring systems, an imbalance is apparent. One area where field tests have continued, and progress has been marked, is the rapid development of distributed acoustic sensing and its pairing with permanent seismic sources. Progress here has the potential to make seismic monitoring cheaper and less intrusive. Interesting proposals have been made for monitoring with pressure data, but most have not been tested. Methods of monitoring in the marine ecosystem are rapidly being adapted to the requirements of M&V. These methods are well adapted to the quantification of leakage that is mandated in some jurisdictions. Overall, the need for testing the numerous good ideas in field experiments is very apparent.

The CO2CRC Otway Project in southwestern Victoria, Australia has injected over 17 months 65,445 tonnes of a mixed CO2-CH4 fluid into the water leg of a depleted natural gas reservoir at a depth of ∼2km. Pressurized sub-surface fluids were collected from the Naylor-1 observation well using a tri-level U-tube sampling system located near the crest of the fault-bounded anticlinal trap, 300m up-dip of the CRC-1 gas injection well. Relative to the pre-injection gas-water contact (GWC), only the shallowest U-tube initially accessed the residual methane gas cap. The pre-injection gas cap at Naylor-1 contains CO2 at 1.5mol% compared to 75.4mol% for the injected gas from the Buttress-1 supply well and its CO2 is depleted in 13C by 4.5‰ VPDB compared to the injected supercritical CO2. Additional assurance of the arrival of injected gas at the observation well is provided by the use of the added tracer compounds, CD4, Kr and SF6 in the injected gas stream. The initial breakthrough of the migrating dissolved CO2 front occurs between 100 and 121 days after CO2 injection began, as evidenced by positive responses of both the natural and artificial tracers at the middle U-tube, located an average 2.3m below the pre-injection GWC. The major CO2 increase to ∼60mol% and transition from sampling formation water with dissolved gas to sampling free gas occurred several weeks after the initial breakthrough. After another ∼3 months the CO2 content in the lowest U-tube, a further average 4.5m deeper, increased to ∼60mol%, similarly accompanied by a transition to sampling predominantly gases. Around this time, the CO2 content of the upper U-tube, located in the gas cap and an average 10.4m above the pre-injection GWC, increased to ∼20mol%. Subsequently, the CO2 content in the upper U-tube approaches 30mol% while the lower two U-tubes show a gradual decrease in CO2 to ∼48mol%, resulting from mixing of injected and indigenous fluids and partitioning between dissolved and free gas phases. Lessons learnt from the CO2CRC Otway Project have enabled us to better anticipate the challenges for rapid deployment of carbon storage in a commercial environment at much larger scales.

Abstract Soil gas baseline and assurance monitoring was performed over the CO2CRC Otway Project demonstration site for six years (2007–2012). The main objective of the monitoring was assurance monitoring, to demonstrate to regulators that there were no adverse environmental impacts on soils. The use of the method for leakage detection was also of research interest. Soil gas sampling was carried out annually over a defined grid, in preparation for and during the experimental storage of 65,000 tonnes of carbon dioxide (CO2) into the Waarre C Formation at a depth of about 2 km. CO2, methane, oxygen and nitrogen were the main gases analysed. Advanced analytics included helium and δ C CO 2 13 measurements. Baseline monitoring allowed us to develop an understanding of the local variability in soil gas CO2 concentrations in anticipation of the CO2 injection. Interpretation of the concentrations of CO2 in terms of possible leakage rates (fluxes) is not possible without more detailed models and measurements of the possible transport mechanisms of CO2 to and through the vadose zone. However, the comparison of post-injection with baseline data, the examination of fixed gas relationships and C/ 13 C CO 2 12 isotopes showed that most CO2 in the soil was of biogenic origin. A deep subsurface source, which would indicate deep gas migration to the surface, was not apparent. This shows that there is little impact of the injected CO2 on the local ecosystem and, in conjunction with other monitoring data, helps to satisfy our regulatory requirements. Our work is a case study of a practical soil gas monitoring programme and illustrates both the utility and the limitations of the technique.

Abstract The ultimate purpose of carbon capture and storage is to keep CO2 out of the atmosphere. However there are some scenarios in which leakage to atmosphere may occur. Because of the large and variable level of naturally-occurring CO2, and rapid dispersion in the atmosphere, leakage to atmosphere can be difficult to detect from concentration measurements. By using prior information from risk assessments about plausible location of leaks, it is possible to design simple yet effective systems for identifying the location of a leak within a pre-defined area of surveillance. We have designed an inexpensive system of autonomous sensors and inversion methods that can locate leaks of CO2 and have tested it during a controlled release at the CO2CRC Otway site. We have implemented a Bayesian inversion method, and carefully modelled the various error sources, to quantify the detection limit of the complete system. The combination of methodologies proved effective and it, and its associated workflow, could be adapted and implemented in a variety of storage settings.

AbstractAtmospheric monitoring can in principle be used to quantify emissions of leakage from geologically stored CO2, if the locations of the leaks are known. However, large-scale industrial storage sites will have a surface footprint of many tens of square kilometres. Atmospheric tomography can be used to determine the location and area of an unknown source using an array of instruments that measure atmospheric concentrations of the leaking gas or its tracer(s). We have completed a detailed simulation study that uses synthetic concentration perturbations downwind of a source at an unknown location within an array of point samplers to test the feasibility of using ‘atmospheric tomography’ to locate the source. The results of this simulation are extremely encouraging and suggest that the limiting factor in a tomographic method of leakage location detection is largely the need to gather enough wind directions to triangulate the location of the leakage source. This may take some months, depending on weather patterns, but by hypothesis one would be searching for a slow, steady leak; intermittent or fast leaks can be detected in other, easier ways. A field experiment is in progress to verify these conclusions.

Abstract Statistical methods will be important tools in the monitoring of CO 2 storage sites, because any signals of leakage are likely to be small compared with measurement and modelling error and natural variability. To conclude that there is no leakage at a storage site necessarily involves consideration of leakage models, as proving “no leakage” as an isolated proposition is impossible. It is important in the circumstances of carbon capture and storage (CCS) to have clear and reproducible methods for testing leakage and no-leakage models against each other in the light of data, and some statistical methods are more useful than others in achieving this. This article reviews three broad approaches to the statistical interpretation of data containing a small leakage signal, with the objective of clarifying the concepts involved for practitioners in CCS who are not statisticians. Bayesian methods are given particular emphasis because they most clearly answer the questions that stakeholders ask, and give a natural framework for dealing with a comprehensive suite of models. The nature and importance of baseline data is discussed in the context of statistical interpretation of monitoring data.