• Energy Research

Abstract The detection and quantification of an underwater gas release are becoming increasingly important for oceanographic and industrial applications. Whilst the detection of each individual bubble injection events, with commensurate sizing from the natural frequency of the acoustic emission, has been common for decades in laboratory applications, it is impractical to do this when hundreds of bubbles are released simultaneously, as can occur with large methane seeps, or leaks from gas pipelines or undersea facilities for carbon capture and storage. This paper draws on data from two experimental studies and demonstrates the usefulness of passive acoustics to monitor gas leaks of this level. It firstly shows experimental validation tests of a recent model aimed at inverting the acoustic emissions of gas releases in a water tank. Different gas flow rates for two different nozzle types are estimated using this acoustic inversion and compared to measurements from a mass flow meter. The estimates are found to predict accurately volumes of released gas. Secondly, this paper demonstrates the use of this method at sea in the framework of the QICS project (controlled release of CO 2 gas). The results in the form of gas flow rate estimates from bubbles are presented. These track, with good agreement, the injected gas and correlate within an order of magnitude with diver measurements. Data also suggest correlation with tidal effects with a decrease of 15.1 kg d −1 gas flow for every 1 m increase in tidal height (equivalent to 5.9 L/min when converted to standard ambient temperature [25 °C] and absolute pressure [100 kPa] conditions, SATP).

Abstract Passive acoustics has been identified as an important strategy to determine underwater gas flux at natural sites, or at locations related to anthropogenic activities. The ability of an acoustic system to detect, quantify and locate a gas leak is fundamentally controlled by the Signal to Noise Ratio (SNR) of the bubble sounds relative to the ambient noise. This work considers the use of beamforming methods to enhance the SNR and so improve the performance of passive acoustic systems. In this work we propose a focused beamforming technique to localise the gas seeps. To achieve high levels of noise reduction an adaptive beamformer is employed, specifically the minimum variance distortionless response (MVDR) beamformer. The technique is demonstrated using an array of five hydrophones collecting data at the controlled CO2 gas release experiment conducted as part of STEMM-CCS (Strategies for Environmental Monitoring of Marine Carbon Capture and Storage) project. The experimental results show that the adaptive beamformer outperforms the conventional (delay and sum) beamformer in undersea bubble localisation. Furthermore, the results with a pair of hydrophone arrays show an improvement of the localisation compared to the use of one hydrophone array.

Carbon capture and storage is a key mitigation strategy proposed for keeping the global temperature rise below 1.5 °C. Offshore storage can provide up to 13% of the global CO2 reduction required to achieve the Intergovernmental Panel on Climate Change goals. The public must be assured that potential leakages from storage reservoirs can be detected and that therefore the CO2 is safely contained. We conducted a controlled release of 675 kg CO2 within sediments at 120 m water depth, to simulate a leak and test novel detection, quantification and attribution approaches. We show that even at a very low release rate (6 kg day−1), CO2 can be detected within sediments and in the water column. Alongside detection we show the fluxes of both dissolved and gaseous CO2 can be quantified. The CO2 source was verified using natural and added tracers. The experiment demonstrates that existing technologies and techniques can detect, attribute and quantify any escape of CO2 from sub-seabed reservoirs as required for public assurance, regulatory oversight and emissions trading schemes.

Legislation for offshore storage has been developing over the last decade or so and is currently most developed in Europe. Although the large-scale operating sites in Europe were started prior to the regulations coming into force, any planned sites will need to meet these regulatory requirements. Our review of monitoring experiences from both the operating sites and research at experimental injection sites and in areas of natural CO2 seepage suggest that broadly, the technical and regulatory challenges of offshore monitoring can be met. A full report reviewing offshore monitoring including tool capabilities, practicalities and costs is available from IEAGHG (released Q1 2016).

Abstract Estimating the range at which an acoustic receiver can detect greenhouse gas (e.g., CO2) leakage from the sub-seabed is essential for determining whether passive acoustic techniques can be an effective environmental monitoring tool above marine carbon storage sites. Here we report results from a shallow water experiment completed offshore the island of Panarea, Sicily, at a natural CO2 vent site, where the ability of passive acoustics to detect and quantify gas flux was determined at different distances. Cross-correlation methods determined the time of arrival for different travel paths which were confirmed by acoustic modelling. We develop an approach to quantify vent bubble size and gas flux. Inversion of the acoustic data was completed using the modelled impulse response to provide equivalent propagation ranges rather than physical ranges. The results show that our approach is capable of detecting a CO2 bubble plume with a gas flux rate of 2.3 L/min at ranges of up to 8 m, and determining gas flux and bubble size accurately at ranges of up to 4 m in shallow water, where the bubble sound pressure is 10 dB above that of the ambient noise.