Why is this research needed?

Once CO₂ is injected deep underground, monitoring its fate is an important challenge to address. This project investigates how gravity sensors could be used as a monitoring method. Gravity sensors measure changes in the Earth’s gravitational field: in this case, these would be produced by changes in mass distribution due to underground flow and accumulation of CO₂.

Quantum technology (QT) based gravity sensors utilising cold atoms have the potential to provide step-change improvements in sensitivity and measurement time, and provide the opportunity to benefit a wide range of applications across a number of sectors. One of these applications is Carbon dioxide Capture and Storage (CCS), where modelling and monitoring the longevity of CO₂ storage and accurately measuring the conditions under which it is stored present significant measurement needs and challenges. Monitoring plans will require the use of multiple sensing modalities, including geophysical technologies. Maximal benefit will be obtained by combining complementary geophysical techniques, principally seismic and gravity alongside geochemical and geological techniques. Gravity measurements offer unique capabilities for relatively long range, time-lapse sensing and have been demonstrated to be important for conformance monitoring since they are sensitive to dissolved CO₂. If the promise of quantum sensors can be realised, a UK-innovated gravity sensing technology can support the future development of CCS and potentially enable more cost-effective monitoring by using more sensitive land-based gravity sensors.

QT gravity sensors are currently being developed by the researchers at the University of Birmingham. Of particular interest is the cold-atom gravity gradiometer, which offers greatly improved rejection of ambient seismic noise compared to conventional gravity sensors. The researchers at Birmingham are currently in discussions with the British Geological Survey (BGS) about the potential deployment of their sensors during an injection trial at GeoEnergy Test Bed – GTB, a joint initiative of the British Geological Survey and the University of Nottingham, supported by the Energy Research Accelerator project. The GTB is a monitoring technology test-bed that will improve understanding and develop techniques to more effectively monitor CO₂ in the shallow subsurface.

What is this research investigating?

This feasibility study will use computer modelling to predict the gravity and gravity-gradient signals produced by the planned injection of CO₂ into the known geological formation at the GTB site. This will be an important de-risking activity which will reveal whether such a trial is likely to give useful data, and support the future application of UK cold-atom gravity sensors to CCS monitoring. Onshore CO₂ storage is undertaken at significant scale in the USA (more than half the CO₂ stored annually in geological formations takes place in the USA, mainly through CO₂-EOR operations) and in Central and Eastern Europe where depleting oil and gas fields are present.

The main objectives of the project are:

• Establish the required performance that a cold atom-based gravity sensor (a gravimeter or a gravity gradiometer) would need to monitor CO₂ injection at the UK based GTB.
• Explore, more generally, the feasibility of surface monitoring of CCS with cold atom-based gravity sensors.

What does the research hope to achieve?

This project will baseline the requirements for quantum technology-based gravity monitoring of CCS and could lead to the first demonstration of a quantum technology-based gravity sensor being used for this purpose. This would open up new applications for the quantum sensor community, with new pathways for commercialisation of ultra-precise quantum sensors. In the medium to long term, the research will benefit the CCS and geophysics communities, as a quantum enhanced sensor would provide an unprecedented amount of high-quality data, which when used in conjunction with existing monitoring technology will enable a significant leap in our ability to understand and manage CCS storage sites, enhancing understanding and making a contribution to preventing the many negative impacts expected from climate change.