What have we learnt from CO2 release field experiments, and what are the gaps for the future?

Roberts, Jennifer J. and Stalker, Linda (2020) What have we learnt from CO2 release field experiments, and what are the gaps for the future? Earth-Science Reviews, 209. 102939. ISSN 1872-6828 (https://doi.org/10.1016/j.earscirev.2019.102939)

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Abstract

Legislation and guidelines developed for Carbon Capture and Storage (CCS) have set performance requirements to minimize leakage risk, and to quantify and remediate any leaks that arise. For compliance it is necessary to have a comprehensive understanding of the possible spread, fate and impacts of any leaked CO 2, and the ability to detect and quantify any CO 2 seepage into marine or terrestrial environments. Over the past decade, a number of field scale CO 2 release experiments have been conducted around the world to address many of the uncertainties regarding the characteristics of near-surface expression of CO 2 in terms of the impact and quantitation of CO 2 leaks. In these experiments, either free phase or dissolved CO 2 was injected and released into the shallow subsurface so as to artificially simulate a CO 2 leak into the near-surface environment. The experiments differ in a number of ways, from the geological conditions, surface environments, injection rates and experimental set-up - including the injection and monitoring strategy. These experiments have provided abundant information to aid in the development of our scientific understanding of environmental impacts of CO 2 while assessing state of the art monitoring techniques. We collated a global dataset of field-scale shallow (depths < ~25 m) controlled CO 2 release experiments. The dataset includes 14 different field experiment locations, of which nine intended to release CO 2 to the surface, and the remaining sites intended for CO 2 to remain in the shallow subsurface. Several release experiments have been conducted at half of these sites, and so in total, 41 different CO 2 release tests have taken place at the 14 sites in our dataset. We scrutinized our dataset to establish: (i) the range of experimental approaches and settings explored to date (such as the environment, subsurface conditions, injection strategy and whether gaseous or dissolved CO 2 were injected and in what quantities); (ii) the range of CO 2 injection and surface release rates at these experiments; (iii) the collective learnings about the surface and subsurface manifestation of the CO 2 release, the spread and fate of the CO 2, rates of CO 2 flux to surface, and methods of measuring these; (iv) the strengths and limitations of current approaches for detecting and quantifying CO 2. This allowed us to highlight where uncertainties remain and identify knowledge gaps that future experiments should seek to address. Further, drawing on the collective experiences, we have identified common issues or complications which future CO 2 release experiments can learn from.

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