Time-lapse imaging of CO2 migration within near-surface sediments during a controlled sub-seabed release experiment

Ben Roche,Jonathan M Bull,Hector Marin-Moreno,Timothy G Leighton,Ismael H Falcon-Suarez,Madeleine Tholen,Paul R White,Giuseppe Provenzano,Anna Lichtschlag,Jianghui Li,Michael Faggetter

doi:10.1016/j.ijggc.2021.103363

Abstract

The ability to detect and monitor any escape of carbon dioxide (CO2) from sub-seafloor CO2 storage reservoirs is essential for public acceptance of carbon capture and storage (CCS) as a climate change mitigation strategy. Here, we use repeated high-resolution seismic reflection surveys acquired using a chirp profiler mounted on an autonomous underwater vehicle (AUV), to image CO2 gas released into shallow sub-surface sediments above a potential CCS storage site at 120 m water depth in the North Sea. Observations of temporal changes in seismic reflectivity, attenuation, unit thickness and the bulk permeability of sediment were used to develop a four-stage model of the evolution of gas migration in shallow marine sediments: Proto-migration, Immature Migration, Mature Migration, and Pathway Closure. Bubble flow was initially enabled through the propagation of stable fractures but, over time, transitioned to dynamic fractures with an associated step change in permeability. Once the gas injection rate exceeded the rate at which gas could escape the coarser sediments overlying the injection point, gas began to pool along a grain size boundary. This enhanced understanding of the migration of free gas in near-surface sediments will help improve methods of detection and quantification of gas in subsurface marine sediments.

Highlights

Since the industrial revolution, the concentration of CO2 in the at mosphere has risen from 277 parts per million to a current level of > 410 ppm (Friedlingstein et al, 2019)
This paper describes in detail the evolution of gas migration pathways, culminating in the for mation of seismic chimneys, with a particular focus on constraining the primary mechanisms by which gas migrates within the near-surface over time
Based on the temporal and spatial development of the acoustic anomalies seen during the CO2 release experiment along with visual seabed seep observations we propose a four-stage model for the evolu tion of gas migration pathways in the sub-surface (Fig. 12): Stage 1 – Proto-migration: the initial migration of gas immediately following the start of CO2 injection where individual bubbles make their own way to the surface via capillary invasion, stable and dy namic fracture propagation with no preferred pathways

Summary

Introduction

The concentration of CO2 in the at mosphere has risen from 277 parts per million (ppm) to a current level of > 410 ppm (Friedlingstein et al, 2019). This rise has been directly linked to anthropogenic sources such as the burning of fossil fuels, the manufacture of cement, and changing land uses (Friedlingstein et al, 2019). The large-scale adoption of Carbon dioxide Capture and Storage (CCS) has been identified as a key factor for reducing anthropogenic greenhouse gas emissions to reach climate goals (IPCC 2014). Robust strategies for leak detection and man agement are still in their infancy despite being a regulatory requirement to comply with international marine legislation (e.g., EU CCS Directive, London Protocol, OSPAR) and must be advanced to make CCS a safe and reliable strategy for the long-term mitigation of atmospheric CO2 increase

Methods

Results

Discussion

Conclusion