Density Driven Convective Instability during CO2 Geological Sequestration

Abstract

At the heart of the global climate change is the rising levels of CO2 in the atmosphere, which is responsible for various adverse environmental problems, such as floods, droughts and irregular weather. Carbon capture and storage (CCS) has been identified as a viable strategy to reduce CO2 emissions while retaining access to fossil fuel energy. Therefore, we focus on some of the key physical processes that occur during CO2 sequestration involving the injection of CO2 into geological formations for long term storage purposes.
In this thesis, we investigated the migration of CO2 after its injection and dissolution in the fluid saturating the formation during CO2 storage. We studied the transition between the two modes of mass transfer: diffusion and convection, transporting and distributing CO2 within the storage formation – a change referred to as the onset of convective instability. Us-ing theoretical analysis, we studied the onset of convective instability in layered and inclined porous media, since most of the previous studies are in homogeneous, horizontal systems. In addition, numerical simulations were used to show the migration patterns beyond the onset time.
Deep saline aquifers, the preferred formations for CO2 storage, typically comprise layers with different permeability. Using linear stability analysis, the effect of a vertically varying, non-monotonic, non-periodic permeability profile on the onset of convective instability is studied. We found certain interactions between the permeability profile and CO2 concentra-tion that affect the onset time. The onset time can either be accelerated or delayed depending on the location of the embedded low-permeability, the sharpness of the transition between regions of different permeability, and the average formation properties. The effect is greatest when the variation is close to the top of the domain, where the onset occurs, while variations near the bottom of the domain do not have a large effect on the onset conditions. This non-trivial dependence underscores the importance of accurate site characterization – capturing small spatial geological features, for CO2 sequestration.
Storage reservoirs often do not exist in an idealised horizontal configuration; they are commonly tilted or inclined due to tectonic activities. Inclination modifies the transport of CO2 due to the simultaneous occurrence of diffusion and lateral buoyancy-driven fluid flow, especially near the top of the storage where CO2 is most concentrated. The linear stability analysis of Darcy’s law and mass conservation for the flow and the concentration field reveals that the onset time is delayed as the inclination angle increases, and no onset is found beyond a certain angle (the cutoff angle) that is dependent on the parameters of the system.
Beyond the onset time, a layered permeability heterogeneity controls the migration of dissolved CO2. Numerical simulations reveal the evolution of the perturbations and their advancement towards the bottom of the storage formations. We find a structural change in the convective flow when it migrates across a permeability difference, and the migration pattern governs the flux of dissolved CO2. At certain times, the dissolution flux in layered systems can be approximated by the flux in homogeneous systems.
These results explain key processes affecting the distribution of CO2 vital to improve CO2 subsurface dissolution, which is of great importance to the geological CO2 sequestra-tion.

Date of Award	Jul 2023
Original language	English
Awarding Institution	Coventry University
Sponsors	Petroleum Technology Development Fund (PTDF)
Supervisor	Michael Dallaston (Supervisor), Seyed Shariatipour (Supervisor) & Ran Holtzman (Supervisor)