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International Journal of Greenhouse Gas Control 109 (2021) 103339Available online 29 April 20211750-5836/Crown Copyright © 2021 Published by Elsevier Ltd. All rights reserved.Numerical modelling of CO2 migration in heterogeneous sediments and leakage scenario for STEMM-CCS field experiments Umer Saleema, Marius Dewara,b, Tariq Nawaz Chaudharya, Mehroz Sanaa, Anna Lichtschlagd, Guttorm Alendalc, Baixin Chena,* aInstitute of Mechanical, Process and Energy Engineering, Heriot-Watt University, Edinburgh, EH14 4AS, UK bPlymouth Marine Laboratory, Prospect Place, Plymouth, PL1 3DH, UK cDepartment of Mathematics, University of Bergen, Bergen, Norway dNational Oceanography Centre, Southampton, UK ARTICLE INFO Keywords: STEMM-CCS, CO2 injection Two-phase flow in porous media Porosity and grain size distribution Gas migration Darcy resistance Carbon Capture and storage CO2 Leakage Pipe flow CO2 dissolution ABSTRACT The dynamics and plume development of injected CO2 dispersion and dissolution through sediments into water column, at the STEMM-CCS field experiment conducted in Goldeneye, are simulated and predicted by a newly developed two-phase flow model based on Navier-Stokes-Darcy equations. In the experiment, CO2 gas was released into shallow marine sediment 3.0 m below the seafloor at 120 m water depth in the North Sea. The pre-experimental survey data of porosity, grain size distributions, and brine concentration are used to reconstruct the model sediments. The gas CO2 is then injected into the sediments at a rate of 5.7 kg/day to 143 kg/day. The model is validated by diagnostic simulations to compare with field observation data of CO2 eruption time, changes in pH in sediments, and the gas leakage rates. Then the dynamics of the CO2 plume development in the sediments are investigated by model simulations, including the leakage pathways, the fluids interactions among CO2/brine/sediments, and CO2 dissolution, in order to comprehend the mechanisms of CO2 leakage through sediments. It is shown from model simulations that the CO2 plume develops horizontally in the sedi-ments at a rate of 0.375 m/day, CO2 dissolution in the sediments is at an overall average rate of 0.03 g/sec with some peaks of 0.45 g/sec, 0.15 g/sec, and 0.3 g/sec, respectively, following the increase in injection rates, when some fresh brine provided. These, therefore, lead to a ratio of 0.90~0.93 of CO2 leakage rate to injection rate. 1. Introduction Carbon capture and storage (CCS) is a vital solution to mitigate the climate change and/or ocean acidification accompanying anthropo-genic carbon dioxide (CO2) level increasing by more than 25% since 1959 (NOAA 2020). The carbon emission has resulted in global warming of 1.5C above pre-industrial levels and affected natural habitats on/off shore (IPCC 2018). CCS offers a solution of the disposal of carbon di-oxide (CO2) in the overburden sub-seabed reservoirs/geological struc-tures instead of emitting the gas into the atmosphere to meet the ‘targetset by the Kyoto Protocol (Freund and Ormerod, 1997; Han et al., 2012). The geological reservoirs can be chosen for long term storage of CO2 in well-designed storage sites (Oleynik et al., 2020). The utmost important concern about implementing CCS, especially for CO2 under seabed reservoir storage, is the leakage risk of the sequestrated CO2 to the ocean due to its environmental physicochemical impacts (Feely et al., 2016). The impacts include the local acidification at the CO2 leakage site (Sokołowski et al., 2020; Yang et al., 2019) and the effects on marine life and ecosystem (Amaro et al., 2018; Jones et al., 2015; Molari et al., 2019). Therefore, it is necessary to understand the leakage mechanisms, then to estimate or predict the potential of the leakage to reduce the associated risks. For under-seabed storage, studies have been made from Lab exper-iments (Li et al., 2020; Uemura et al., 2011), the natural CO2 migration through geoformations into the ocean, to the designed filed experi-ments, in collaboration with the studies of CO2 ocean storage (Dewar et al., 2013; Caudron et al., 2012; McGinnis et al., 2011; Esposito et al., 2006). It has been recognized that CO2 leakage developments are at a range of spatial scales from pore (~mm) in the geoformation, the bub-ble/droplet (~ cm) once leaked into ocean, then the regional (~ 102 km) to global in the ocean. Liquid and gas phase CO2 plume developments in turbulent ocean has been observed from small scale field experiments * Corresponding author. E-mail address: (B. Chen). Contents lists available at ScienceDirect International Journal of Greenhouse Gas Control journal homepage: Received 11 December 2020; Received in revised form 1 April 2021; Accepted 12 April 2021
International Journal of Greenhouse Gas Control 109 (2021) 1033392(Kim et al., 2019; Rhino et al., 2016) and modeled by means of computational fluid dynamics (CFD) (Dewar et al., 2013; Drange et al., 1993) (Umer, add this paper to here, Sato, T.; Sato, K. Numerical pre-diction of the dilution process and its biological impactsonCO2 ocean sequestration. J. Marine Sci. Technol. 2002, 6, 169-180.) and a kind of integral plume models (Dissanayake et al., 2018). The data of changes in pH from these small-scale plume models has been successfully imple-mented, as the input parametric data for plume further movements, in the large-scale ocean models (Blackford et al, 2020) once CO2 leaks from seabed. On the other hand, the CO2 mitigation from storage reservoirs to sediments were widely investigated from the field observations (Furre et al., 2017), the Lab experimental studies (Rillard et al., 2015; Tongwa et al., 2013) and numerical simulations (Discacciati et al., 2002; Du et al., 2016; Cai et al., 2009; Chidyagwai and Rivi`ere, 2010). However, leakage from sediments into the ocean turbulent bottom boundary layers (TBBL) has been less focused, which, however, is one of the key processes for assessments of the biological impacts of leaked CO2 on the ocean, as a rich population of marine organisms resides within both the ocean TBBL and shallow sediments. It is the dynamics of CO2 migration and dissolution crossing the interface between shallow sediments and oceanic TBBL that dominate the leakage sources to the plumes evalua-tions in the ocean (Caudron et al., 2012). In order to ensure the effective environmental monitoring of offshore CCS storage sites, the Strategies for Environmental Monitoring of Marine Carbon Capture and Storage (STEMM-CCS) project was launched in 2016 (Blackford et al., 2018), following previous field experiments QICS (Taylor et al., 2015) and ECO2 (Furre et al., 2017). STEMM-CCS is a scientific research project to simulate a sub-seafloor CO2 leak under real-life conditions in the North Sea (Blackford et al., 2018). One of the primary objectives of the project is to produce experimental data for the development and calibration of numerical models to simulate the leakage dynamics of CO2 out of the geoformations with the knowledge of dispersion time, pathways through faults and high permeable zones. In terms of marine physiochemical and biological impacts, for instance, the changes in pH due to leaked CO2 is one of the most significant data for the assessment of leakage and model calibrations. Adequate knowledge of CO2 dispersion through complex structure geoformation and dissolution characteristics of developed plume are mandatory and vital towards the development of the leakage prediction models. Sup-ported by the project, a so-called Arbitrary Navier-Stokes-Darcy mul-ti-fluid flow model (AnsdMF) has been developed for simulations of CO2 transportations through the sediments with complex structures into the turbulent ocean. In this study, the AnsdMF model is applied to simulate and predict the dynamic processes of initiations and developments of CO2 and CO2 solution plumes from injection ports through sediments to the ocean current in STEMM-CCS experiment. The numerical model settings, including the data collections, analysis, and reconstruction of sediments, are discussed in Section 2. The overall methodology of the numerical model including sub models of mass, momentum, and interfacial in-teractions among fluid-fluid-solid are explained in Section 3. The anal-ysis and discussions on the model diagnostic simulation results are made in Section 4 with the CO2 injection through pipe and leakage scenario results. Finally, the conclusions are drawn in Section 5. 2. Model sediment setup and data collection The CO2 release experiment of STEMM-CCS project was carried out in the vicinity of the Goldeneye platform located in a sandstone for-mation of Early Cretaceous near Scotland beneath Moray Firth (56-60 ̊N) (Dean and Tucker, 2017). In this project, gas CO2 supplied from gas tanks was controllably released from 3m underneath the seabed (Flohr et al., 2020). Ships, remotely operated vehicles (ROVs), autonomous underwater vehicles (AUVs), the gas sampling system, and related equipment and sensors were employed to measure the changes in physicochemical properties in seawater and brine and monitor the leakages of CO2. The gas release started on 11/05/2019 at 15:19hrs, which is set as the start of day 0 of the experiment (Flohr et al., 2021) in this study. The data of CO2 injection rate and the total CO2 injected, as shown in Fig. 1, are collected from field experiment as the input data of the modeling. 2.1.Porosity and grain size distribution To set up the model sediments, AnsdMF requires data of particle size distributions and the porosity distributions of sediments. The data ob-tained from a project pre-survey were collected, which are those sampled in 18 boxes at different locations around CO2 injection site down to 5.0 meters. The porosity data from core samples used in this study are plotted in Fig. 2, of which are the data from landers (POS527/ si83 and gravity corer GC-06-Station-102), pockmarks (GC-01-Station- 97 st.90/91 Obj 103), and well (GC-07-Station-103). The data shows that the average porosity is around 53% with +10% and 20% for the sediments deeper than 16 cm (Fig. 2 left), while, the decent distributions from surface to the depth of 16cm for shallow sediments (Fig. 2 right). Another set of data requested from setting the model is the particle size distributions, which are taken from Particle size analysis (PSA) for various samples collected at different depths of sediments (Lichtschlag et al., 2021). A brief discussion on the characteristics of the particle distribution will help for model setting and for modelling the dynamic process of CO2 gas penetration through sediments. From the data, shown in Fig. 3, it can be found that the deeper sediments, 34 m below the seabed (data of GC3 398-412cm), are mostly fine sand or laminated mud. The surface sediments down to about 0.5 m, however, are mostly particles of 60-100 μm with some smaller particles of less than 10 μm. In between those layers is a mixture of substances with varied grain sizes dominated within 8 to 100 μm. It must be noted that the very small particles within the sediments are more sensitive to additional distur-bances, such as the penetration of CO2 into the sediments, while the larger particles have a relatively larger inertia to withstand the distur-bances and keep their original positions. This behavior of particles can be utilized for diagnostic setting of the model sediments for prediction of CO2 dispersion with comparisons of observation data, such as the eruption time. 2.2.Reconstruction of model sediments by data The heterogenous sediments are sediments with complex structure and various pore throat size distributions. The model requires the permeability distribution, intrinsic permeability at the first stage, to predict the resistant forces of fluids and porous solids. For the hetero-geneous sediment or rock, the throat size varies depending on the inter- Fig. 1.Gas injection rate and total gas injected over the days from start in-jection collected from field experiment (Flohr et al., 2021). U. Saleem et al.