Overburden Analysis and Seal Integrity Study for CO2 Sequestration in the North Sea

Anthropogenic atmospheric carbon dioxide (CO2) is a major greenhouse gas responsible for human-induced global warming. The impact of global warming may trigger the critical thresholds called tipping points if warming increases to equal or greater than 1.5oC. Crossing the critical threshold might be devastating for the human race. Despite the pledges made under the 2015 Paris Agreement, global warming will reach about 2.8oC by the end of the century unless emissions are cut by half by 2030, and net-zero emissions are achieved by 2050. Subsurface storage of CO2 is one of the viable solutions that cut emissions with the lowest possible cost, where the CO2 will be captured from the point sources, then transported, and permanently stored in suitable subsurface geological formations. However, injecting CO2 into the subsurface has several failure risks. Therefore, a detailed integrated analysis is needed to build confidence for safe and permanent subsurface geological CO2 storage. The OASIS research project focuses on CCS site characterization in the northern North Sea. The study area (northern North Sea) will be the future CCS hub, where the Norwegian government already awards three CCS licenses (EL001, EL002 & EL004). In addition, two injection wells (31/5-A-7 AH & 31/5-C-1H) have been drilled targeting megaton level injection from 2024. Wireline log suites from 64 exploration wells, two long offset 2D seismic surveys (NSR and SG8043) and two site-specific 3D cubes (GN1101 and GN10M1) from the area were used as the input database. In addition to GN1101 (Smeaheia) and GN10M1 (Aurora) 3D seismic cubes, a set of partial stacks with angles 0-100, 10-200, 20-300, 30-400, and 40-500 are also available for both 3D surveys. These angle stacks are used to carry out the stochastic seismic inversion analysis. Additionally, 12 core and cuttings caprock shale samples from the Upper Jurassic Viking Group were collected from the North Sea for petrographic (XRD & SEM) and laboratory (nano-indentation) analysis. An extensive analysis was carried out throughout the project timeframe (2018-2023). A description of the major activities under the OASIS project is given below: To evaluate the transition zone, which is the boundary between mechanical and chemical compactions, the temperature gradient of each well has been estimated using bottom hole temperature (BHT) and total vertical depth (TVD). Then the paleo and present temperature on top of interested intervals have been estimated. Study area regional temperature maps were generated using the well data points to identify the lateral variations, mainly the mechanical and chemical compaction zones. Petrographic analysis such as X-ray diffraction (XRD) of core and cuttings sample has been carried out to identify the mineral assemblages and estimate the percentage of each mineral present. Additionally, Scanning Electron Microscope (SEM) images of the same samples are also assessed to identify the type of minerals and have a qualitative understanding of the grain size of the samples. This information is then used to help define the mineral assemblages during XRD interpretation and also in paleo-depositional environment study and estimation of mineralogy-based brittleness indices. The results obtained from XRD and SEM analysis are used together to make an overall understanding of the mineralogy and depositional environment of the caprock samples. Wireline logs are extensively used in geological and geophysical characterization investigations, providing important information about the subsurface. As part of the petrophysics analysis, the volume of shale (Vsh) for all the wells has been estimated using a combination of methods. The gamma-ray log is used to calculate the Vsh using both the "Young & old rock" method proposed by Larionov (1969), and the combination of density (RhoB) and neutron porosity (NPHI) logs are also used. Vsh is important in characterizing the subsurface and understanding the depositional environment. To understand the lateral variation of Vsh, well based regional map was also generated. This activity involves testing two different petrophysical methods for estimating the total organic carbon (TOC) from P-sonic, deep resistivity, and density logs. The first technique is called the ?logR method, which utilizes P-sonic and deep resistivity logs as input (Meyer and Nederlof, 1984; Passey et al., 1990). A second method is also a petrophysical approach that predicts TOC from a bulk density log (Vernik and Landis, 1996; Carcione, 2000). The results of these methods were used to understand the distribution and concentration of TOC in the subsurface. This activity is focused on understanding the geomechanical properties of the rocks, primarily the reservoir, caprock, and overburden, by utilizing wireline logs. The properties of interest are Young's modulus (E) and Poisson's ratio (?), which are important indicators of rock stiffness.

1. Generate new knowledge, expertise, and innovations to i) a better understanding of storage capacity, injectivity, and long-term effects of stored CO2, ii) a better understanding of the mechanisms that prevent leakage of CO2, and iii) increased understanding of how pressure builds in a CO2 storage can be handled. 2. The research techniques developed in the project will be widely disseminated. 3. Building competency in the CCS sector by training Postdoc, Ph.D., and MSc students. 4. Knowledge about the geological storage of CO2 significantly impacts climate by mitigating anthropogenic greenhouse gases.

This proposal is directly in response to the thematic area "New knowledge that facilitates large-scale CO2 storage in the North Sea". The overall objectives are also targeted towards addressing some important topics under the themes of i) increasing knowledge about storage capacity, injectivity and the long-term effects of stored CO2, ii) gaining a better understanding of the mechanism that prevent leakage of CO2, and iii) increasing knowledge of how pressure build in a CO2 storage can be handled. The seal rock integrity and the overburden rocks will be analysed for proposed CO2 storage sites Smeaheia (east of the Troll Field) in the North Sea in order to identify critically stressed faults to predict slip on potential active faults as a function of pore pressure change or buildup, evaluate risk for fracturing, and risk of vertical and lateral CO2 migration. The project will be carried out as a collaborative effort between the University of Oslo, Norwegian Geotechnical Institute, Norwegian Computing Center, Colorado School of Mines and Curtin University with Statoil & TOTAL. OASIS will generate site specific seismic data derived 3D geomechanical models focusing on the overburden and seal rocks for the potential CO2 storage reservoirs. All the other necessary information (e.g. available cores, cuttings, wireline logs, laboratory test data, and pressure data from the wells) can be subsequently used to calibrate and iterate the models. In addition to the innovative procedure for the integrated computational 3D geomechanical modeling of the CO2 storage seal and overburden, caprock leakage potential, and the overburden pore pressure and pore fluid distributions from seismic inversion, the project will produce much needed high-quality hydro-mechanical data on overburden and cap rocks from advanced laboratory testing. This makes the project directly applicable and of benefit to existing and future large-scale CO2 storage projects in the Norwegian Continental Shelf (NCS).