Caprock Stress Contrast Measurement, Mechanism Investigation and Development of Area In-Situ Stress Characterization, Northern Lights CCS, Offshore Norway

This work, focused on the Northern Lights CCS project (offshore Norway) builds upon the recognition by Thompson et al. (2022) of stress contrasts within a CO2 storage caprock shale by introducing newly collected in situ data and laboratory testing. Specifically, carefully designed in situ testing (microfrac) was performed to validate the stress contrasts suggested from the exploration/appraisal well observations, as well as full log suites obtained to investigate stress profiles. Laboratory tests, focusing on the degree of relative creep between the identified shale intervals, was performed to better understand the mechanism behind the stress contrasts observed. A viscoelastic stress relaxation approach is then used to characterize in situ stress from experimentally obtained creep parameters. In total, the stress contrasts are again observed through all methods, with a high consistency from well to well. A connection to mineralogy is further validated, where increased creep higher clay content is observed to result in higher relative minimum formation stress. These collective observations bring confidence that higher minimum formation stress is present in the caprock, allowing the project to define CO2 safe storage potential more confidently in the structure. 1. INTRODUCTION As a means of achieving IPCC targets of reducing net atmospheric carbon emissions, carbon capture and storage (CCS) is experiencing rapid growth; project development activity has increased significantly in recent years, towards a projected storage need of upwards of 10 Gt/yr by 2050 (IPCC [2018], IEA [2021] Global CCS Institute [2023]). The Longship (including CO2 capture) and Northern Lights projects (CO2 transport, temporary storage and subsea pipeline transport to permanent subsurface storage) are frontrunners in this industry, realizing the full CCS value chain through establishment if capture sites at various industries, transport ships, a temporary storage facility and subsea infrastructure off the coast of Norway (Northern Lights JV DA [2024], Gassnova [2024]). Phase 1 of the project (up to 1.5 Mt/yr storage, for 25 years) is ready to receive CO2 in 2024.