Acoustic Emission Testing of Shales for Evaluation of Microseismic Monitoring of North Sea CO2 Storage Sites

Summary This study focuses on the geomechanical and Acoustic Emission response of caprock lithologies in the North Sea CO2 storage sites. Two multistage triaxial tests perform on caprock lithologies. One shallow (700 m) mudstone/shale from Nordland Group, Southern Viking Graben and another one deeply buried (2581 m) Draupne shale from well 16/8-3S from the Ling Depression, central North Sea. The Nordland shale is a representative caprock for the shallow CO2 storage formations Utsira and Skade. The Draupne shale is a caprock overlying the reservoir sandstone of Sognefjord Formation in the Troll and Smeaheia area. The elastic properties from the tests compare with the existing empirical correlations and datasets of several tested shales from the North Sea, and correlations are applied to well logs in the Smeaheia area. Triaxial testing shows no microseismic activity during any of the phases, including effective stress unloading corresponding to CO2 injection, shearing of the intact rock, and remobilization of the induced fractures. The small strain elastic response in the two tested shales are in agreement with previous datasets and empirical trendlines and is the most relevant input for geomechanical modelling to evaluate CO2 storage sites.