Laboratory Evaluation of Mechanical Properties of Draupne Shale Relevant for CO2 Seal Integrity

Abstract

The mechanical integrity of caprocks overlying injection formations is one of the key factors for safe storage of carbon dioxide in geological formations. Undrained effects caused by CO2 injection on strength and elastic parameters should be properly considered in the operational design to avoid fracture creation, fault reactivation and unwanted surface uplift. This study presents results from eleven undrained triaxial compression tests and one oedometer test on the Draupne shale, which is the main caprock of the Smeaheia site in the North Sea, to extract parameters relevant for seal integrity. Tests have been performed on samples oriented perpendicular to and parallel with the horizontal layering of the rock to study the effects of sample orientation relative to the loading direction. Results from undrained triaxial tests showed only minor effects of sample orientation on friction and cohesion. However, when loading during undrained shearing was parallel with layering (horizontal samples), measured Young’s modulus was roughly 1.4 times higher than for the vertical samples. Undrained shearing of vertical samples generated 30–50% more excess pore pressure than for horizontal samples with similar consolidation stress owing to more volume compaction of vertical samples. With apparent pre-consolidation stress determined from a high-stress oedometer test, the normalized undrained shear strength was found to correlate well with the overconsolidation ratio following the SHANSEP (Stress History and Normalized Soil Engineering Properties) procedure.

Experimental evaluation of predicted undrained pore pressure generation as function of stress path and orientation in the Draupne shale

“…The mineralogy and mechanical properties of the Draupne core material from this well have been characterized by several authors in recent studies (e.g. Bohloli et al, 2020;Koochak Zadeh et al, 2017;Skurtveit et al, 2015;Smith, 2019;Soldal et al, 2021a). The clay content of the Draupne shale is approximately 50 % and the total organic content is between 6 and 8 wt.%.…”

“…The manuscript submitted to replace this text with name of AGU journal porosity ranges between 13 -17 % and the average bulk density is 2.25 g/cm 3 . Assumed vertical and horizontal effective in-situ stresses for the core material used in the current study is 26.1 and 17.2 MPa, respectively (Koochak Zadeh et al, 2017), and the material is considered normally to slightly overconsolidated (Soldal et al, 2021a).…”

“…Ewy, 2015Ewy, , 2018Valès et al, 2004), the Draupne core material has been kept submerged in mineral oil since retrieval to prevent drying. Recent fluid content measurements after several years of storage showed no change in saturation (Soldal et al, 2021a). Core sections from which triaxial rock samples were prepared were initially separated from the remaining core using a circular saw.…”

“…Four vertical side drains connecting the porous filters at the top and bottom were used for radial drainage to speed up pore pressure equilibration. A more detailed description of the equipment used can be found in Berre (2011) and Soldal et al (2021a). During undrained testing, the measured pore pressure will be affected by the presence of a non-zero dead-volume with a certain compressibility between the sample and the closed valve(s) (e.g.…”

Third‐order elasticity of transversely isotropic field shales

“…Shales, abundant in the overburden, are known for their low permeability. It is commonly assumed that shales exhibit undrained static stiffness (Bauer et al, 2008;Delle Piane et al, 2011;Islam & Skalle, 2013;MacBeth & Bachkheti, 2021;Sarout & Guéguen, 2008;Soldal et al, 2021;Thompson et al, 2021). In low-permeability formations, the immediate response to reservoir depletion is a heterogeneous undrained pore-pressure change that occurs within a substantial volume surrounding the reservoir, encompassing the entire overburden (Duda et al, 2023;Yan et al, 2023).…”