Consolidation Behaviors of Clays, Mudstones and Shales -CO2 Storage Sites in Horda Platform, Offshore Norway
Authors: Grande L. , Skurtveit E. , Choi J. C. , Mondol N. H. , Mondol N. H. , Thompson N. 
Published: August 2024

ABSTRACT:
Clays, mudstones and shales above CO2 storage reservoirs are all together defining the top seal. In this paper, we evaluate the constrained modulus (M) and permeability (k) of clay sediments, mudstones and shales from the Horda Platform area, offshore Norway, where the CO2 storage sites Aurora, Smeaheia and Luna are located. We utilize a dataset of uniaxial strain tests in depth range 0-2.6 km including Quaternary clays from geotechnical site investigations (depth &lt;220m), three mudstones (depth ca 1.1-1.4km) and one deep shale (depth ca 2.6km) and combine with the larger NGI database and previous tests on synthetic mixtures of clay and silt. M and k are key input to a range of analysis i.e. settlement of structures (geotechnical), burial compaction in sedimentary basins, surface settlement or heave from petroleum production or subsurface CO2 storage. New empirical relationships for M, based on modulus number (m), volume of clay (Vcl), plasticity index (Ip) and water content (wi) are suggested, and new method for upscaling M using Vcl from gamma ray logs are demonstrated from clays at geotechnical through mudstones down to depth of caprock for Smeaheia CO2 storage site. Permeabilities (k) in the mechanically compacted natural clays and mudstones aligns well into an exponential trendline when compared against maximum burial depth.

1. INTRODUCTION
The impact of geological history on rheology and stresses in sedimentary basins has been evaluated for various CO2 storage sites in the SHARP project (www.sharp-storage-act-eu). This includes burial compaction, erosion and uplift, glacial loading during the last ice age. Constrained modulus (M) and permeability (k) are key parameters in modelling deformation responses during compaction, tectonic uplift, erosion and glacial loading/unloading in sedimentary basins including time-dependent undrained vs. drained behaviors and overpressure developments. M and k are also key parameters in field scale ground models, i.e. to evaluate reservoir deformation from CO2 injection using predictions based on monitoring of surface heave (i.e., Bohloli et al., 2022). In such cases, parameters covering the entire lithology from the seabed to the CO2 storage reservoirs are needed. Empirical correlations for M vs. index properties have been developed for geotechnical applications (i.e. Andersen and Schjetne, 2013; DeGroot et al., 2019). Empirical correlations stiffness based on velocity logs are normally used for mudstones and shales (i.e. Horsrud et al., 2001, Grande et al. 2008 and 2022). However, the transition zone at ca 0.2 - 1 km, deeper than geotechnical borehole and shallower than what is normally cored in petroleum and CCS projects, are not well constrained in empirical correlations. High-stress uniaxial strain tests (UST) on mixtures of sediments with well-known mineralogy are useful to constrain engineering parameters, i.e. systematic studies of sand (Chuhan et al., 2002 and 2003) and silt-clays mixtures (Mondol et al., 2009 and Grande et al., 2013). Ageing and mineral transformation can occur in the transition of clays to mudstones (&lt;2km); however chemical compaction process (diagenesis) and porosity reduction and stiffening from cementation, starts at temperatures &gt;70°C, further transforming mudstones to shales (Bjørlykke and Høeg, 1997, Bjørlykke, 1998). Nevertheless, the initial composition of the sediment defines the behavior of deeper mudstones and significant parts of sealing units for CO2 storage.