The Influence of Burial History and Capillary Suction on Geomechanical Properties of Clay Shales

ABSTRACT: Geomechanical properties of clay shales significantly depend on mineralogical composition, saturation, and burial history. In this contribution, we provide insights into a unique opportunity to study the influence of burial history on geomechanical parameters of a rock unit in a distinctive area in Northern Germany where clay shales were retrieved at shallow depths. In this area, the mineralogical composition is relatively homogeneous, but thermal maturity increases substantially over a lateral distance of approximately 50 km. Using an empirical correlation between vitrinite reflectance and temperature, and assuming a geothermal gradient of 30 °C/km, initial estimates suggest that maximum burial depths range between 1300 m and 3600 m. Systematic uniaxial compression tests and consolidated undrained triaxial tests under well-defined saturation conditions suggest a major impact of thermal maturity on geomechanical properties. Although peak strength and stiffness are functions of porosity, which is the main parameter influenced by maximum burial, the general hydro-mechanical behavior remains similar. While comparing the first results from two degrees of thermal maturity in this contribution, the overall goal of this systematic analysis is to develop functions that allow predicting geomechanical properties for a clay shale formation that has undergone different burial histories. 1 INTRODUCTION Shales and similar clay-rich rocks have increasingly attracted attention for utilization as host rocks for the final disposal of radioactive waste in deep repositories. A precise characterization of such rocks in terms of their (thermo-)hydro-mechanical behavior is therefore fundamental for evaluating their suitability as a geological barrier. This task is, however, challenging since the properties of clay-rich rocks are highly influenced by, for instance, structural and mineralogical heterogeneities. They exhibit state-dependent properties, meaning that mechanical strength or permeability are functions of saturation. Another important factor influencing the geomechanical properties of shales is their geological burial history, i.e., thermal maturity. During burial, the rock experiences different diagenetic processes such as compaction or cementation as a result of elevated stresses and temperatures.