Summary
Elastic moduli of clastic rocks with a load-bearing clay framework, such as shale and mudstone differ from other clastic rocks. The small platelets of the clay framework have a large surface area and high reactivity with the saturating fluid. A change in fluid saturation affects the elastic moduli and mechanical strength of the clay framework. Rock physical models of clay borne rocks attempt to account for the fluid sensitivity and, if possible, relate changes in elastic moduli to mechanical strength predicting the stability of shales for instance. Rock physical models proposed so far postulates a continuous matrix medium perturbed by a complex pore-structure. The mineralogy of the clay defines the background medium and the pore-space causes the observable moduli of a specific sample. We argue that a clay framework is closer to a granular assembly of individual grains than a continuous solid frame. As such, the interactions between fluid and solid clay can affect the solid structure itself. Therefore, any background medium assumed comes to depend on both the solid mineralogy and the saturating fluid. We demonstrate the sensitivity of the solid frame to the saturating fluid in compacted clay powders. We normalize elastic moduli measured in compacted clay powders to a load-bearing solid cross-section using the iso-frame model. Comparing the moduli of the solid framework in the dry and brine-saturated states shows that brine saturation does affect the solid structure. Brine saturation decreases the elastic shear modulus of the framework while increasing the elastic bulk modulus. This moduli change is associated with the solid clay and not the pore-space. We interpret the change as water coating of the clay platelets affecting inter-platelet contacts.
Introduction
The elastic moduli of shales and mudstones depend on both mineral composition and fluid saturation. The type and composition of the saturating fluid affects the mechanical and elastic properties of a load-bearing clay matrix in a manner not fully understood by the present state of rock-physics.
The sensitivity to fluid saturation has direct practical consequences. Shale formations exposed to a brine with a different composition than the formation fluid can develop shear failures. Such failures are a major concern to well-bore stability. Prediction of shale behavior without coring and testing is of great value during drilling.
