Effects of Anisotropy and Saturation on Geomechanical Behavior of Mudstone

• Published in: Journal of geophysical research. Solid earth Vol. 124; no. 8; pp. 8101 - 8126
• Main Authors: DeReuil, A. A., Birgenheier, L. P., McLennan, J.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-08-2019
• Subjects: Anisotropy; Clay; Compression; Compression tests; Compressive strength; Correlation analysis; Crack propagation; Deformability; Deformation; Deformation effects; Dependence; Drilling; Earth; Earth crust; Earthquakes; Formability; Fractures; fracturing; Geomechanics; Geophysics; Geothermal energy; Heavy metals; heterolithic; Horizontal orientation; Hydrocarbons; Industrial applications; Interfaces; Levels; Mechanical properties; Mechanics; Metals; Modulus of elasticity; mudstone; Parameters; Physics; Plugs; Preservation; Radioactive wastes; Reduction; Rocks; Sandstone; Saturation; Sedimentary rocks; Seismic activity; Seismic engineering; Shale; Tensile strength; Waste disposal
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2018JB017034

The abundance of mudstone in Earth's crust and its academic and industry applications has led to advancements in the understanding of mudstone deposition and preservation. However, there are few quantitative geomechanical studies on mudstone. To test correlations between anisotropy, fluid saturation levels, and deformability and strength parameters in mudstone, a suite of indirect tensile and unconfined and triaxial compression tests was completed on samples from the Mancos Shale and the Agrio Formation. Discs or plugs were tested with axial loading parallel or perpendicular to bedding, at various confining pressures, and at saturation levels from 0 to 17%. Tensile strength, compressive strength, Young's Modulus, Secant Modulus, stress drop, and energy released exhibit mechanical anisotropy and decrease due to saturation. Tensile strength anisotropy revealed the weakness of heterolithic bedding in mudstone, as tensile fractures commonly propagated along bedding interfaces. Saturation increased mechanical anisotropy for tensile and compressive strength. A compressive strength reduction of 39% to 75% was observed between dry and partially saturated plugs. Lithofacies were used to classify lithologic variations in different types of mudstone. When partially saturated, sandstone‐bearing facies revealed a greater reduction in strength and moduli than clay‐rich mudstone facies. Empirical relationships between partially saturated and dry sample strength and moduli are proposed, to allow for estimation of deformation and failure behavior under in‐situ conditions. The effects that saturation, orientation, and lithofacies have on mechanical behavior are of interest in numerous engineering and academic applications, including fault behavior, induced fracturing of hydrocarbon and geothermal reservoirs, and nuclear waste disposal.
Plain Language Summary
Mudstone is a fine‐grained rock that makes up a large proportion of Earth's crust. Studying the physics and mechanics of mudstone is important for assessing fault and earthquake behavior, and for industry applications in which mudstone is drilled into or propped open. Mudstone is typically composed of grains oriented along repeated horizontal layers, like a layer cake. This layering results in different rock strengths depending on the orientation of the layers. The level of fluid saturation (wetness or dryness) of the mudstone also results in different strengths. In our study, we applied loads to mudstone samples at various sample orientations and saturation levels to mimic various environments in the subsurface. Our study found that distinct types of fractures and strength parameters have varying degrees of dependence on the orientation of the layering, the level of saturation of each mudstone sample, and on the amount of coarse‐grained material composing each mudstone sample. Studies such as these are ongoing, as understanding the mechanics of mudstone informs fault slip and earthquake behavior and has direct applications to drilling holes for water, hydrocarbons, geothermal energy, heavy metals, and, potentially, for nuclear waste disposal.
Key Points
Indirect tensile and unconfined and triaxial compression tests were completed on dry and partially saturated mudstone samples at varying orientations
Orientation and saturation exhibit first‐order control on deviatoric stress‐axial strain behavior during loading and failure
The degree of reduction in strength and/or deformability due to saturation depends on the lithofacies of each sample