The impact of different clay minerals on the anisotropy of clay matrix in shale

Abstract

Although clay is composed of disconnected anisotropic clay platelets, many rock physics models treat the clay platelets in shale as interconnected. However, the clay matrix in shales can be modelled as anisotropic clay platelets embedded within a soft isotropic interplatelet region, allowing the influence of disconnected clay platelets on the elastic properties of the clay matrix to be analysed. In this model, properties of the interplatelet region are governed by its effective bulk and shear moduli, whereas the effective properties of the clay platelets are governed by their volume fraction, aspect ratio and elastic stiffness tensor. Together, these parameters implicitly account for variations in clay and fluid properties, as well as fluid saturation. Elastic stiffnesses of clay platelets are obtained from the literature, including both experimental measurements and first‐principles calculations of the full anisotropic (monoclinic or triclinic) elastic stiffness tensors of layered silicates. These published elastic stiffness tensors are used to compile a database of equivalent transverse isotropic elastic stiffness tensors, and other physical properties, for eight common varieties of layered silicates. Clay matrix anisotropy is then investigated by examining the influence of these different elastic stiffnesses, and of varying model parameters, upon the effective transverse isotropic elastic stiffness tensor of the clay matrix. The relationship between the different clay minerals and their associated anisotropy parameters is studied, and their impact on the resulting anisotropy of the clay matrix is analysed.

Rock Physics Model of Shale: Predictive Aspect

“…Sayers (2013b) demonstrated that the elastic properties of organic-rich shales can be reasonably described by this model using elastic stiffnesses of kerogen as a soft interplatelet region. On the other hand, the model assumes that the spatial distribution of clay platelets as defined by a two-point correlation function has the same aspect ratio as the clay platelets (Sayers and den Boer, 2019). This may not be a reasonable assumption for some shale rocks, however, as demonstrated by Ponte Castaneda and Willis (1995), the effect of the spatial distribution of the inclusions on the effective modulus tensor is relatively small.…”

“…First, clay platelets are assumed to consist of muscovite or ideal kaolinite, whose properties are based on the best-fitting TI approximation of theoretical calculations of Militzer et al (2011). Muscovite properties were used for most of samples since muscovite is both structurally and compositionally similar to the clay mineral illite, thus may be expected to have similar elastic properties (Katahara, 1996;Sayers & den Boer, 2019;Tosaya, 1982), while ideal kaolinite properties were used for Norwegian Sea shale because it is dominated by kaolinite. Although the assumption gave reasonable results, actual clay mineral properties, which have not been measured experimentally, might be largely different from the theoretical calculations, and clay platelets could actually consist of multiple minerals (or there may be domains of different clay minerals).…”

“…Scanning electron micrographs of several shales indicate that the shale rocks are comprised of stacks of locally aligned clay platelets (Hornby et al, 1994;Sayers and den Boer, 2019). The stacks are often called domains (Aylmore and Quirk, 1960), and this local alignment is a key source of shale anisotropy (Sayers and den Boer, 2019). Sayers and den Boer (2018) proposed a rock physics model based on the anisotropic Hashin-Shtrikman estimates of Willis (1977) and Ponte Castaneda and Willis (1995).…”

“…Sayers and den Boer (2018) proposed a rock physics model based on the anisotropic Hashin-Shtrikman estimates of Willis (1977) and Ponte Castaneda and Willis (1995). This model is chosen here because (a) it allows the domain to be modeled as anisotropic clay platelets embedded within a soft isotropic interplatelet region; this seems to be a reasonable assumption for shale rocks, and (b) calculations can be performed using a convenient closed-form analytic formulation (Sayers & den Boer, 2019). The model gives a strict lower bound since the matrix (interplatelet region) is the most compliant phase.…”

Comparison of Biotite Elastic Properties Recovered by Spherical Nanoindentations and Atomistic Simulations — Influence of Nano‐Scale Defects in Phyllosilicates

“…make rocks more compliant and weaker with increasing phyllosilicate content (e.g., Nishizawa & Kanagawa, 2010;Sayers & den Boer, 2019;Sone & Zoback, 2013a). Phyllosilicates are also abundant in fault rocks and are believed to impose critical limits on the strength of crustal faults zones.…”