Mechanical Properties of Lamellar Shale Considering the Effect of Rock Structure and Hydration from Macroscopic and Microscopic Points of View

Abstract

It is well known that the effect of shale hydration causes wellbore instability due to water phase invasion of drilling fluid to lamellar shale rich formations. This is because of that the mechanical properties (compressive strength, elastic modulus, etc.) of lamellar shale surrounding the borehole, which is rich in clay minerals, will decrease significantly after hydration. In this study, using the lamellar shale in the continental stratum of the southern Ordos Basin, the mechanical properties of lamellar shale were studied by compression tests considering the effect of lamellar structure and hydration from a macroscopic point of view. In addition, the mechanical mechanism was discussed combined with the CT scanning tests results from a microscopic point of view. The results demonstrate the following points. Lamellar shale has stronger anisotropy than bedding shale, the compressive strength (deviatoric stress) and elastic modulus of lamellar shale are both lower than those of bedding shale, and it is more prone to tension fracture. With the increase in the angle (β) between the lamina and the axial direction from 0° to 90°, the compressive strength of lamellar shale decreases when β < 30° and then increases, the elastic modulus of lamellar shale decreases greatly when β < 30° and then tends to flatten. With the increase in hydration time, the compressive strength and elastic modulus of lamellar shale both gradually decrease, and the rates of their decrements reduce. The mechanical properties of lamellar shale are more affected by hydration than those of bedding shale. The hydration of lamellar shale leads to the formation of new fractures and the expansion of existing fractures in the junction area between the laminae and rock matrix, resulting in easy tension fracture along the laminae of shale.

Improvement of Emulsion Stability and Plugging Performance of Nanopores Using Modified Polystyrene Nanoparticles in Invert Emulsion Drilling Fluids

“…With the increasing shortage of conventional oil and gas resources, unconventional sources, such as resources derived from shale oil and gas, have become important replacements worldwide. However, the process of drilling shale formations for oil and gas is prone to challenges such as wellbore instability ( Zeng et al, 2021 ; Zhang et al, 2022 ), which significantly prolongs the drilling cycle and adversely impacts the economic viability. Compared with water-based drilling fluids (WBFs), inverse emulsion oil-based drilling fluids (OBFs) have unique advantages in shale oil and gas drilling and are widely used ( Guo et al, 2012 ; Li et al, 2014 ).…”

Dynamic response and damage evolution of red sandstone with confining pressure under cyclic impact loading

“…19 Zhang described the influence law of lamellar structure and hydration, and computed tomography (CT) scanning showed tensile fracturing along the laminae of shale. 20 Deng et al 22 derived the constitutive relationship of rock under freezing-thawing cycles based on nuclear magnetic resonance (NMR) test results. 21 NMR is a nondestructive testing technology that is highly precise, quick, and repeatable and has applications in medical treatment, petroleum engineering, 22,23 etc.…”

“…Wei found three crack types (oblique shear cracks, quasi‐coplanar cracks, and shear cracks) in sandstone fragments by scanning electron microscopy (SEM) under dynamic triaxial loading 19 . Zhang described the influence law of lamellar structure and hydration, and computed tomography (CT) scanning showed tensile fracturing along the laminae of shale 20 . Deng et al 22 derived the constitutive relationship of rock under freezing–thawing cycles based on nuclear magnetic resonance (NMR) test results 21 .…”

A direct shear apparatus for intact rock under dynamic moisture content

“…In the field, the moisture content of rock changes with the weather conditions [1], and water would highly affect the physical (microstructures) and mechanical properties of rock [2][3][4][5]. Accurate evaluation of the shear properties of rock under dynamic moisture content conditions is essential for construction safety in geotechnical engineering.…”