Abstract
Understanding the mechanisms of strain localization leading to brittle failure in reservoir rocks can shed light on geomechanical processes such as porosity and permeability evolution during rock deformation, induced seismicity, fracturing, and subsidence in geological reservoirs. We perform triaxial compression tests on three types of porous reservoir rocks to reveal the local deformation mechanisms that control system-size failure. We deformed cylindrical samples of Adamswiller sandstone (23% porosity), Bentheim sandstone (23% porosity), and Anstrude limestone (20% porosity), using an X-ray transparent triaxial deformation apparatus. This apparatus enables the acquisition of three-dimensional synchrotron X-ray images, under in situ stress conditions. Analysis of the tomograms provide 3D distributions of the microfractures and dilatant pores from which we calculated the evolving macroporosity. Digital volume correlation analysis reveals the dominant strain localization mechanisms by providing the incremental strain components of pairs of tomograms. In the three rock types, damage localized as a single shear band or by the formation of conjugate bands at failure. The porosity evolution closely matches the evolution of the incremental strain components of dilation, contraction, and shear. With increasing confinement, the dominant strain in the sandstones shifts from dilative strain (Bentheim sandstone) to contractive strain (Adamswiller sandstone). Our study also links the formation of compactive shear bands with porosity variations in Anstrude limestone, which is characterized by a complex pore geometry. Scanning electron microscopy images indicate that the microscale mechanisms guiding strain localization are pore collapse and grain crushing in sandstones, and pore collapse, pore-emanated fractures and cataclasis in limestones. Our dynamic X-ray microtomography data brings unique insights on the correlation between the evolutions of rock microstructure, porosity evolution, and macroscopic strain during the approach to brittle failure in reservoir rocks.
Highlights
IntroductionStrain localization in rocks and geomaterials (e.g., cement) is observed over a broad range of length scales, varying from centimeter-sized laboratory rock specimens (e.g., Lockner et al, 1991; Paterson & Wong, 2005; Renard et al, 2019a) to crustal fault zones stretching over several kilometers (e.g., Cilona et al, 2012; Fossen et al, 2007, 2018; Rotevatn et al, 2016)
Strain localization in rocks and geomaterials is observed over a broad range of length scales, varying from centimeter-sized laboratory rock specimens (e.g., Lockner et al, 1991; Paterson & Wong, 2005; Renard et al, 2019a) to crustal fault zones stretching over several kilometers (e.g., Cilona et al, 2012; Fossen et al, 2007, 2018; Rotevatn et al, 2016)
Our results present a detailed analysis of strain localization observed in three kinds of porous rocks, Adamswiller sandstone (23% porosity), Bentheim sandstone (22.8%) and Anstrude limestone (20%)
Summary
Strain localization in rocks and geomaterials (e.g., cement) is observed over a broad range of length scales, varying from centimeter-sized laboratory rock specimens (e.g., Lockner et al, 1991; Paterson & Wong, 2005; Renard et al, 2019a) to crustal fault zones stretching over several kilometers (e.g., Cilona et al, 2012; Fossen et al, 2007, 2018; Rotevatn et al, 2016). Recent studies (Baud et al, 2017b; Huang et al, 2019; Ji et al, 2015; Louis et al, 2007; Renard et al, 2019a; Wong & Baud, 2012) have argued for a systematic microstructural characterization including X-ray computed tomography (lCT) and digital volume correlation technique to provide a consistent description of the multiscale mechanics of strain localization in porous reservoir rocks With this motivation, we present here triaxial deformation tests on Adamswiller sandstone, Bentheim sandstone and Anstrude limestone (of comparable porosities 20–23%) subjected to low and intermediate confining pressures (5–30 MPa). Dilatational bands may increase the permeability while the compaction bands decrease it (Zuluaga et al, 2014)
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