Abstract

We deformed organic-samples from the Alum shale, Denmark, under the scanning electron microscope (SEM) using a deformation stage to observe the micromechanical deformation occurring during fracturing and creep deformation. In the first set of experiments, a disc-shaped sample was compressed to observe the formation of a mode I fracture as in a Brazilian disc test. In most cases, we were able to see the formation of a thin opening fracture before the macroscopic failure by a through-going fracture. Tensile strengths estimated from the mechanical data are in good agreement with those from normal-scale Brazilian disc tests. In another configuration, a rectangular sample was compressed under controlled loads in order to observe the elastic and creep deformation of the shale samples. Digital image correlation (DIC) analysis using the acquired microscope images show that elastic deformation within the rock is spatially heterogeneous as expected, but also the creep deformation occurs in a heterogeneous manner. Organic materials in the shale appear to be relatively compliant compared to other mineral components for both elastic and ductile deformation. While difficulties exist in capturing images during the deformation processes, these attempts give additional insights to the micromechanical processes governing the brittle and ductile deformation of shales.

1. INTRODUCTION

Revealing the micromechanical processes that occur during rock deformation can help understand the macroscopic mechanical behavior observed during rock mechanics experiment and its controls. In brittle fracturing, observations of the gradual coalescence of acoustic micro-fractures revealed the gradual damage accumulation prior to rock failure and lead to damage rheology models which can quantify the gradual loss of stiffness as rocks approach failure [1, 2]. It is also common in ductile deformation of rocks that certain relatively weak phases in the rock dictate the overall strength of a rock as evidenced by strain localization features observed frequently in geological samples [3].

In this study, we investigate the possibility of visually observing the micromechanical deformation process by conducting in-situ deformation experiments under the scanning electron microscope (SEM) and applying digital image correlation (DIC) analysis to the acquired digital images. DIC has been applied to rock mechanics studies primarily utilizing optical images [4-6] but still rarely with SEM images due to technical challenges [7]. Utilization of SEM images has the advantage of greatly enhancing the magnification while challenges lie in the longer image acquisition times and image instability compared to optical images.

We apply this technique to characterize the micromechanical deformation of organic-rich shales. Shales are characterized by a fine heterogeneous fabric which makes it difficult to observe deformation structures by simply comparing microscope images of pre- and post-deformed samples. Moreover, shales are clay-rich poly-mineralic rocks suggestive of complicated rheological behaviors. Laboratory studies have shown the mixed brittle-ductile behavior of these rocks even at unsaturated room temperature conditions suggested to be mainly dependent on the amount of clay and organic contents [8, 9]. However, much remains unclear including the exact physical mechanism responsible for these deformational behaviors. Thus a direct observation identifying the mineral constituent responsible for the overall behavior would enhance our understanding and capability to predict the long-term rheological behavior of these rocks. Such insights are of great importance for engineering purposes as ductile shale behavior is related to the stress variations observed in shale gas reservoirs [10] and shales are considered as potential sites for nuclear waste disposals [11].

This content is only available via PDF.
You can access this article if you purchase or spend a download.