Abstract
Creep is a term used to describe the time dependent behavior of rock. Knowledge of creep can be used to predict different phenomena such as reservoir subsidence associated with production, wellbore stability and proppant embedment problem. In this work, we investigate creep behavior at small scale using Nanoindentation. After the initial Nanoindentation loading stage, the indenter tip is held under constant maximum force for a period of time while deformation is recorded. The measured creep values are compared for different materials including soda-lime glass, Lyons sandstone, Sioux Quartzite, pyrophyllite, and Indiana limestone. The measurements on shales are then presented. The experiments measure comparably higher creep values for shale, i.e. 10 times higher than the one for fused silica. This creep depends strongly on the composition. It correlates positively with Total Organic Carbon (TOC) and clay content, while correlates negatively with carbonates. The measurements reveal a directionally dependent (anisotropic) creep with a higher value in the direction perpendicular to the bedding. Mercury Injection Capillary Pressure (MICP) and acoustic velocity measurements are correlated with the creep. The measured creep value could be an indicator of the volume of smaller pore sizes (nanometer scale) which are extensively found in the organic and clay components.
1. INTRODUCTION
Time dependent deformation of rock is an important factor for considerations of underground dynamics. This phenomenon can be predicted based on laboratory scale creep measurements under constant stress conditions. The behavior of rock under this constant stress usually consists of three distinct strain response stages. Figure 1 symbolically illustrates these three different stages: 1) primary or decelerating, 2) secondary or "steady state", and 3) tertiary or accelerating creep (Heap e al., 2009). The creep strain rate is derived using the linear portion (secondary stage). Nanoindentation method has been used successfully to evaluate various aspects of material mechanical behavior at a smaller scale. To name a few, Oliver and Pharr (1992) deduced hardness and modulus of the tested materials using the loading and unloading curves. Fracture toughness can be estimated after modification (Lawn an Marshall, 1979). Fatigue and impact load can be applied using some modifications (Beake et al., 2001). Bower et al., (1993) examined a creeping solid under Nanoindentation. In this report, the Nanoindentation method is employed to evaluate the creep behavior of the candidate rocks in a finer scale.
