Quantifying the Uncertainties of Macro- and Micro-Mechanical Measurements on Sequestration Reservoir Formations
Authors: Wu Zhidi , Edelman Eric , Ritterbush Kathleen , McPherson Brian 
Published: August 2024

ABSTRACT:
Geomechanical properties of rocks exhibit significant variation across different measurement scales, primarily due to heterogeneity. Micro-scale geomechanical tests offer a fresh perspective, with minimal sample volume requirements, low cost, and quick measurement turnaround time. However, previous research implies significant variance and uncertainty in micro-mechanical measurements and tries to address this issue by correlating the mineral phase mapping to elastic modulus mapping; attribution, in part, is the need for standard testing procedures on rocks. The objectives of this study are: 1) quantify chemical and physical controls on micro-mechanical property and 2) determine the source of uncertainties associated with nanoindentation measurements. To reach these goals, we conducted mechanical tests on three different scales: triaxial compression test, scratch test, and nanoindentation test. We found that mineral phase weight percentage is highly correlated with nanoindentation elastic modulus distribution. Finally, we conclude that the nanoindentation test is a mineralogy and microstructure-based method. The uncertainty of the testing method is associated with overlooking the pore space. An improved testing protocol that utilizes mineral mapping and substantial and random data sampling must be developed to quantify the uncertainty of nanoindentation measurements.

1. INTRODUCTION
Micro-mechanical tests are widely used to characterize the mechanical properties of rocks primarily because of lower testing costs compared to conventional mechanical testing methods, especially in highly-fractured caprocks or ultra-deep reservoir rocks (Vanorio et al., 2008; Yang et al., 2024). Micro-mechanical methods, such as nanoindentation and scratch tests, have the advantages of obtaining large statistical sampling, high precision of local measurement, flexible sample volume, and cost-effectiveness compared to conventional mechanical tests (Auvray et al., 2017; Bennett et al., 2015; Hampton et al., 2022; Khan et al., 2021; Kolawole & Ispas, 2020; Ma & Huang, 2021; Naeimipour et al., 2015; Randall et al., 2009; Sakulich & Li, 2011; Suárez-Rivera et al., 2002).
Among micro-mechanical tests, nanoindentation test has gained special attention and popularity in the geomechanical field (Oliver & Pharr, 1992, 2004). Adequate studies show that the relative size of the indenter, cutter geometry, contact depth (or the loading force), and the mineral grain size are critical to determining the quality of the nanoindentation measurements(Auvray et al., 2017; Thom, 2019). The reliability of nanoindentation measurements relies on adequate statistical sampling (Kumar et al., 2012). Most recent studies on applying nanoindentation tests on shales are driven by the need for mechanical characterizations on highly fractured and poorly reserved caprocks in unconventional formations. Micro-mechanical properties measured of these rocks shows a significant range in elastic modulus, attributable to heterogeneity in shales. (Bennett et al., 2015; Charlton et al., 2021; Goral et al., 2020; Hampton et al., 2022; Kumar et al., 2012; Liu et al., 2016; Liu et al., 2021; Mighani et al., 2019; Shi et al., 2019; Shukla et al., 2013). Such heterogeneity in mineralogy and microstructure may explain variability in nanoindentation elastic modulus of up to orders of magnitude, similar to the variations observed on the field scale (Goral et al., 2020).