Geomechanical characterisation of organic-rich shale properties using small scale experiments and homogenisation methods

Abstract

Shale, or mudstone, is the most common sedimentary rock. It is a heterogeneous, multimineralic natural composite consisting of clay mineral aggregates, organic matter, and variable quantities of minerals such as quartz, calcite, and feldspar. Determination of the mechanical response of shales through experimental procedures is a challenge due to their heterogeneity and the practical difficulties of retrieving good-quality core samples. Therefore, in recent years extensive research has been directed towards developing alternative approaches for the mechanical characterisation of shale rocks. In this study, a nanoscale mechanical mapping technique called PeakForce QNM R has been combined with imaging and chemical analysis in order to investigate the mechanical response of each constituent of the shale microstructure. Isotropic elastic behaviour was observed for silt inclusions while a highly anisotropic response was found in the clay matrix. Organic matters with different levels of thermal maturity were investigated and the elastic moduli were determined. These information are essential and useful in order to predict or understand the macroscopic mechanical response of shale rocks. Indentation testing was then carried out in order to scale up the nano-mechanical measurements. This test allows for generating data related to the mechanical behaviour of shale rocks from shale cuttings. Shale samples with a range of mechanical behaviour, from soft to hard, and mineralogical compositions were used in these tests. Issues related to indentation testing such as loading and unloading rate, tip shape and creep behaviour were studied. The capabilities and limitations of this test applied to shale rock were further clarified. Aside from these experimental studies, the Micromechanical modelling (rock physics), a mathematical description of composite-like material, was theoretically and practically studied as an alternative approach for predicting the elastic response of shale rocks. The limitations and the ranges of applicability of the micromechanical formulations were evaluated using direct numerical modelling of shale microstructure. Suitable formulations for homogenisation of shale composite structure were determined. Finally, the data obtained in the nano-scale experiments, as input data, and the results of indentation testing, as the validation III data sets, were adopted for these mathematical formulations. In the last step, numerical modelling of indentation test was undertaken to back-calculate the plastic response of shale samples using the load-displacement curves obtained from this test. The recently developed Material Point Method has been implemented to simulate the large deformation that can occur when pressing the indenter into the shale surface. The nonuniqueness problem of the indentation curve for pressure-sensitive materials was addressed using two different indenter geometries. Inverse analysis was conducted simultaneously until a set of parameters was found matching both experimental curves.