Micro-mechanical Properties of Niger Delta Sandstone Rock using Advanced Experiments and Multi-scale Modelling

The focus of this investigation is to understand the micromechanical characteristics of the oil-bearing Niger Delta sandstone at different length scales. Initially, the sandstone samples are experimentally characterised to understand their morphological, physical, chemical and mechanical properties at grain scale and bulk scale where applicable. In spite of a significant level of scientific advancements made so far, sensing stress distribution characteristics of opaque and anisotropic materials such as sandstone rock remains as a stiff challenge in a wide range of science and engineering fields including geotechnical, geophysics, petroleum, mining, minerals, advanced materials and particulate science and engineering. Here we present an original framework for simulating and quantifying the strength characteristics of real sandstone samples using combined measurements and modelling strategy. Using photo-stress analysis methodology, first we sense elastic shear stress (or strain) distribution and its components along orthogonal directions on the surface of a V-notch sandstone sample under mechanical loading. Using this and applying a classical grain-scale model, the stiffness ratio of the sandstone is evaluated. This measure is also compared with using ultrasound sensors and a good level of agreement is obtained. Thereafter, the grain-scale stiffness ratio which characterises the signature of material anisotropy is fed as an input in to the discrete element modelling (DEM) of cylindrical sandstone rock samples subjected to uni-axial and tri-axial compression loading. Physical experiments are also conducted to evaluate their load-displacement characteristics and bulk fracture strength of sandstone sample under these loading conditions. A good level of agreement is obtained between the results of simulations and experiments. Taking advantage of the validated DEM simulations, an extensive level of parametric studies are conducted to evaluate the influences of different grain-scale properties on the bulk strength and fracture characteristics of sandstone. Thus the current multi-scale framework can be applied in future to quantify the strength characteristics of such complex and anisotropic materials in a reliable manner.