ELASTIC MODULUS CALCULATION FOR VARIOUS PORE GEOMETRY OF ROCKS MODELS USING FINITE ELEMENT METHOD

• Main Author: Firmansyah
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/37415
• Institution: Institut Teknologi Bandung

Understanding physical properties of rocks was important in exploration geophysics especially in data interpretation e.g. mapping of hydrocarbon resources. The earlier study showed that physical properties of rocks (elastic, electric, magnetic, and etc.) depend on complexity of microstructure and pore geometry of porous rock. In this study, the calculation of elastic properties was carried out in various pore geometry based on numerical models which calculations using finite element method. Numerical modeling was conducted as preliminary study to determine the possible effects on elastic properties due to the complexity of microstructures and pore geometry. The pore geometry in this study was classified into two types i.e. cracks (in 2D shape: lines and bar; in 3D shape: needles, cylinder and penny coin) and caused of grain sortation (in 2D shape: diamond, ellips with varying aspect ratio ????; in 3D shape: diamond, ellipsoida, sphere). Besides of that, the effect of orientation, size, and distribution of pore was investigated. Extended study has done to investigate the effects of clay, clay-type, grain stiffness, and clay distribution, and also effect of fluid saturation. An alternative calculation has been carried out using homogenisation method (upscalling) by comparing the effective bulk modulus in smaller sub-samples to larger sub-samples. The results shows that increasing pore size will increasing Young modulus. Different of pore shape (in 3D) shows that Young modulus from higher to lower is Penny coin > Sphere > Ellipsoida > Cylinder > Polihedral > Diamond. The orientation effect of a single inclusion shows that maximum decrease trend of Young modulus while the angle was ????=90°. The symmetrical property only experienced by single pore bar-shape, ellips-shaped, and diamond-shape 1. While for diamond-shape 2 was not symmetric, its conclude that not all single pore has symmetrical properties. The aspect ratio effect with constant porosity shows there was misalignment to Kachanov’s theory gradually with change of aspect ratio. Griffiths et al., 2017 shows the same trend with this study, so its important to consider the porosity value while varying aspect ratio, we can not arbitrary varying value of a and b. When the angle ????=0° there was decreasing of Young modulus together with change value of aspect ratio to 1. While the angle ????=90° there was increasing of Young modulus together with change value of aspect ratio to 1 which it was a good agreement with Kachanov et al., 1994. The effect of the pore distribution of both crack and granular shows the same properties, that is, if the pores are well distributed parallel to each other it will have an impact on the effective bulk modulus which is the smaller than the random distribution. And also, for smaller porosity (< 15%) crack distribution was more sensitive. The higher clay fractions present in the rock will increase the value of the effective bulk modulus. The difference in intrinsic bulk modulus of clays also influences its effective bulk modulus, which is related to the clay type. In addition, the density level of the granular/ solid constituent of rock also affects the value of the effective bulk modulus, where the effective bulk modulus of stiffer granular is greater than the softer granular. And also, influence of clay distribution was quite significant when clay fraction under 0.25 from total pore. The upscalling approach that has been carried out is by comparing the average effective bulk modulus of smaller sub-samples to larger sub-samples showing the effective bulk modulus is almost the same, so this approach can be applied in future research for large image data.