Estimation of Petrophysical Parameters of Heterogeneous Carbonate Rock Sample with Multi-Scale CT Images

Abstract

AbstractThe understanding of petrophysical properties is essential for the assessment and exploitation of hydrocarbon reserves. Reservoir rock is one of the main sources used to derive petrophysical properties by laboratory measurements. However, probing complex pore structure and heterogeneity of carbonate rocks are difficult with conventional experimental methods. Digital rock physics (DRP) is able to compute the petrophyscial parameters (e.g. porosity, permeability, elastic modulus) from three dimensional (3D) computed tomography (CT) images, which can make up for some disadvantages of traditional experiments. It also provides an understanding on how the large variations in petrophysical properties are caused by wide variations in pore type, pore shape, and pore interconnectivity.This paper is a case study on analysis of heterogeneity and estimation of petrophysical parameters in a heterogeneous rudstone sample using multi-scale CT images with three resolutions of 39.48 μm/voxel, 1.0357 μm/voxel and 65 nm/voxel. The Back-scattered Electron Image (BSEI) analysis that is used to locate subsamples in low-resolution image for high-resolution CT scan shows the range of pore-throat radius size of this sample is from 0.1μm to 10μm and most pores are under 1μm. The low-resolution images (here resolution of 39.48 μm/voxel and 1.0357 μm/voxel) show a poor pore connectivity. The high-resolution image (65 nm/voxel) provide a more detailed pore structure that contributes to estimation of petrophysical properties. The estimated porosity (21.92%) and absolute permeability (6.54mD) from the high-resolution images are close to the lab measurements (22.7% and 4.9mD), which shows the accuracy of DRP in this particular sample. The relative errors of elastic modulus between the simulation and the measurements are 77.23% and 87.68% for bulk moduli and shear moduli, respectively. The image registration locates the Nano-CT images in the low-resolution image (1.0357 μm/voxel), which better characterize the pore geometry in different 3D image and provide a new way to do the upscaling work. The multi-scale images, the visualization of fluid velocity distribution, the lab measurements, and the simulated values show the heterogeneity of this carbonate sample.