New insight of correlation between digital rock analysis and petrographic study for rock type classification within carbonate reservoir transition zone

The characterization of capillary transition zone (TZ), which contains large amount of oil, is challenging, because of the rock types in it affecting the fluid flow behavior. The thickness of TZ may vary from a few feet to hundreds of feet, spanning extremely low permeability and very high permeability reservoirs. Conventional core analysis (CCA) and special core analysis (SCAL) techniques can be used to study and model TZs, but the detailed pore structure information governing the rock properties rarely can be obtained. Digital rock analysis (DRA) technique can directly capture the images of rock microstructures and accurately simulate the rock properties based on digital rock images. Consequently, DRA technique can be well used for reservoir characterization. In this paper, thin sections (TS), scanning electron microscope (SEM) images, and X-Ray computed tomography (XCT) scans are used to study the rock properties for reservoir characterization. The petrography and diagenesis of TZ are also analyzed based on these digital images and simulations. DRA technique is used to simulate the rock properties. The results show that DRA can be well used to predict the rock properties such as porosity and P- and S-wave velocity. The permeability of TZ samples is found to be less than 1 mD. The TZ rock samples are qualitatively and quantitatively classified into three types such as matrix, fractured, and vuggy by an improved carbonate rock index (ICRI) method. These show that DRA is a powerful tool for carbonate reservoir characterization.