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3D Digital Rock Imaging Enhances Mercury Injection Capillary Pressure Analysis

Mercury injection capillary pressure (MICP) laboratory tests are used on a regular basis to characterize reservoir and seal rocks in terms of pore space, pore throat sizes and capillary pressure. Standard MICP tests take core cylinders (plugs) of 1 inch in length and diameter that are intended to represent the same rock type. Yet, the complexity of reservoir rocks makes the selection of MICP plugs very challenging. So, integrating multiple sources of information is beneficial for selecting proper plugs, which, in turn, leads to a more robust interpretation of MICP data for reservoir characterization. The standard MICP plug selection includes the visual inspection of plug’s outer surface for identifying typical features such as visible vugs, fractures, and composite nature. This study focused on plug’s internal structure and presents various metrics based on 3D rock imaging to quantify rock heterogeneity (or homogeneity) and its spatial distribution, which could then be used for an optimal plug selection for MICP testing. A total of 20 carbonate plugs of 1 inch in diameter and length were used in this study. The plugs were first cleaned and dried and their conventional core analysis was conducted. The plugs were then imaged using micro-computed tomography (micro-CT) to get grayscale 3D images with a voxel size down to 10 µm. These images were segmented using a semi-supervised machine-learning (ML) method to produce corresponding digital rock models (DRM) of pore space. Both grayscale images and pore-space DRMs were analyzed using a set of heterogeneity measures of rock texture (one-dimensional histogram statistics, two- dimensional histogram statistics) and pore-space structure (Minkowski parameters, mean chord length and chord fraction length percentiles). As a result, each rock sample had a corresponding vector of normalized heterogeneity metrics ready to be analyzed and clustered. Finally, MICP tests were carried out on all plugs. In summary, digital rock images were examined to assess the rock heterogeneity and its distribution at plug scale using various techniques and validate against the MICP data to determine how the proposed image-based techniques could be utilized for a better plug selection for MICP tests. The present study revealed significant carbonate pore system heterogeneity at MICP plug scale and reaffirmed the importance of advanced plug screening before MICP testing. The described 3D imaging-based approach provided a solid basis to identify heterogeneous plugs before destructive lab tests, therefore saving both valuable rock samples, boosting the quality of MICP data and minimizing uncertainty related to the rock heterogeneity in subsequent petrophysical interpretation and reservoir modelling. The approach was validated against the MICP results acquired on the plugs, which confirmed that the pore throat size distributions followed different trends when the heterogeneity assessment indicated that the plugs were dissimilar. Advanced plug screening and a comprehensive heterogeneity assessment would be most beneficial for highly heterogeneous rocks (thin-bedded, vuggy, diagenetically altered). In addition, the authors are of the opinion that these results pave the way for a significant improvement of quantitative characterization of sample heterogeneity through 3D rock imaging.
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