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Integrating Cores, CT, NMR, HPMI Data and Well Logs to Characterize the Pore Structure of Carbonates - A Case Study From the Mishrif Formation in Southeast Iraq


Carbonate reservoirs typically contain a wide pore throat sizes ranging from the macro-scale to nano-scale, and have complex pore types, pore geometry and pore throat connectivity. Microscopic pore structures are the most important factors affecting the macroscopic reservoir quality and the capacity for fluid storage and flow in carbonates. Evaluation and characterization quantitatively the microscopic pore structures, including pore types, pore geometry, pore size distribution, and pore connectivity, are of great importance for development and enhancing petroleum recovery. This study took the Middle Cretaceous Mishrif Formation, one of the richest oil-bearing strata and widely distributed in the Middle East, in a supergiant oilfield in southeast Iraq as example. The observations of cast thin sections, lab tests of 2-D image analysis, porosity, permeability, cores CT scans, high pressure mercury injection (HPMI) and nuclear magnetic resonance (NMR) as well as analysis of field conventional and special well logs responses were conducted to comprehensive characterize and describe the entire pore structure of the Mishrif carbonates. Six pore types were recognized based on the cast thin section and CT scans, the pore size, pore geometry of each pore type were obtained on the basis of 2-D image analysis and NMR. Pore throat size distribution and capillary parameters of the Mishrif carbonates were examined based on HPMI. The quantitative relationship between pore geometry parameters and pore throat size in microscopic as well as the relationship between microscopic pore structure parameters (pore size and pore throat size) and macroscopic reservoir quality parameters, especially permeability, were discussed and clarified. The fractal theory was introduced to evaluate the pore structure and two fractal features were obtained for characterizing the strong heterogeneity Mishrif carbonates. Then depositional and diagenetic imprints controls on the pore structure were discussed. Lastly, the conventional and special well logs responses features were determined and the identification model of pore structure types for uncored wells was built and show good prediction in blind wells testing. This study provide comprehensive and quantitative characterization of the microscopic pore structure, bridge the gap between the microscopic pore structure and macroscopic reservoir quality, which will improve the understanding of strong heterogeneity Mishrif Formation and similar carbonates.