Digital and Conventional Techniques to Study Permeability Heterogeneity in Complex Carbonate Rocks

Most carbonate reservoirs are characterized by multiple-porosity systems that impart petrophysical heterogeneity to the gross reservoir interval. This heterogeneity complicates the task of reservoir description and thus necessitates the establishment of accurate and detailed understanding of the geological heterogeneities and their impact on petrophysics and reservoir engineering. One of the fundamental input parameters into reservoir models is permeability. The challenge would be to select appropriate samples that represent reservoir heterogeneity for accurate acquisition of vertical to horizontal permeability K-v/K-h data. In an unpublished work, thousands of plug permeability measurements were performed to obtain K-v/K-h ratios across a large carbonate field in the Middle East. The results were largely influenced by reservoir heterogeneity and yielded large K-v/K-h ratios (greater than unity). Such data would need to be acquired on the same rock volumes for proper K-v/K-h ratios. In this work, permeability measurements were investigated using digital and conventional techniques to determine the effect of heterogeneity. Detailed thin-section descriptions and mercury injection capillary pressure ( MICP) tests were used to understand the different rock types. Advanced three-dimensional (3D) X-ray CT imaging was acquired at multiple scales for detailed digital rock characterization. Permeability was computed directly on the 3D images by the lattice Boltzmann methodology and upscaled to the plug and whole-core levels. Permeability varied largely among different scales/locations and was clearly linked to complex geological features in the rock samples. Integration of the CT images and thin-section photomicrographs provided geological variation in 3D and showed that permeability was influenced by macroscale heterogeneity that may only be examined through multiscale imaging. Larger volume samples were vital in capturing the reservoir heterogeneity, which gave proper K-v/K-h ratios less than unity. Our understanding of the comparisons among different scales will be crucial for upscaling laboratory-measured properties to grid-block scale in reservoir geological models.