Permeability estimations based on core-to-log relationships in dual-porosity carbonate reservoirs usually fail to reach the permeability measured on drillstem tests (DST). The reasons behind this inaccuracy are mainly related to the inherent low representability of rock samples in such systems and to the limitations of the logs in the presence of structures, such as caves and vuggy fractures. Conversely, high-resolution ultrasonic borehole image logs provide not only an image of the mega- and gigapore system present in dual-porosity reservoirs but also have been used empirically to identify the permeable facies within them. These logs have been widely used to identify fractures and caves normally related to fluid losses during the drilling operations but so far have not been used to estimate the permeability of such structures. Thus, the challenge of this study was to develop a quantitative method for permeability estimation using ultrasonic image logs as an input and to reach an acceptable calibration with matrix permeability measured in the laboratory as well as the whole system permeability measured in the DSTs. Ultrasonic-image-derived estimated permeability curves were calculated for three different wells drilled in karstified carbonate reservoirs and reached a very satisfactory calibration with matrix permeability measurements and permeability estimated on DSTs.
Carbonate reservoirs have a wide range of textural and structural variations in their framework elements and pore systems that may have been created by syngenetic, diagenetic and superimposed deformational processes. This is the case for naturally fractured reservoirs and karstified reservoirs, in which different scales of porosity are present: the matrix scale and the fractures/vugs scale (Warren and Root, 1963). Such variations on the scale of porous media address heterogeneities to the reservoir flow properties that cannot be efficiently quantified in the laboratory, making it difficult to tie those properties to the ones registered by DSTs. Permeability estimation based on the NMR log also fails to reach the order of magnitude measured in DSTs due to the log/tool limitation in the presence of caves and vuggy fractures.