Case Study of Permeability, Vug Quantification, and Rock Typing in a Complex Carbonate

Abstract

Abstract This case study demonstrates a new method to compute continuous permeability and estimate reservoir rock type from logs in a complex, heterogeneous, Middle Eastern carbonate reservoir. The 795 ft conventionally cored interval consists of interbedded limestones and dolomites with anhydrite cement and features a wide variety of textures. In some intervals the depositional textures are preserved, in others they are highly altered by diagenesis. Vugs are developed in several intervals. Computation of permeability from porosity alone yields scatter of a factor of 700. Rock typing using only conventional logs was unsatisfactory due to the poor permeability estimation. The effect of geological complexity on the log based prediction is overcome by including pore size distribution data from a combination of NMR and borehole electrical image logs. This data is sufficient to partition the porosity according the pore size, compute permeability and assess the rock types, independently of mineralogy, facies, and other variables. The results are validated by comparison to core derived properties and formation tester mobilities. Incorporation of the pore size information into the log based interpretation reduces the scatter in computed permeability to a factor of less than 10. The assumptions and principles of the log analysis method were validated in the lab through extensive characterization of the pore system over a range of scales. Data from a variety of methods including MICP, lab NMR, BET surface area, thin section analysis, continuous vuggy porosity analysis from the core slab, minpermeametry and other advanced research methods are included. A key result of this study is that a relatively simple method for log derived permeability and rock type analysis in carbonates, first developed in high porosity limestones, can be successfully applied in this lower porosity (10 – 20 p.u.) carbonate. This gives us confidence to project that the method could be applicable to many other carbonates worldwide.