An Integrated Approach To Obtain Reliable Permeability Profiles From Logs in a Carbonate Reservoir

Abstract

Summary Permeability is one of the key petrophysical parameters in reservoir evaluation. Information about permeability is commonly derived from cores and test data, which generally cover only part of the reservoir section, but can also be derived from logs and then extrapolated to uncored intervals and wells. Two logs provide such information: acoustic and nuclear magnetic resonance (NMR). In the Karachaganak field, an approach based on the acoustic tool was preferred because of the textural characteristics of the vuggy carbonate reservoir. The approach relies also on the use of image logs to obtain a detailed description of the reservoir-rocks texture and to discriminate between rocks with primary interparticle porosity or very small vugs and lithotypes with multimodal distribution of pores, enlarged vugs, and touching vugs. More than 900 m of core have been used to validate the permeability log derived from the analysis of Stoneley waves in 25 wells from this field. A correlation between the validated log-derived permeability and the textural facies from image logs has allowed the relationship between permeability variations and the geological framework to be established. The results have been compared with dynamic data from production logging through the definition of flow units from Stoneleywave-derived permeability and porosity-log data and the use of a stratigraphic-modified Lorenz plot (SMLP) to identify possible fluid-entry points. Three main permeability trends have been identified: For undolomitized or patchily dolomitized biohermal deposits for pervasively dolomitized lithologies characterized by lower mean permeability values for facies characterized by well-developed vuggy porosity with enhanced dissolution phenomena (i.e., touching vugs and microfracturing) Also, well-test results have highlighted the occurrence, in some wells, of very high k values in the biohermal deposits, which are clearly unmatchable by log-derived matrix permeability. Open fractures have been observed in these wells on the image logs, thus suggesting that the enhanced permeability and the improved well performances have to be related to their presence. The comparison of log-derived permeability and well-test results has given a new perspective in the interpretation of well-test results with respect to the geological framework and a well-defined sequence-stratigraphic model. The extension of this methodology to new wells could improve knowledge of the petrophysical characteristics of the reservoir and allow better prediction of reservoir productivity.