Litho-Facies and Permeability Prediction From Electrical Logs Using Fuzzy Logic

Abstract

Summary Several areas of the geosciences have benefitted from the application of the new mathematics of "fuzzy logic."1,2 The oil industry now routinely uses new interpretation techniques, based on fuzzy logic, to predict permeability and litho-facies in uncored wells. Litho-facies and permeability prediction have presented a challenge to formation evaluation due to the lack of tools that measure them directly. The method described can be used as a simple tool for confirming known correlations or as a powerful predictor in uncored wells. Fuzzy logic is simply an application of recognized statistical techniques. Whereas conventional techniques deal with absolutes, the new methods carry the inherent error term through the calculation rather than ignoring or minimizing it. This retains the information associated with the error and gives surprisingly better results. One clear application is to litho-facies determination. Litho-facies typing is used in well correlation and is important for building a three-dimensional model of an oil or gas field. The technique makes no assumptions and retains the possibility that a particular litho-facies type can give any well log reading although some are more likely than others. This error or fuzziness has been measured and used to improve the litho-facies prediction in several North Sea fields. In one study, descriptions from 10 cored wells were used to derive litho-facies descriptions in 30 uncored wells. This technique gave near-perfect differentiation between aeolian, fluvial, and sabkha rock types. In addition, it went some way towards differentiating between sandy, mixed, and muddy sabkhas. Using the fuzzy logic technique gives much better predictions than more complicated methods such as neural networks or cluster analysis. A second application is permeability calculation. Knowledge of permeability is important in determining the well completion strategy and the resulting productivity. The problem with permeability prediction is derived from the fact that permeability is related more to the aperture of pore throats rather than pore size, which logging tools find difficult to measure. Determining permeability from well logs is further complicated by the problem of scale, well logs having a vertical resolution of, typically, 2 ft compared to the 2 in. of core plugs. The new techniques quantify these errors and use them, together with the measurement, to improve the prediction. This new approach has been used in several fields to obtain better estimates of permeability compared to conventional techniques. In addition, the method uses basic log data sets such as gamma ray and porosity rather than depending on new logging technology.