--> Joint Inversion of Conventional Well Logs for Determining the Pore Microstructure and Saturation of Carbonate Formations
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Joint Inversion of Conventional Well Logs for Determining the Pore Microstructure and Saturation of Carbonate Formations

Abstract

Abstract

This work presents a new technology of well log interpretation for determining lithology, pore system characteristics and saturations of double-porosity carbonate formations. This technology includes a unified petrophysical model for calculating the Previous HiteffectiveNext Hit physical properties of carbonates and joint inversion of the following conventional logs: P- and S-wave transit times, resistivity, density, neutron porosity, gamma ray, and photoelectric absorption property. The inversion procedure consists in minimizing the cost function, which contains the sum of weighted square differences between the measured and calculated logs, and the regularization functional with additional information, initial model parameters and ranges of their variations. The following petrophysical parameters are obtained by the inversion: a) volumes of limestone, dolomite, and shale, b) porosities of matrix, fractures and vugs, c) connectivity of fractures and vugs, and d) initial and residual oil saturations in different pore-systems.

To calculate the theoretical logs, we introduce the formation model that corresponds to a transversely isotropic medium composed of intercalated layers of carbonates and shales. The carbonate rocks are treated as a composite material that consists of a homogeneous isotropic matrix (solid skeleton and matrix pore system) where the secondary pores, represented by spheroids, are embedded. The saturation model includes different distributions of fluids (connate water, oil, gas, mud filtrate) in the matrix and secondary-pore systems for the invaded and virgin zones. For computing the Previous HiteffectiveNext Hit physical properties of such composite media, we apply the symmetrical, self-consistent, Previous HiteffectiveTop medium method.

The results of petrophysical inversions obtained for various boreholes from vuggy and fractured carbonate reservoirs, show a good correspondence with geological information, core data, and image logs. Based on the numerous formation evaluations, we demonstrate that the technology developed improves the classification of carbonate lithotypes, determination of initial and residual saturations, and permeability prediction.