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Resistivity logging across carbonate reservoirs which have relatively homogeneous porosity will generally show a gradually increasing resistivity profile from bottom to top. If the reservoir has high permeability and the mud filtrate has a contrasting resistivity with respect to the formation water, the invasion of mud filtrate will induce a lateral differential resistivity response. In some instances the oil water contact is not clearly evident directly from the log responses. Generally we interpret the water contact level as being located in the resistivity response transition zone.
The High Resolution Induction (HRI#) has three resistivity measurements with different depths of investigation. Based on geometrical factor modeling it is possible to compute five resistivities which have average depths of investigation radially at 24 inches, 30 inches, 40 inches, 60 inches and 90 inches. By processed interpolation of the resistivity responses and mapping in color format, a resistivity image (HRIMAP#) can be created. The HRIMAP# provides characterization of the gross matrix and fluids resistivity responses. This is useful for graphically showing the mud filtrate invasion profile around the borehole.
These resistivity images can be further enhanced to provide an apparent fluid resistivity image (RWAMAP#). This is accomplished by incorporating factors which are dependent on porosity, mineral and shale contributions to the measured log responses. If this can be accurately modeled, the enhanced resistivity image will allow correlation to in situ fluid resistivity. This leads to a more precise definition of the reservoir behavior including fluid typing.
It is proposed that by studying the patterns of the apparent fluid resistivity images a more detailed understanding of reservoir characteristics can be achieved. From response environmental correction and mud filtrate invasion effects the images may give an improved fluid type identification.
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