Computation of pseudosonic logs in shallow fresh/brackish water wells :a test case in Brunswick, Georgia
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Due to the usefulness of sonic logs in formation evaluation, efforts have been made to develop a method for calculating pseudosonic logs for wells in which sonic logs were not originally obtained. These efforts attempt to use electrical resistivity data in the calculation of pseudosonic logs by means of empirical scale functions. The purpose of this study is to examine ways of applying these relationships in relatively shallow wells where the principal formation fluid is fresh or brackish water. Data from four wells situated in Brunswick, Georgia were used in this study.
Conventional focused resistivity logs are sensitive to beds as thin as one foot and can provide detail similar to that seen on sonic logs. Focused resistivity logs should be best for conversion to pseudosonic logs in shallow wells, where invasion is minimal and the water used for drilling fluid has electrical resistivity close to that of formation water. Sonic and resistivity logs from a representative well are needed in the procedure for finding an empirical relationship between sonic transit time and resistivity. Values of transit time plotted versus resistivity are read from corresponding depths on both types of logs. The graphs obtained in this study reveal significantly more scatter than previously published graphs based upon deep well data.
An important feature clearly evident in the graphs is the presence of groups of points which me offset from each other. A separate scale function relating transit time and resistivity can be obtained from each group of points. It is noted that the different groups correspond to differences ir1 the chlorinity of the formation water. The results of this study indicate that it is necessary to consider the salinity of the formation water as well as electrical resistivity for purposes of calculating pseudosonic logs. In previous studies three constant coefficients were deterrnined experimentally in order to obtain an empirical scale function. The present study suggests that it may be possible to replace these constants with chlorinity dependent coefficients. The final results of this study indicate that reasonably reliable pseudosonic logs can be obtained only by using high quality focused resistivity logs from wells where information about the salinity of the formation water is also available.
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