Porosity Balance Verifies Water Saturation Determined From Logs
- M.P. Tixier (Schlumberger Well Surveying Corp.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- July 1958
- Document Type
- Journal Paper
- 161 - 169
- 1958. Original copyright American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Copyright has expired.
- 4.3.4 Scale, 5.6.1 Open hole/cased hole log analysis, 2.4.3 Sand/Solids Control, 2.2.2 Perforating, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 5.2 Reservoir Fluid Dynamics, 5.5.2 Core Analysis
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In several log interpretation methods, water saturation is evaluated by use of the ratio of the readings of a short spacing resistivity device and a long spacing resistivity device plus information on the mud filtrate and connate water resistivities, which is often derivable from the SP. These methods are valid over a certain range of conditions, usually specified by moderate invasion and Rxo greater than Rt. Since these methods involve no explicit evaluation of the formation factor, F, the saturation so found may be used in the standard Archie equation to derive a computed formation factor which, as a check, may be compared with the formation factor known from other independent measurements. Discrepancies in the two values of formation factor generally indicate that the ratio method is not within its range of applicability. If the computed F is too low, the corresponding Sw is too low; if the computed F is too great, the corresponding Sw is too great. By means of this "porosity balance" check, and some knowledge of the probable conditions of invasion, the interpretation can often be improved.
The porosity balance check is discussed for the cases of the Induction-Electrical Log Interpretation Method, the Rocky Mountain Method, and the Rxo/Rt Method. For the first method, a discussion is also given for the case of shaly sands.
Although many logging tools are available to obtain a practical value of porosity, the only logging method reflecting the water saturation is electrical logging.
Water saturation is commonly computed from electrical log data by means of the basic Archie formula,
where F is the formation factor, Rw the resistivity of the formation water, and Rt the true resistivity of the formation.
One difficulty in this procedure is a possible error in the evaluation of Rt from the logs - for example, in the case of very deep invasion. There is no possibility, inherent in the procedure, which would provide a check of the accuracy of Rt and, hence, of the computed saturation.
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