The Relation Between Electrical Resistivity and Brine Saturation in Reservoir Rocks

Abstract

In connection with electric log interpretation it is important to know thevalue of the saturation exponent. For example, if in a given reservoir it isfound that the resistivity is three timcs the resistivity observed when thereservoir is 100 per cent saturated with water, this fact would be interpretedas indicating a water saturation of 33 per cent if the saturation exponent were1 and a water saturation of 64 per cent if the saturation exponent were2.5. Experimental Method In the work to be described it was assumed that reservoir conditions aremost nearly obtained when core plugs are desaturated by the capillary pressuretechnique referred to in numerous places in the literature, as for example, inBruce and Welge's paper. In this technique the core, saturated 100 per centwith brine, is placed in contact with a ceramic disc permeable to brine but notto the displacing medium for the displacement pressures used. Pressure is thenapplied to the displacing medium and brine forced out of the core through theceramic disc. Fig. 1 shows the core plug in place in the cell in whichresistivity and saturation measurements are made. Fig. 2 shows the schematicelectrical diagram used to make resistivity measurements on the core plug. Afour-electrode type circuit is used, employing a Hewlett-Packard model 400A, ACvacuum tube voltmeter. The 60-cycle AC current through the core is adjusted to1 milliampere and measured by noting the voltage drop across the calibrated100-ohm resistor. The voltages appearing at probes 1, 2, 3, and 4 are thensuccessively measured. Voltage drops across the top, center, and bottomportions of the core are obtained by subtracting the voltages appearing atsuccessive probes. This technique avoids any polarization or other high contactresistance phenomena which may develop at the current input electrodes.Resistances which may develop between the core and the probes, and which aresmall compared to the 1-megohm input impedance of the vacuum tube voltmeterwill obviously not affect the measurements appreciably. Any very appreciableresistances which may develop at any of the probe wires are detected andallowed for by inserting a 1-megohm resistor in series with the voltagemeasuring probe. If the probe resistance is actually zero, the new voltagemeasured after insertion of the 1-megohm resistor will be approximatelyone-half of that previously measured- since the input impedance of the vacuumtube voltmeter is itself 1 megohm. If any appreciable probe resistance hasdeveloped, the new voltage is found to be appreciably greater than one-half ofthe previously measured voltage. T.P. 2711