GB1519977A - Method of determining partial water saturation in shaly formations - Google Patents

Abstract

1519977 Borehole logging EXXON PRODUCTION RESEARCH CO 16 Oct 1975 [3 March 1975] 42450/75 Heading G1N The partial water saturation of a formation in a borehole is determined from the dielectric constant of the formation. Shallow and deep dielectric constant measure- ments are made and the equation K/K O =S<SP>P</SP> w is employed where K is the dielectric constant of a shaly sand formation, Ko is dielectric constant at 100% water saturation, Sw is water saturation and p is shaliness exponent. Sw is determined from the above equation and the partial hydrocarbon saturation SH obtained from the relation S H =1-Sw. The dielectric constant is obtained using a logging tool carrying a pair of bow shaped electrodes 7A, 7B, Figs. 1 and 2 (not shown) which is lowered down the borehole with the electrodes in contact with the formation. Squarewaves from generator 27 are applied to 7A and via low resistor 39 to 7B and the voltage appearing across resistor 39 is applied to the input of gated amplifier 29, the gating signal being derived from 37 and the output from 29 being fed to a surface oscillograph. The capacitance of the earth formation between the electrodes 7A, 7B is obtained from the area of the pulse on the oscillograph, the applied voltage from the generator, voltage across resistor 39 and the value of resistor 39. The dielectric constant is then calculated from the obtained capacitance value. Alternatively the instrument may be calibrated to measure dielectric constant directly in which case the dielectric constant of a number of samples corresponding to the drilled formation is obtained by known methods, the samples are placed successively in contact with the electrodes 7A, 7B and the area of the pulse on the oscillograph measured and a calibration curve plotted relating area and dielectric constant. The measurements on the borehole are then made and the results correlated with the curve.