Geophysical research: article

I. Chebotareva1
A. Kamshilin2
1 Oil and Gas Research Institute, Russian Academy of Sciences
2 Schmidt Institute of Physics of the Earth, Russian Academy of Sciences
Journal: Geophysical research
Tome: 20
Number: 3
Year: 2019
Pages: 87-102
UDK: 550.34.016; 537.868; 534-18
DOI: 10.21455/gr2019.3-6
Full text
Keywords: seismoelectric effect, rocks, type of fluid saturation, heterogeneous media, laboratory tests
Аnnotation: Experiments on studying the characteristics of the seismoelectric effect of electrokinetic origin were carried out using sandstone cores with strong lateral nonuniform saturation with kerosene or NaCl solutions. The liquid was added drop wise through the puncture of the protective film and was 6 % of the volume of the pore space, which created a strong lateral heterogeneity at the side edge of the cylindrical core. All core samples were pre-dried for a long time at 110 °C and covered with a protective film. Nevertheless, in the initial state they contained residual water. The acoustic impact was created by a piezoelectric vibrator in the form of a signal linearly modulated in frequency in the range 0.5-10 kHz. For all types of core saturation high amplitude second harmonics were observed in the spectra of electrical oscillations. An equal strong increase in the first harmonic of electric response spectrum, up to 10 dB, occurred both with the injection of a weakly concentrated NaCl solution and with the injection of a non-polar liquid - kerosene into the core. However, the character of the change in frequency spectrum and the temporal dynamics of the electric response for the polar and non-polar fluids are different. The amplitude spectrum of the core response after injection of kerosene varies greatly over the entire frequency range with a range of deviation more than 30 dB and remains stable in shape, but decreases in amplitude after long vibration. The shape of the electrical response spectra after injection of NaCl solutions is well preserved, except for narrow intervals of strong variations: 2.45-4.95 kHz for NaCl solution of 0.05 mol/l and 5.45-7.15 kHz for NaCl solution of 0.5 mol/l. After a long vibration, the amplitude of the spectrum decreases and varies greatly in shape. The amplitude of the second harmonics for kerosene first increases after injection, then decreases after a long vibration to the level of the initial state of the core. For NaCl solution of 0.05 mol/l, the amplitude of the spectrum of the second harmonics increases after injection and continues to increase after vibration. For solution of 0.5 mol/l no significant changes are observed. The detected features of the seismoelectric effect of electrokinetic origin are apparently caused precisely by the presence of strong lateral heterogeneity in the saturation of the rock, which generates a very non-uniform electric and magnetic field during the passage of a longitudinal wave along the core axis.