Studies of passive seismic data in the frequency range below 20Hz have shown that the frequency content of the ever-present seismic background noise changes above hydrocarbon reservoirs. Different explanations for this observation have been proposed. In this study, the effect of oscillating pore fluids, i.e. oil, on the seismic background noise is investigated. A non-wetting fluid drop entrapped in a pore can oscillate with a characteristic eigenfrequency. Capillary forces act as the restoring force driving the oscillations. A 1D wave equation is coupled with a linear oscillator equation, which represents these pore fluid oscillations. The resulting linear system of equations is solved numerically with explicit finite differences. The most energetic part of the seismic background noise, i.e. frequencies around 0.1-0.3Hz, is used as the external source. This part is presumably related to seismic surface waves generated by ocean waves. It is shown that the resulting elastic wave initiates oscillations of the fluid drops. The oscillatory energy of the pore fluid is transferred continuously to the elastic rock matrix. In consequence, seismic waves in the elastic rock carry a second frequency, the eigenfrequency of the pore fluid oscillations on top of the applied external frequency. Both frequencies can be measured at the earth surface. The presented model is considered as a possible explanation for observed spectral modifications above hydrocarbon reservoirs. Time evolution of the pore fluid oscillations seems to be related to the thickness of the hydrocarbon reservoir.
Spectral modifications of seismic background noise in the frequency range below 20Hz have been observed above hydrocarbon reservoirs (Dangel et al., 2003; Bloch and Akrawi, 2006). A new direct hydrocarbon indication method was developed using spectra of low frequency seismic noise measurements. The physical explanation for these modifications is the subject of current discussions (Graf et al., 2007). Seismic attenuation phenomena in poro-elastic media, subsurface reflection patterns and phase transition effects (Suntsov et al., 2006) have been discussed as possible causes. The behavior of non-wetting fluids entrapped in capillary tubes and in idealized pore spaces were thoroughly studied in the past (Dvorkin et al., 1990; Graham and Higdon, 2000a, 2000b). The main finding of these studies is the oscillatory movement of fluids when an external force is applied. The frequencies of these oscillations can be reasonably low. The driving force is the surface tension force acting on the interface between the wetting and the non-wetting fluid phase. The results of these works were used by the oil and gas industry to develop a new enhanced oil recovery (EOR) method termed “wave stimulation of oil” or “vibratory mobilization” (Iassonov and Beresnev, 2003; Beresnev et al., 2005; Li et al., 2005). The general idea of the method is to excite oscillations of the entrapped oil with a vibratory device. Inertial forces occurring with oscillations eventually are strong enough to overcome the capillary pressure. This way the oil drops are enabled to leave the pore constrictions. The method and many application results are reviewed in Beresnev and Johnson (1994).