A correlation between oil rates and earthquakes was observed in the 1970s. It resulted in the first attempts to use the energy of seismic waves to mobilize residual oil spread in the reservoir in the form of drops of different sizes, and to rehabilitate depleted oil fields with high water production. Laboratory tests showed that oil displacement can be increased by application of low-frequency vibration. It was also confirmed by field tests (results reported at VI EOR ES, 1991): oil production due to vibroseismic stimulation increased by 30-40%. However, estimations show that such an increase in oil production does not occur with low-frequency vibration. Most probably some unknown phenomena exist which impact on the mechanism of seismic vibration-induced increases in oil production. The object of this study was the development of an analytical model of vibroseismic stimulation to explain the mechanism of residual oil mobilization. Such a model can provide the basis for effective use of vibration stimulation. The model oil-saturated formation consists of blocks of different sizes. Each large block includes several blocks of smaller sizes. Block sizes depend on the geological processes taking place during their formation, while correlation between their sizes does not depend on properties of constituent materials. Core measurements taken in the millimetre range and analysis of photographs of layer distribution in open hole taken in the metric range show that correlation between the large and small blocks occurs over a rather small range and varies between 2.91 and 3.43. Simulation of vibroseismic impact on a formation shows that a process of low-frequency energy transfer from larger blocks to smaller ones, producing high-frequency vibration, takes place. These high-frequency vibrations provide conditions under which capillary forces, retaining oil drops in the porous media, are destroyed, improving conditions for oil mobilization. These results agree with experimental data of noise measurements recorded during pilot commercial tests of vibroseismic action at different oil fields in Russia. Thus the possibility of residual oil mobilization by applying low-frequency vibroseismic stimulation was confirmed both theoretically and in laboratory and field tests. The results received were an essential part of the creation of the technology for vibroseismic stimulation of waterflooded, depleted oil reservoirs.

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