[1] We describe how experimentally measurable acoustic and hydroacoustic events occur when one fluid displaces another within a porous medium. We show that the events are directly related to discrete fluid bursts (''Haines jumps'') that occur at the pore scale. We find that the events show power law statistics with respect to magnitude for drainage (air invading) but not for imbibition (water invading). These observations agree with simple conceptual arguments on how the displacements take place. The results show that acoustic and hydroacoustic measurements can be an excellent technique for observing Haines jumps in-situ.

Research related to weak elastic wave stimulation of oil reservoirs started in the late 1950's. Activity peaked in the 1970's and 1980's in the US and in the Soviet Union. However in recent years there has been a resurgent interest and research. This interest derives form observations made in some fields near areas affected by earthquakes and, even heavy traffic, where changes in water level and oil production have been observed. For example, some variations in oil production were noticed in Kern County during the Southern California earthquake of July 1952. However, mechanism associated with earthquakes are very complex and variations in oil production or water level could be consequences not only of seismic vibrations, but also of rock fracturing and other effects associated with earthquakes. Two different types of waves are usually distinguished: high power frequency (around 20 kHz) and low power frequency waves (e.g. 40 Hz). High power frequency waves have a local effect in the reservoir and are mainly applied for wellbore stimulation. Their effects are limited to a region close to source due to higher sound absorption in the porous medium at higher frequencies. Low frequency acoustic waves effects can cover a larger region and are consequently used for reservoir, rather than well stimulation. They are generally applied by using surface vibrators, but there are also example of downhole applications. This paper is primarily focused on waves used for reservoir stimulation, as opposed to well stimulation. It aimed at understanding the main mechanisms associated with the application of vibrational energy in porous media, and assessing laboratory and field studies. In this paper, together with potential mechanisms, laboratory and field trials are discussed. Mechanisms When a fluid saturated porous medium is exposed to elastic vibrations the following waves are dispersed into the medium:Two longitudinal waves, compression shock and pressureA lateral shear wave The compressional shock wave is mostly attenuated in the proximity of the source. The pressure wave advances much further into the reservoir producing changes in the saturated fluid. Under the effect of such waves, due to inertial forces and differences in density, the saturated phases start to vibrate with respect to their centres and to the other phases. Part of the elastic vibration produced at the pore surface is attenuated and lost in the medium due to several irreversible processes associated with shear and volumetric viscosity in the medium, e.g. conversion of vibrations are produced at the pore level due to micro-heterogeneity of both the porous medium and the oil-water interface. The high frequency volumetric pulsations are accompanied by hydrodynamic microflows about the dispersed phases, as well as driftage of the diffusive sublayers around them. This results in loosing and thinning of the surface adsorbed layers around the phases. Some authors believe that this is likely to be the reason why reductions in interfactial tension and contact angle occur. This technology is believed to have a number of enhancing effects on oil recovery through:Decreasing the cohesive and adhesive bonding and capillary forcesPoroelastic motion giving rise to changes in pore volumeHeating the oil, causing its viscosity to decreaseRelease the trapped that could lead to an increase in oil mobility by changing rock wettability or to an increase in gas recovery. Following these mechanisms are discussed in detail.