Application of Acoustic Waves for Reservoir Stimulation

Abstract

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.

Modified model of porosity variation in seepage fluid-saturated porous media under elastic wave

“…Low-frequency vibration production technology could improve the oil recovery through the follow mechanisms (Cidoncha 2007;Ariadji 2005;Kurawle et al 2009): (1) reducing the capillary forces and fine particles blocking rate inner pore throats could increase the absolute permeability of rocks; (2) crude oil viscosity decrease, hydrophilicity increase on rock surface, and an additional drive force added by vibration could improve the fluid seepage rate; (3) velocity difference between solid and fluid might increase crude oil detachment or extraction in bypassed throats, so as to reduce residual oil saturation and improve oil recovery. Thereby, the real pore volume under wave is only part of the whole pores.…”

Efficiency Improvement of Vibrowave Devices Impact on the Bottomhole Formation Zone for the Intensification of Oil Recovery

“…Further field studies have shown that in order to achieve a high success in method implemenation in complicated mining and geological wells operating conditions it is necessary to conduct theoretical, laboratory and field studies, design and technological research (Gil Cidoncha, 2007, Kostrov et al, 2008.…”

“…In a radius of 5-6 meters from the well several branched cracks are formed without closing after the the pressure is removed, that is why there is no need for consolidation of sand unlike the hydraulic fracturing. In the oil fields of Texas in 1958, the fracturing method known as vibrofrac has been successfully applied for the first time (Gil Cidoncha, 2007, Kostrov et al, 2008, Westermark et al, 2001. The essence of the method is to create harmonic shock waves in BFZ due to special placement of charges.…”

Velocity and attenuation of elastic wave in a developed layer with the initial inner percolation in the pores

“…This method is called (artificial) seismic production technology or lowfrequency vibration oil extraction technology (Cidoncha 2007;Kurawle et al 2009), which has been tested in many oil fields with certain effects, including increase in injection rate, production rate improvement, and plugging removal. When the technology is used, two processes are involved because of the fluid-solid coupling.…”

“…Comprehensive experimental studies on the influence of lowfrequency vibration have been conducted by Nikolaevskiy et al (1996), Dusseault et al (2000), Ariadji (2005), Wang et al (2005), Cidoncha (2007), and Liu et al (2012) in terms of the following factors: whole permeability, O/W relative permeability, capillary force, residual oil saturation, connate water saturation, oil viscosity, porosity, and O/W separation. Nevertheless, fewer studies on dynamic model building and mathematic analysis are conducted than on the experimental method.…”