Influence of intensity and frequency of ultrasonic waves on capillary interaction and oil recovery from different rock types

Ultrasonics Sonochemistry 17 (2010) 500–508

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Influence of intensity and frequency of ultrasonic waves on capillary interaction and oil recovery from different rock types Khosrow Naderi, Tayfun Babadagli * University of Alberta, Department of Civil and Environmental Engineering, School of Mining and Petroleum Engineering, 3-112 Markin CNRL-NREF, Edmonton, AB, Canada T6G 2W2

a r t i c l e

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Article history: Received 14 May 2009 Received in revised form 24 September 2009 Accepted 28 October 2009 Available online 31 October 2009 Keywords: Ultrasonic frequency and intensity Rock wettability Capillary interaction Ultrasonic penetration

a b s t r a c t Oil saturated cylindrical sandstone cores were placed into imbibition cells where they contacted with an aqueous phase and oil recovery performances were tested with and without ultrasonic radiation keeping all other conditions and parameters constant. Experiments were conducted for different initial water saturation, oil viscosity and wettability. The specifications of acoustic sources such as ultrasonic intensity (45–84 W/sq cm) and frequency (22 and 40 kHz) were also changed. An increase in recovery was observed with ultrasonic energy in all cases. This change was more remarkable for the oil-wet medium. The additional recovery with ultrasonic energy became lower as the oil viscosity increased. We also designed a setup to measure the ultrasonic energy penetration capacity in different media, namely air, water, and slurry (sand + water mixture). A one-meter long water or slurry filled medium was prepared and the ultrasonic intensity and frequency were monitored as a function of distance from the source. The imbibition cells were placed at certain distances from the sources and the oil recovery was recorded. Then, the imbibition recovery was related to the ultrasonic intensity, frequency, and distance from the ultrasonic source. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Using acoustic waves as an enhanced oil recovery technique has been of interest in past decades. The main idea behind these studies was to observe how and to what extent acoustic energy might affect oil recovery. In a pioneering study, Duhon and Campbell [16] conducted waterflood tests under ultrasonic energy and showed that ultrasonic energy causes an improvement in oil recovery. They also related the ultimate recovery to the frequency. Nosov [46] observed a decrease in the viscosity of polystyrene solution under sound waves. Chen [14] and Fairbanks and Chen [18] reported an increase in the oil percolation rate through porous media. Chen [14] also showed that the effect of heat generated by ultrasonic radiation was minimal on the observed oil recovery increase. Johnston [34] studied the influence of ultrasound on decreasing polymer viscosity. Cherskiy et al. [15] described a sharp increase in permeability of core samples saturated with fresh water in the presence of an acoustic field. Neretin and Yudin [41] observed an increase in the rate of oil displacement by water through loose sand under ultrasound. Pogosyan et al. [47] showed that gravitational separation of water and kerosene accelerates due to an acoustic field.

* Corresponding author. Tel.: +1 780 492 9626. E-mail address: [email protected] (T. Babadagli). 1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2009.10.022

In addition to laboratory research, Beresnev and Johnson [4] then pointed out four case studies of downhole reservoir ultrasonic stimulation. Morris [38] used a very high-frequency device in a field in Texas and observed an increase in oil production. Kuznetsov and Efimova [35] and Simkin et al. [49] reported a similar response to ultrasonic excitation tests in Siberia. Shaw Resources Services Inc. [48] applied ultrasonic radiation in two field tests in California which stimulated oil production remarkably. The oil recovery mechanism during acoustic applications has also been the subject of numerous studies. Ganiev et al. [22] proposed that ultrasound would deform the pore walls and alter the radius of the pore. This vibration causes fluctuations in capillary pressure and expansion of surface films. Traveling waves along pore walls may cause a ‘‘peristaltic transport” of fluid displacement. This is a possible explanation for permeability changes observed by Cherskiy et al. [15]. More recently, Aarts and Ooms [1] showed that the mechanism of peristaltic transport works only at ultrasonic frequencies, and also the intensity of the ultrasonic field should be more than a specific amount. With these conditions, the effect will occur only near a well bore due to the high attenuation of the ultrasound. Later, Aarts et al. [2] showed that ultrasonic radiation deforms the pore walls and enhances the fluid velocity in porous media. They used a rubber stopper as the porous medium and water as the fluid, and a 20 kHz ultrasonic source. They analyzed the

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‘‘peristaltic transportation” mathematically. Both their numerical and experimental results showed that, by increasing ultrasonic power, the velocity of fluid inside the porous media increases. They suggested that ultrasonic energy may facilitate well acidizing. In their work the liquid phase was water and they used an artificial porous medium which was not a representative of natural porous media. Gadiev [21] radiated ultrasound to oil saturated unconsolidated sand packs and observed a considerable increase in both the cumulative oil production and the production rate. He assumed that this effect was due to a phenomenon called ‘‘sono-capillary effect”, in which the depth of fluid penetration into pores is raised due to the pressure created from cavitation, which collapses bubbles. Tamura et al. [51] performed a series of experiments showing that liquids may adhere to flat ultrasonically driven surfaces. The shape of the droplet depends on the amplitude of vibration. They developed a simple model to predict drop shapes. The predicted drop dimensions matched closely with the experimental observations. The research in acoustic waves’ application was also focused on seismic waves, which are low-frequency sound waves. Beresnev and Johnson [4] provided a broad list of efforts that were done in elastic waves’ experimental works and a few field cases including both ultrasonic and seismic methods, concluding in a positive effect in most cases on the oil production rate. Nikolaevskii [42], Nikolaevskii [43] reviewed the possibility of a mechanical (acoustic and seismic) stimulation of an oil reservoir. The positive effect of vibration on oil recovery was demonstrated and was attributed to the restoration of permeability as a result of drop clusterization. A mathematical model was also proposed to illustrate the dominant vibration frequency. Such a model is based on the nonlinear effect associated with viscoelastic resonance. Elastic wave fields may reduce capillary forces by vibrating, and consequently breaking surface films adsorbed on the pore walls. Ultrasonic vibrations may mobilize oil droplets into pores, or may result in coalescence due to the Bjerknes forces. The Bjerknes forces acting between particles can be attractive or repulsive depending on the droplet’s location relative to the wave field. Hence, its oscillating phase will be attractive if oscillations are in phase and repulsive if they are out of phase [8,10,37]. The magnitude of such force depends on the density of the continuous phase and the radius of the droplet. A series of pendant drop experiments was performed by Hamida and Babadagli [31]. From these experiments, they concluded that the ultrasound affects the dripping rate of water through a capillary into various oleic phases. This dripping rate reaches a maximum at a characteristic intensity depending on oil viscosity and interfacial tension. In addition to that, they conducted different types of experiments involving ultrasonic effects on capillary imbibition recovery of oil and also on immiscible and miscible displacement in porous media [23–32]. In one of their first attempts, Hamida and Babadagli [24] performed imbibition experiments to test the ultrasonic effects on capillary imbibition. They used an ultrasonic bath operating at the frequency of 40 kHz and generated power up to 2 kW. The cores were epoxy coated to create different matrix boundary conditions, i.e. one and all sides open to flow. Different fluid pairs were used in the experiments and all experiments were performed with and without ultrasonic radiation to compare the results. They observed that ultrasonic waves may enhance oil recovery due to capillary imbibition depending on the fluid and matrix-fracture interaction type, i.e. co- or counter-current. In a series of subsequent works, Hamida and Babadagli [25], Hamida and Babadagli [29], Hamida and Babadagli [30] studied the capillary interaction between the matrix and fracture under different ultrasonic intensities for different fluid types. They used an ultrasonic source with

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frequency of 20 kHz and power of 25 and 45 W/cm2. Experiments showed that the rheological properties of polymers may be altered and the surfactant solubility may be increased under ultrasonic energy. Also investigated were the ultrasound effects at the interface of fluids during immiscible and miscible displacement in porous media [26,28,32]. They performed both capillary imbibition and Hele–Shaw experiments using an ultrasonic bath with a frequency of 40 kHz and power of 35 W/cm2 and also an ultrasound source whose horn transmitted ultrasound energy with a frequency of 20 kHz and power up to 250 W/cm2. On the displacement pattern images obtained through the Hele–Shaw experiments, they performed fractal analysis to analyze them quantitatively. They concluded that ultrasonic waves alter the shape of the interface between two immiscible fluids and reduce the interfacial tension. The ultrasonic waves also enhanced the molecular diffusion at low injection rates during miscible displacement. A series of static and dynamic experiments investigating the displacement of oil by water were performed by Naderi and Babadagli [39]. For dynamic experiments, they used 2-D sand pack models which were saturated with different viscosities and with oil- and water-wet wettabilities. The patterns of injected water to the models were changed by ultrasonic waves. The fingers were shorter and thicker and the cluster was more compact under ultrasonic radiation. In the oil-wet cases, more oil was produced after breakthrough compared to the identical non-ultrasound cases.

2. Wave penetration into porous media An important factor controlling the feasibility of acoustic waves’ stimulation in wells is its depth of penetration through porous media. The basics of the theory of elastic wave propagation in fluid-saturated porous media were first studied by Frenkel [20] and Biot [5–7]. Biot developed a comprehensive analytical model to describe the propagation of stress waves in poroelastic solids containing a viscous and compressible fluid. He found that there are two compressional waves namely the slow (P2-wave) and the fast wave (P1-wave). Attenuation occurs mainly with the slow wave due to the dissipative forces generated by pore fluid viscosity. A great deal of work has been carried out based on Biot’s theory following his pioneering papers. For example, a mathematical model was developed by Norris [45] which described the propagation of low-amplitude waves in saturated sediments. Zimmermann and Stern [52] presented several closed-form analytical solutions of the Biot’s equations, such as radiation from a harmonically driven plane wall. Buckingham [11–13] extended the Biot’s theory for consolidated and unconsolidated porous material. According to Hamida and Babadagli [32], Biot’s theory predicts that the penetration depth for ultrasonic waves at a frequency of 20 kHz is 2–10 cm. Because the attenuation is proportional to the square of frequency, low-frequency waves are more applicable in penetrating to the reservoir for longer distances. If a periodic signal with frequency (-f-) is distorted, in a frequency domain, it will be a composition of the main frequency and its harmonics with higher frequencies, which are integer multiples of f. However, the harmonics are usually smaller in amplitude as their frequency increases and the dominant frequency (with the greatest amplitude) is normally the first harmonic with frequency f [50,3]. The propagation of nonlinear acoustics such as finite amplitude plane sound waves will generate higher harmonics with a frequency of integer multiples of the initial frequency, which can be mathematically shown by the Fourier series [19,33,9]. Therefore, during the penetration of nonlinear low-frequency elastic waves

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T1

Amplitude

Amplitude

through porous media, the wave shape might be distorted due to energy loss and attenuation, and make harmonics with higher frequencies. Then, one may expect to have waves on the order of ultrasonic frequencies in the reservoir when sending low-frequency elastic waves such as seismic waves, from the surface or due to earthquakes nonlinear plane waves under the surface. However, they might be negligible in intensity comparing to main harmonic and also due to their higher frequencies they attenuate faster. Fig. 1a and b show how a wave shape might be distorted after traveling a certain distance in a medium. Suppose that a low-frequency wave (e.g. a seismic wave) is generated at the surface and sent down through the formations. Its frequency at the surface is f1 = 1/T1. If the distortion imposed on the wave due to environmental conditions is similar to Fig. 1b, it will produce harmonics with a higher frequency of f2 = 1/T2. Hence, the wave becomes a combination of the original wave with a frequency of f1 and its harmonics with higher frequencies of f2, which can be ultrasonic waves. A common and general proposed waveform after distortion due to propagation (mostly in fluids) after a specific distance (depending on wave and medium) is shown in Fig. 1c [36,40,33]. That is a deformed shape of the original wave and since it is still periodic with the same period of T1, it can be presented as a combination of harmonics of f1 and its integer multiples so we can still suggest that higher harmonics are included. Dunin and Nikolaevskii [17] and Nikolaevskii and Stepanova [44] also proposed a theory about the ultrasonic waves generation as result of nonlinear effects associated with seismic and low-frequency acoustic waves in porous media saturated with fluid. Under conditions of long-short-wave resonance, the nonlinear generation of high ultrasonic frequencies by seismic waves is possible. Based on the above explained theory, ultrasonic energy (highfrequency waves) could be the main reason for enhancement of oil recovery after artificial or natural seismic activities. Hence, one has to understand how ultrasonic energy affects oil recovery at the core and pore scale, and clarify what type of reservoir is more suitable for this type of application. One of the possible ways of doing this is to directly apply ultrasonic energy that is believed to be generated as harmonics of low-frequency seismic waves as they penetrate into the reservoir. This might eventually help clarify the reasons for observed improved oil recovery after earthquakes. This constitutes the first objective of this paper and a series of experiments were conducted as a continuation of previous attempts in our institution [24-32,39]. In those studies, mostly water-wet rocks were tested with low viscosity oil. In the present

T1

time

time

T2

Amplitude

(a)

(b)

T1

study, our focus was mostly on the oil-wet medium and much higher oil viscosities. The second objective is to test ultrasonic waves as a well stimulation tool for direct application of high energy waves through the wellbore. Penetration of high-frequency waves in different media and their effect on oil recovery were the main concern. 3. Experimental procedure 3.1. Capillary imbibition experiments We performed imbibition experiments using cylindrical cores of Berea sandstone. These cores were all the same size with a length of 7 cm and a diameter of 2.5 cm. All sides were open and in contact with the surrounding fluid to ensure a co-current interaction. The cores without initial water saturation were directly saturated by oil under a vacuum. The cores with the initial water saturation were first vacuum saturated by brine in a core holder using a pump and then oil was injected to drain the water out until irreducible water saturation was reached. Then cores were placed inside a graduated imbibition cell filled with 3 wt.% NaCl brine. For ultrasonic experiments, these cells were put inside a bath filled with water which surrounds the cell below its neck. An ultrasonic generator provided ultrasonic energy which was delivered to the bath through a half-immersed horn. The generator used was SonicatorÒ 3000 ultrasonic processor which generates and emits ultrasonic waves at a frequency of 20 kHz. The generator transforms an AC line power to a 20 kHz electrical signal. There is a piezoelectric converter or transducer which converts this electrical signal to a mechanical vibration. This vibration is amplified by the horn and transmitted by its tip to the surrounding area. Fig. 2 shows a schematic of the setup designed for the imbibition experiments. We used mineral oils with three different viscosities: 40, 500, and 1600 cp, at ambient conditions. The output power of the ultrasonic generator was set to 45 and 84 W/cm2 for different cases. The frequency of the ultrasonic wave generated by this generator is fixed at 20 kHz. We also performed four experiments with a low power output of 1 W to see the effect of frequency. In those experiments, we used another generator (MicrosonTM XL-2000). This device is designed for lower wave intensities and generates ultrasonic waves at the frequency of 22.5 kHz. For the higher frequency tests, we used Prowave ultrasonic ceramic transducers which work at a center frequency of 40 kHz. This device is connected to a function generator which emits an electrical signal at arbitrary frequency, amplitude and wave shape. We set the function generator to produce a sinusoidal signal with 10 V amplitude at a frequency of 40 kHz. Having output matched impedance of 50 X (ohm), the function generator transmits 1 W mean power. For each case, two experiments were run with and without ultrasonic energy for comparison. Table 1 displays experiments conducted throughout the study. To obtain oil-wet cores, dichlorooctamethyltetrasiloxane (SurfaSilTM, a siliconizing fluid) was applied. This chemical was added to pentane by vacuum saturation to dry cores before the saturation process. Pentane evaporates under the vacuum and pore walls are covered with a monolayer of SurfaSilTM which makes the core oil wet. The quantity of oil wet is dependent on the concentration of SurfaSilTM in the mixture. The oil-wet cores were treated using a very high concentration of siliconizing fluid to make them strongly oil-wet.

time

(c) Fig. 1. (a) Wave shape at surface with period of T1, (b) distorted wave shape after penetrating into the reservoir; one produced harmonic with period of T2 is shown, (c) a general distorted waveform at a specific distance from origin.

3.2. Penetration experiments A series of experiments were run to measure the effect of the distance to the ultrasonic source. Higher frequencies result in higher attenuation. Therefore, penetration into porous media is

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Fig. 2. Experimental setup for ultrasonic core experiments.

radiate and travel through the surrounding medium. For the experiments in water and slurry, we used a setup shown in Fig. 3. The generator creates ultrasonic waves; three imbibition cells were placed inside the medium (slurry or water) in different distances from the horn.

Table 1 Capillary imbibition experiments conducted throughout the study. Experiment number

Initial water saturation

Oil viscosity (cp)

Ultrasound intensity (W/cm2)

Ultrasonic frequency (kHz)

Wettability

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

0 0 0 0 0 0 0 0 0 0.4 0 0.4 0 0 0 0

500 500 500 500 500 1600 1600 1600 35 35 35 35 35 500 35 500

0 45 84 0 45 0 45 84 0 0 45 45 1 (W) 1 (W) 1 (W) 1 (W)

20 20 20 20 20 20 20 20 20 20 20 20 22.5 22.5 40 40

Water wet Water wet Water wet Oil wet Oil wet Water wet Water wet Water wet Oil wet Oil wet Oil wet Oil wet Water wet Water wet Water wet Water wet

4. Results and analysis 4.1. Capillary imbibition experiments Fig. 4 shows the recovery curves for experiments #1 to #5. The oil viscosity in all cases is 500 cp. The first three experiments were water-wet cases and the other two were oil wet. The trends in the recovery rate for water-wet cases were very similar. The recovery rate was only affected in the early stages of the experiment but the ultimate recovery increased by increasing the ultrasonic energy intensity. A slight increase in the ultimate recovery with increasing intensity of ultrasonic waves was observed. For oil-wet experiments, the effect of ultrasonic energy on the recovery is much more critical. Due to the oil-wet nature of the cores, it takes much longer for recovery by capillary imbibition to start. Under no ultrasound energy, oil recovery is very minimal

lower for ultrasonic waves due to their high frequency. We tested ultrasonic waves in air, water and a slurry mixture of sand and water to observe how the properties of waves change while they

Imbibition Cells

Converter and Horn

Ultrasonic Generator

10cm

Slurry (sand + water mixture) or water

20cm 40cm Fig. 3. Penetration experiments setup for slurry and water medium.

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Experiments with Heavy Oil ( viscosity = 500 cp ) 60

WW: Water Wet 50

Oil Recovery(% OOIP)

Oil Recovery (% OOIP)

60

Heavy Oil ( Oil Viscosity = 1600 cp ) Water Wet

OW : Oil Wet 40

WW - NUS WW - US 45 W/cm²

30

WW - US 84 W/cm² OW - NUS

20

OW - US 45 W/cm²

10

50 40 30

NUS US 45 W/cm²

20

US 84 W/cm² 10

US 84 (re) US 45 (re)

0 1

10

100

1000

10000

0

100000

t (min)

1

100

1000

10000

100000

t (min)

Fig. 4. Experiments #1–5, heavy oil with viscosity of 500 cp.

Fig. 5. Experiments #6–8, 1600 cp oil and water-wet cores.

Oil Wet Experiments ( Oil Viscosity = 35 cp )

Oil Recovery(% OOIP)

30

on/off started

Swi=40% , NUS Swi=40% , US Swi=0 , NUS Swi=0 , US

25 20 15 10 5 0 10

100

1000

10000

100000

1000000

t (min) Fig. 6. Oil-wet experiments with light mineral oil, with and without initial water saturation (experiments #9–12).

saturation, we observe a large increase in recovery after ultrasonic radiation but this recovery improvement is even higher in the cases with initial saturation. In the oil-wet medium, water has more tendencies to flow because it has less affinity to the rock surface. The peristaltic movement of the initial water could facilitate faster transportation of oil resulting in more recovery. At 160,000 min, we exposed all four cases to ultrasonic energy; half a day on and half a day off. As seen, the recovery of the case with initial water saturation which was not under ultrasound energy

Water Wet Experiments, Low Power (1 Watt)

70

Oil Recovery (% OOIP)

(5% OOIP). Only a 10% change in the ultimate recovery was observed by applying ultrasound energy with an intensity of 45 W/ cm2 in the water-wet case, whereas, with the same amount of ultrasonic intensity, the oil-wet cases yielded more than a 100% change in ultimate recovery. One may first think of increased temperature due to dissipated acoustic energy as an effective parameter to rule the recovery, however we measured the temperature for a moderate intensity (45 W/cm2) and it was observed to increase to 31 °C, which doesn’t seem to have a massive effect on viscosity or density. Obviously, different recovery mechanisms governed the process in the water- and oil-wet cases. Favorable changes in the interfacial properties are expected in both cases as also observed by Hamida and Babadagli [29]. The Bjerknes forces are also expected to be an effective mechanism for the water-wet cases, resulting in higher ultimate recovery under ultrasonic energy [8,10]. This explains the difference in the ultimate recoveries that started to become obvious after 100 min. As the oil droplets would not be in a ganglion shape in the oil-wet case, the Bjerknes forces are not expected to be an effective mechanism. The IFT change is expected and this causes a change in the wettability. The observations suggest that more attention will be given to oil-wet cases with lower viscosities as the ultrasonic effect is more pronounced in those cases. Fig. 5 compares the recovery curves for heavier oil (1600 cp) with and without ultrasonic energy (experiments #6, #7 and #8). The change in the recovery rate and ultimate recovery due to ultrasonic energy is trivial compared to the lighter oil cases given in Fig. 4. This observation shows that when the oil viscosity increases, the effect of ultrasonic waves decreases. This is in agreement with the observations reported by Hamida and Babadagli [28]. Ultrasonic experiments (i.e. experiments #7 and #8) were repeated to check the reproducibility. Although there is a little early fluctuation, ultimate recovery is almost equal for these cases. Also, looking at general trend, a similar behavior was observed. The slight differences in the very early times of the experiments could be attributed to the changes in the core properties as different cores from the same block were used in the experiments. The observations suggest that more attention will be given to oil-wet cases with lower viscosities as the ultrasonic effect is more pronounced in those cases. In addition to the previous water-wet cases, the effect of initial water was added in the new set of experiments. Peristaltic movement of the water phase is expected to cause additional oil recovery [22,2] as the water will not be in the wetting phase and be more mobile in the oil-wet case. Fig. 6 shows the comparison of the recovery curves of the oil-wet cores with and without initial water saturation for the light mineral oil (35 cp) (experiments #9, #10, #11 and #12). For two experiments without initial water

10

60 50 40 30

35 cp , 40 kHz 500 cp , 40 kHz

20

35 cp , 22.5 kHz 35 cp , 22.5 kHz (re)

10

500 cp , 22.5 kHz 500 cp , 22.5 kHz (re)

0 1

10

100

1000

10000

t (min) Fig. 7. Experiments #13–16, low power, different frequencies.

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jumps up and gets close to the equivalent case which has been under stimulation since the starting time (t = 0). This big change verifies the effect of ultrasonic energy causing peristaltic movement of water that results in more oil production. Change in the interfacial properties (IFT, wettability) is also expected for this case. The last group of experiments was performed to observe the effect of frequency. Experiments #13 and #14 are the cases with oil viscosity of 35 and 500 cp, respectively and both were exposed to ultrasonic waves with a frequency of 22.5 kHz. Oil viscosity in experiments #15 and #16 was 35 and 500 cp, respectively and both experiments were under ultrasonic radiation of 40 kHz fre-

Oil Recovery (% OOIP)

12

quency. Fig. 7 shows the recovery curves of these experiments. For reproducibility we repeated experiments #13 and #14. Observation shows that the rate of recovery increases at higher frequencies but finally all experiments converge to a similar ultimate recovery value. Recovery for cases with higher viscosity oil is less during the experiments but the ultimate recovery is almost the same for both viscosities. The repeated experiment #13 shows a similar recovery trend but its ultimate recovery is a few percent higher than the first experiment #13. This change might be due to a difference in core properties or experimental conditions. For experiment #14, the recovery curve is in a very close match with its equivalent.

Penetration Experiments Oil Wet, (Viscosity=500cp) Water, 10cm

10 8 6

Water. 20cm Water. 40cm Sand, 10,20,40cm

4 2 0 100

1000

10000

505

100000

t (min) Fig. 8. Recovery curves for penetration experiments in water and slurry.

Fig. 11. Received signal in air at a distance of 40 cm.

Fig. 9. Received signal in air at a distance of 10 cm.

Fig. 12. Received signal in water at a distance of 10 cm.

Fig. 10. Received signal in air at a distance of 20 cm.

Fig. 13. Received signal in water at a distance of 20 cm.

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Fig. 14. Received signal in water at a distance of 40 cm.

Fig. 15. Received signal in slurry (sand + water).

4.2. Penetration experiments 4.2.1. Imbibition recovery We chose oil-wet imbibition experiments because the effect of ultrasonic was more critical on them as discussed above. The pro-

P=1W f = 40 kHz

Vm = 1 V f ≈ 40 kHz

duction curves are shown in Fig. 8. Due to the oil-wet nature of the sample, the production needed quite a long time to start. As seen, until around 20,000 min, there was no production in all three experiments inside the slurry mixture, while for the water cases

Vm = 500 m V f ≈ 40 kHz

Vm = 200 m V f ≈ 31 kHz

Air

I = 45 W/cm 2 f = 20 kHz Vm = 4 V f ≈ 20 kHz

Vm = 2 V f ≈ 20 kHz

Vm = 1 V f ≈ 19 kHz

Water

I = 45 W/cm 2 f = 20 kHz

Sand

10cm 20cm 40cm Fig. 16. Penetration experiments diagram (P, power; I, intensity; f, frequency; Vm, amplitude).

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there was a small amount of production. Cells closer to the source yielded faster and higher recoveries. 4.2.2. Wave propagation The wave properties as it propagates through each medium was also measured. In air, we used ceramic ultrasonic transducers set at 40 kHz. The function generator sends a sinusoidal electrical signal to the transmitter and it converts the electrical signal to ultrasonic waves. On the other hand, the receiver converts the received ultrasonic wave to an electrical signal which can be detected by an oscilloscope. The shapes of the waves received at three different distances of 10, 20 and 40 cm are shown in Figs. 9–11 as they appear on the oscilloscope screen. At the distance of 10 cm, a signal with amplitude of 1 V and frequency of 39.997 kHz was detected (Fig. 9). At the 20 cm distance, the amplitude decreased to 500 mV and frequency was read as 39.995 kHz (Fig. 10). This wave will have an amplitude of almost 200 mV and frequency of 30.870 kHz when it reaches the distance of 40 cm. (Fig. 11). Comparison of these three measurements gives us an idea about attenuation of ultrasonic waves radiating in air. From 10 to 20 cm, the wave amplitude decreased to almost half. As the power is proportional to the square of amplitude, at the 20 cm distance, the wave power is a quarter of the power at the 10 cm distance. Coming to the third point at 40 cm, the wave amplitude is about 200 mV, which shows a decrease in the power at around one fifth of the previous point (20 cm). For the experiments in the water bath, we used a SonicatorÒ 3000 ultrasonic generator to obtain higher power. We also measured the wave properties inside the bath using the same ceramic receivers. Figs. 12–14 show the received signal under water at the distances of 10, 20 and 40 cm, respectively. In this case, the wave shapes are not as ‘‘clean” as the air experiments. This could be due to a different source specification, different propagation medium, and using the transducer at a frequency far from its center frequency. As seen in Figs. 12–14, one may observe noise in the wave but its general form can still be recognized. For the 10 cm case (Fig. 12), the signal has an average amplitude of almost 4 V and a frequency of 20.26 kHz. At the 20 cm distance, the amplitude decreases to 2 V and the frequency is 20.08 kHz (Fig. 13). When the wave reached 40 cm distance from the source, its average amplitude was 1 V and frequency was as low as 18.79 kHz (Fig. 14). The situation inside the slurry was completely different. No wave information was received. The acquired wave shape is shown in Fig. 15. What was received was almost a noise and the receiver could not detect any specific frequency in its range. A schematic diagram of all penetration experiments is shown in Fig. 16. 5. Discussion The paper focused on and questioned two issues: (1) Does ultrasound improve oil recovery? The answer is positive based on previous observations over the last four decades. We provided additional data and laboratory scale observations for specific cases like heavy-oil, oil-wet rocks for different ultrasonic frequency and intensities. The reality, however, is that it is limited due to penetration of high-frequency waves for practical applications. Also studied in the paper was the effect of medium and distance from the ultrasonic source on oil recovery. (2) What does cause an increase in oil recovery rate after seismic activities? Would that be due to harmonic oscillations that create ultrasonic (high frequency) waves as the low-frequency seismic waves penetrate deeper?

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Limited field scale applications for the former are available but restricted to well stimulation. The latter needs more experimental and theoretical research and further analysis of production and seismic data collected in tectonically active areas. Previous experience as well as the present paper showed that irradiation of ultrasonic energy results in faster motion of oil in porous media at laboratory scale. This may eventually yield higher ultimate recovery. But, there exists penetration problem due the level frequency of the waves and this possibly prevents this method from becoming a conventional enhanced oil recovery technique. It may still, however, have potential for well stimulation. Actual field/well trials are not abundant yet but among the few reported cases of downhole ultrasonic stimulation Beresnev and Johnson [4], Morris [38], Kuznetsov and Efimova [35], Simkin et al. [49] and Shaw Resources Services Inc. [48], one may conclude that ultrasonic energy show high potential for well stimulation, or more specifically increasing the well flow performance. In this study, we reported the application conditions of ultrasonic stimulation (frequency and intensity) for a wide range of oil viscosities and rock wettabilities. Although the application of ultrasonic radiation in reservoirs is restricted by its low penetration depth, there still might be a presence of high-frequency waves in reservoirs as a result of propagation of nonlinear low-frequency waves. A speculation can be made here to partly explain the effects of large amplitude excitations such as earthquakes on oil recovery increments in reservoirs located in a long distance from their origin. The generated waves’ frequency, amplitude and hence energy are highly dependent on properties of both medium and original wave. Therefore, the effectiveness of possible generated waves must be tested based on actual combinations of medium and wave. Future research can be focused on the wave propagation in consolidated media and generation, viability and effectiveness of high-frequency harmonics. Our observations in this paper inferred the potential effect of these harmonics on oil recovery, also specifying the rock and fluid properties that show high potential for applications. These results are highly encouraging to open a new research direction on the generation of high-frequency harmonics during the travel of low-frequency waves and its effect on oil recovery.

6. Conclusions This study identified some of the mechanisms which could be the reasons for additional oil recovery after seismic (low-frequency waves) activities based on the theory that harmonics of low-frequency waves create high-frequency waves as they penetrate into the formation. We also tested the penetration of high-frequency waves and its effect on oil recovery for near wellbore stimulation. The following specific conclusions can be extracted from the study conducted: 1. Imbibition experiments showed that ultrasonic radiation increases recovery. Similar trends of recovery were observed in water-wet cases with and without initial water saturation. Comparing different viscosities in water-wet experiments (experiments #1–3 and #6–8 in Figs. 4 and 5), one can conclude that the effect of ultrasonic energy on recovery change is less with higher viscosities. 2. The effect of ultrasonic radiation on recovery is clearly more significant in oil-wet cases. Oil-wet experiments also indicated that the presence of initial water saturation facilitates oil recovery under ultrasound. 3. Higher frequency showed a higher rate of recovery compared to a lower frequency. However, the ultimate recovery was not

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changed significantly (at the low power used for different frequency experiments). 4. Running a series of imbibition experiments inside water and slurry (sand + water) at different distances from the ultrasonic source and also measuring the wave shape and properties at these distances using an ultrasonic transducer connected to an oscilloscope, we observed that water behaves similarly to air. Around the source (up to 20 cm in our experiments), the reduction in intensity was as expected, which was based on a normal intensity relation to the distance from source, i.e. it decreased proportional to 1/r2 and the frequency change was certainly negligible. Beyond this distance (e.g. at 40 cm), attenuation in the medium showed changes particularly in the frequency of propagating waves. Inside the slurry, attenuation was much higher so that we did not acquire any wave shape in the receiver even at the point very close to the source. This might depend on source frequency and power, and also on the sensitivity of transducers. But based on our observations, we can state that the wave loss in the slurry medium was much higher than in the air and water cases. Acknowledgements This research was partly funded by an NSERC (Grant No. G121210595). The funds for the equipment used in the experiments were obtained from the Canadian Foundation for Innovation (CFI) (Project # 7566) and the University of Alberta. We gratefully acknowledge these supports. This paper is the revised and improved version of SPE 117324 originally presented at the 2008 SPE Int. Thermal Oper. and Heavy Oil Symp. held in Calgary, AB, Canada, 20–23 October, 2008. References [1] A.C.T. Aarts, G. Ooms, Net flow of compressible viscous liquids induced by traveling waves in porous media, J. Eng. Math. 34 (4) (1998) 435–450. [2] A.C.T. Aarts et al., Enhancement of liquid flow through a porous medium by ultrasonic radiation, SPE J. 4 (4) (1999) 321–327. [3] J. Arrillaga, N.R. Watson, Power System Harmonics, John Wiley and Sons, Chichester, England, 2003. [4] I.A. Beresnev, P.A. Johnson, Elastic-wave stimulation of oil production – a review of methods and results, Geophysics 59 (6) (1994) 1000–1017. [5] M.A. Biot, Theory of propagation of elastic waves in a fluid-saturated porous solid. 1. Low-frequency range, J. Acoust. Soc. Am. 28 (2) (1956) 168– 178. [6] M.A. Biot, Theory of propagation of elastic waves in a fluid-saturated porous solid 2. Higher frequency range, J. Acoust. Soc. Am. 28 (2) (1956) 179–191. [7] M.A. Biot, Mechanics of deformation and acoustic propagation in porous media, J. Appl. Phys. 33 (4) (1962) 1482. [8] V.F.K. Bjerknes, Fields of Force, Columbia University Press, New York, 1906. [9] D.T. Blackstock, Fundamentals of Physical Acoustics, John Wiley and Sons, Chichester, England, 2000. [10] F.G. Blake, Bjerknes forces in stationary sound fields, J. Acoust. Soc. Am. 21 (5) (1949). 551–551. [11] M.J. Buckingham, Theory of acoustic attenuation, dispersion, and pulse propagation in unconsolidated granular materials including marine sediments, J. Acoust. Soc. Am. 102 (5) (1997) 2579–2596. [12] M.J. Buckingham, Theory of compressional and transverse wave propagation in consolidated porous media, J. Acoust. Soc. Am. 106 (2) (1999) 575–581. [13] M.J. Buckingham, Compressional and shear wave properties of marine sediments, comparisons between theory and data, J. Acoust. Soc. Am. 117 (1) (2005) 137–152. [14] W.I. Chen, Influence of Ultrasonic Energy Upon the Rate of Flow of Liquids Through Porous Media, Chemical Engineering, West Virginia University, Ph.D. Thesis, 141, 1969. [15] N.V. Cherskiy et al., The effect of ultrasound on permeability of rocks to water, Trans. (Doklady) USSR Acad. Sci. Earth Sci. Sect. 232 (1977) 201–204. [16] R.D. Duhon, J.M. Campbell, The effect of ultrasonic energy on flow through porous media, in: SPE 1316, 2nd Annual Eastern Regional Meeting of SPE/ AIME, Charleston, WV, 1965. [17] S. Dunin, V.N. Nikolaevskii, Nonlinear waves in porous media saturated with live oil, Acoust. Phys. 51 (1) (2005) S61–S66. 1. [18] H.V. Fairbanks, W.I. Chen, Ultrasonic acceleration of liquid flow through porous media, Chem. Eng. Prog. Symp. Ser. 67 (1971) 108.

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