Introduction

The degree of irreducible water saturation has an important impact on reservoir evaluation and oil-water two-phase seepage1,2,3. The predecessors have carried out in-depth and systematic research on the irreducible water saturation. The factors affecting irreducible water mainly involve the radius of rock particles, pore throat, physical properties and other factors4,5,6,7. The worse the reservoir physical property, the greater the irreducible water saturation. The existence of reservoir water film is an important cause of irreducible water, and some scholars have also studied reservoir water film and its thickness calculation method8.

Ultrasound is a green and environmentally friendly technology, which is increasingly used in the oilfield development industry. At present, scholars mostly focus on the application of ultrasound in viscosity reduction9and water sensitivity removal10. Wang et al.11 designed a static experiment and carried out a study on ultrasonic removal of reservoir water sensitivity, and found that increasing ultrasonic power was conducive to the removal of core water sensitivity. Khan et al.12 proposed that the optimal ultrasonic treatment time was 100 min, and the corresponding reservoir permeability recovery rate was 24.4%.

Compared with other stimulation measures13,14, ultrasonic treatment is more green.In the carbon capture, utilization and storage (CCUS) technology, the injection of the captured carbon dioxide into the low permeability reservoir is an effective way of storage. By reducing the saturation of irreducible water, ultrasonic technology improves the permeability of the reservoir, making carbon dioxide easier to be injected and stored in the reservoir, thus enhancing the storage capacity of carbon dioxide15.

The above research proves that ultrasonic technology has a certain effect on viscosity reduction and water sensitivity removal in oilfield development. However, there are few studies on the influence of ultrasound on the irreducible water saturation of low permeability reservoir, so there is insufficient understanding of the influence of ultrasound on the irreducible water saturation of low permeability reservoir, and the study of the above problems has a positive significance for the systematic understanding of the mechanism of ultrasonic action on low permeability reservoir.

Therefore, based on the shortcomings of existing studies, this paper designs relevant experiments, and uses ultrasound with different parameters to treat natural low-permeability core, so as to study the effect of ultrasound on the irreducible water saturation of low-permeability reservoir in the flow state, in order to further explore the influence of ultrasonic action on the irreducible water saturation of low-permeability reservoir, and provide theoretical guidance for promoting the application of ultrasonic technology in the development of low-permeability oilfield.

Ultrasonic reduction of irreducible water saturation in low permeability reservoir

In order to study the influence of ultrasound on the irreducible water saturation of low permeability reservoir in the process of seepage, natural cores with different physical properties were selected by self-developed ultrasonic generator to carry out laboratory displacement experiments under different experimental conditions.

Experimental equipment and materials

Experimental equipment

(1) Core displacement experimental device: HKY-20 C, maximum displacement pressure 40 MPa, ring pressure 0 ~ 50 MPa, flow range 0.01 ~ 10 mL/min, working temperature 0 ℃~150 ℃.

(2) Ultrasonic generator: US-GDS-1036 A, independently designed and developed by Key laboratory of sound field-assisted oil and gas exploitation of Puyang, ultrasonic frequency is 17 kHz ~ 125 kHz, electric power is 0 W ~ 2000 W, operating area is 20 cm×30 cm, operating temperature is 10 ℃~110 ℃.

(3) Core gas permeameter: STY-2, test core diameter of 2.5 cm, test core length of 2.0 cm ~ 7.0 cm.

(4) Electronic analysis balance: Li Chen FA124, accuracy 0.1 mg.

Experimental materials

The experimental core samples were the siltstone of Zhongyuan oilfield with diameter of (2.5 ± 0.1) cm, a lengths of 3.2 cm ~ 6.7 cm and permeability of 3.26 × 10−3µm2 ~ 45.81 × 10−3µm2 (Table 1). Kerosene was used as simulated oil, the simulated formation water adopted in the experiment has a salinity of 20,000 mg/L and a composition ratio of NaCl: CaCl2: MgCl2•6H2O = 7.0:0.6:0.4.

Table 1 Experimental core physical properties data.
Full size table

Experimental procedure

(1) Each core was numbered, cleaned and placed in a 50 ℃ oven for drying for 24 h to test the gas permeability, porosity and other physical parameters.

(2) The core saturated with simulated formation water was put into the core clamp, and the simulated oil was injected at a maximum confining pressure of 8 MPa and the speed was set at 0.02 mL/min to carry out the oil displacement experiment. 。.

(3) When other conditions remain unchanged, gradually increase the temperature to 20 ℃, 25 ℃, 30 ℃, 35 ℃, 40 ℃, 45 ℃, 50 ℃, 60 ℃, 70 ℃, to establish the irreducible water saturation of different temperatures during the heating process.

(4)Clean the core, complete step (1), and complete step (2) at 70℃. With other conditions unchanged, the temperature is gradually lowered to 60 ℃, 50 ℃, 45 ℃, 40 ℃, 35 ℃, 30℃, 25 ℃, 20 ℃ to establish the irreducible water saturation under different temperature conditions during the cooling process.

(5) Repeat steps (1) to (2), using different parameters to change ultrasonic frequency, ultrasonic power, core permeability, temperature and treatment time in turn, and test the irreducible water saturation of low-permeability core under various conditions.

Figure 1 shows the schematic diagram of the experimental process.

Fig. 1
figure 1

Experiments flowchar.

Full size image

Experimental results and discussion

In order to compare the influence of ultrasonic action on the irreducible water saturation of low permeability reservoir, 6PV oil flooding was carried out on core YX-6 first, and then ultrasound was started to continue the experiment. The ultrasonic frequency used in the experiment was 25 kHz, the power was 400 W, the ultrasonic treatment time was 20 min, and the experimental temperature was 25 ℃.

Fig. 2
figure 2

The relationship between irreducible water saturation and the presence or absence of ultrasound.

Full size image

It is found that ultrasound can significantly promote the reduction of irreducible water saturation in low-permeability reservoirs, as shown in Fig. 2. Water saturation decreases with the increase of injection volume multiple. In the absence of ultrasonic action, water saturation reaches stability after 4PV displacement, and the corresponding water saturation is the irreducible water saturation (40.49%). After continuing to displace 2PV, the ultrasonic generator is started at 6PV, and the irreducible water saturation decreases. On the basis of the previous displacement, after ultrasonic action of 4PV, the irreducible water saturation remained 37.64%, and after ultrasonic action, the irreducible water saturation decreased by 2.85%. After 4PV displacement, the irreducible water saturation of the low-permeability reservoir under pure ultrasonic action decreases to 38.49%. At the initial stage of ultrasonic action, water saturation of the low-permeability reservoir has little change compared with that without ultrasonic action. With the continuous ultrasonic action, water saturation shows a trend of continuous decrease until after 4PV displacement no more irreducible water is expelled and maintains a stable state, which fully indicates that ultrasonic action can accelerate the expulsion of irreducible water.

Effect of ultrasonic frequency on the irreducible water saturation of the low-permeability reservoir

Under ultrasonic conditions at different frequencies, the oil displacement experiment of core YX-4 was carried out to analyze the influence of ultrasonic frequency on the irreducible water saturation of low permeability reservoir. The ultrasonic power used in the experiment was 800 W, the ultrasonic treatment time was 30 min, and the experimental temperature was 50 ℃. The irreducible water saturation of YX-4 before ultrasonic treatment was 49.71%.

Fig. 3
figure 3

Relationship between irreducible water saturation and ultrasonic frequency.

Full size image

It is found that the ultrasonic frequency presents a relatively complex relationship with the irreducible water saturation of low-permeability reservoirs, as shown in Fig. 3. The overall performance is a three-stage feature of “rapidly decreasing, slowly decreasing, and slowly increasing” with the increase of ultrasonic frequency. When the ultrasonic frequency is in the range of 17 kHz ~ 25 kHz, the irreducible water saturation decreases rapidly with the increase of ultrasonic frequency, and the gradient is −0.7963/kHz. When the ultrasonic frequency is in the range of 25 kHz ~ 33 kHz, the irreducible water saturation decreases slowly with the increase of ultrasonic frequency, and the gradient is −0.1767/kHz. When the ultrasonic frequency exceeds 33 kHz, the irreducible water saturation increases slowly with the increase of ultrasonic frequency, and the gradient is 0.0239/kHz.

There is an optimal range of ultrasonic frequency in the law affecting irreducible water saturation in low permeability reservoir. When the ultrasonic frequency of 17–25 kHz is used to treat the low permeability reservoir core, the corresponding reduction of irreducible water saturation is 6.37%. Different frequencies of ultrasound have different effects on reservoir core irreducible water saturation. On the one hand, if the ultrasonic frequency increases, the cavitation threshold will increase, and the cavitation process will become more difficult. On the other hand, the increase of ultrasonic frequency will lead to the increase of ultrasonic attenuation16,17. Specifically, when the ultrasonic frequency exceeds 33 kHz, ultrasonic action will not significantly reduce the irreducible water saturation.

Effect of ultrasonic power on the irreducible water saturation of the low-permeability reservoir

The influence of different ultrasonic action frequencies (28 kHz and 50 kHz) on the irreducible water saturation of core YX-3 was tested respectively. The ultrasonic power used in the experiment was 0–2000 W (freely adjustable), the ultrasonic treatment time was 40 min, and the experimental temperature was 35℃.

Fig. 4
figure 4

Relationship between irreducible water saturation and ultrasonic power.

Full size image

It is found that with the increase of ultrasonic power under the same conditions, the degree of reduction of irreducible water saturation in low permeability reservoir is greater. Increasing the ultrasonic power can effectively reduce the irreducible water saturation, as shown in Fig. 4. When the ultrasonic frequency is 28 kHz, the irreducible water saturation keeps decreasing with the increase of power. When the ultrasonic power is lower than 800 W, the irreducible water saturation presents the characteristics of “first rapid decrease, then slow decrease”, and the ultrasonic power is more than 800 W, and it remains relatively stable. When the ultrasonic frequency is 50 kHz and the ultrasonic power is lower than 1200 W, the irreducible water saturation of the low permeability reservoir under the action of high frequency ultrasound also presents the characteristics of “first rapid decrease, then slow decrease” similar to the low frequency ultrasound. When the ultrasonic power exceeds 1200 W, the irreducible water saturation remains relatively stable and no longer decreases. Compared with the effect of different ultrasonic frequencies, the curve segment before the irreducible water reaches stability at 50 kHz is longer than of 28 kHz, indicating that under the condition of the same power, low-frequency ultrasound is more conducive to the expulsion of irreducible water. The difference of irreducible water saturation corresponding to the two ultrasonic frequencies is significantly different with the change of power. When the ultrasonic power is less than 1000 W, the difference of irreducible water saturation is larger, ranging from 3.20 to 4.90%, and the difference of irreducible water saturation gradually decreases with the increase of power. When the ultrasonic power is higher than 1000 W, the irreducible water saturation difference is small, ranging from 2.11 to 2.40%, and the irreducible water saturation difference remains relatively stable. When the ultrasonic power is low, the difference of irreducible water saturation corresponding to the two frequencies is large (200 W, the difference is 4.90%). When the ultrasonic power is high, the difference of irreducible water saturation corresponding to the two frequencies is small (1600 W, the difference is 2.10%), indicating that high-power ultrasound has a comparative advantage in reducing the irreducible water saturation of low permeability reservoir, and will not be too limited by ultrasonic frequency in use. With the increase of ultrasonic power, the more abundant the energy supply, the more intense the cavitation18, which is more conducive to the reduction of irreducible water saturation. Increasing the ultrasonic power can make up for the adverse effect caused by the increase of frequency.

The water film thickness in this experiment was calculated using the method in Ref19, as shown in Fig. 5. At the initial stage of ultrasonic action, the water film thickness was 331.74 nm, with a reduction of 46.88%. With the further increase of ultrasonic power, more and more water in the reservoir was displaced by oil, and the water film thickness in the pore throat further decreased When the ultrasonic power is greater than 600 W, the decreasing trend of water film thickness gradually becomes gentle. After treatment with the ultrasonic power of 1200 W, the water film thickness reached 204.90 nm, and the reduction rate of water film thickness was 67.19%.

Fig. 5
figure 5

Relationship between water film thickness and ultrasonic power.

Full size image

Effect of temperature on irreducible water saturation of low permeability reservoir

The ultrasonic power used in the heating and cooling experiments was 600 W, the ultrasonic frequency was 22 kHz, and the ultrasonic treatment time was 40 min, the experimental sample was YX-6.

Fig. 6
figure 6

Relationship between irreducible water saturation and temperature.

Full size image

In the temperature rise experiment, due to the weakening of the reservoir’s ability to bind water after the temperature rise, the thickness of the water film attached to the inner wall surface of the reservoir gradually becomes thinner, this leads to the water being driven out, and the saturation of irreducible water gradually decreases, as shown in Fig. 6. In addition, for the oil displacement process of hydrophilic reservoirs, capillary force is the resistance. As the temperature increases, the viscosity of simulated oil and injected water decreases, resulting in smaller displacement resistance and easier displacement of irreducible water, resulting in a gradual decrease in irreducible water saturation. In the cooling experiment, the irreducible water saturation stabilized at 39.37%, which is basically consistent with the research results in Ref20. Although the decrease of core temperature will lead to the increasing water binding capacity of the reservoir, the irreducible water saturation should be increased, but in the pure simulated oil displacement without the involvement of foreign water to cause this result.Since this result could not truly reflect the influence of temperature on the irreducible water saturation, supplementary experiments were conducted. After the irreducible water was stabilized, a fixed oil-water ratio was adopted for displacement, and then simulated oil was used to displace the core. In the simultaneous injection of oil and water without ultrasonic under cooling conditions, the irreducible water saturation gradually rose with the decrease of temperature, which basically coincided with the experimental curve without ultrasonic treatment under heating conditions.

The ultrasonic effect has no obvious effect on the curve shape of irreducible water saturation with temperature. When the reservoir is at a lower temperature, the irreducible water saturation decreases rapidly with the increase of temperature. When the reservoir temperature exceeds 40 ℃, the irreducible water saturation decreases slowly with the increase of temperature. The effect of ultrasound on the reduction of irreducible water saturation in hydrophilic reservoirs gradually decreases with the increase of temperature. At a higher temperature of 70 ℃, the effect of ultrasound can still reduce the irreducible water saturation, indicating that it is feasible to reduce the irreducible water saturation under the formation temperature. In the oil-water simultaneous injection experiment with ultrasonic treatment under cooling conditions, the irreducible water saturation gradually rises with the decrease of temperature. At the relative high temperature above 40℃, the difference of irreducible water saturation corresponding to the heating and cooling process under ultrasonic action is 0.33–1.28%, and the difference between the two is small. At a relatively low temperature below 40℃, the difference of irreducible water saturation corresponding to the heating and cooling process under ultrasonic action ranges from − 0.76 to 3.04%, with a large difference between the two, and the irreducible water saturation difference increases gradually with the decrease of temperature, indicating that low temperature treatment under ultrasonic condition has strong adaptability.

With or without ultrasound, the difference of irreducible water saturation decreases gradually with the increase of reservoir temperature, which fully indicates that ultrasound has a stronger influence on the irreducible water saturation at low temperature, and ultrasonic action is more suitable for low temperature treatment of hydrophilic reservoirs.When the temperature is low, the mechanical effect of ultrasound is more obvious. The pore diameter of the reservoir increases and decreases with the effect of ultrasound. When the pore diameter increases, the irreducible water under the control of capillary force gets rid of the attraction of the pore wall and becomes movable water, which is an important reason for the decrease of the irreducible water saturation caused by ultrasound at low temperature.However, when the temperature is high, as the thermal action gradually weakens, the mechanical action eventually dominates. In addition, most of the irreducible water has been driven out under the early low temperature conditions, resulting in insufficient influence of thermal action on the irreducible water saturation.

Influence of the reservoir permeability on the irreducible water saturation of the low-permeability reservoir

There is a good linear relationship between the decrease of irreducible water saturation and permeability in the absence of ultrasound. However, when the permeability is lower than 11.35 × 10−3 µm2, the irreducible water saturation of the reservoir without ultrasonic action does not decrease significantly with the increase of permeability, as shown in Fig. 7.The reason is that the permeability of YX-1 and YX-2 varies very much(8.09 × 10−3µm2 difference), but the porosity of the two is basically the same (0.53% difference), indicating that the pore throat structure of YX-1 is more complex than that of YX-2, and the inner wall of the pores of the reservoir has a stronger binding effect on water molecules, with a higher proportion of small pore throats, and a smaller throat leads to a worse fluid flow efficiency, which in turn leads to a larger thickness of the irreducible water film, and hinders the further reduction of the irreducible water saturation. Compared with YX-1, the throat diameter of YX-2 is larger, and the force between the irreducible water and the inner wall of the pore is smaller, so the irreducible water is easier to get rid of the attraction of the inner wall of the pore and fall off. Therefore, the decline of the irreducible water saturation of core 2 is more in line with the linear relationship between the permeability.

Fig. 7
figure 7

Relationship between irreducible water saturation drop and permeability.

Full size image

There is a good logarithmic relationship between the decrease of irreducible water saturation and permeability under ultrasonic action. Especially for the core with permeability of 3.26 × 10−3µm2, the difference of irreducible water saturation between the two conditions with and without ultrasonic action is the largest, which is 9.26%. In both cases, the difference of irreducible water saturation decreases gradually with the increase of reservoir permeability, indicating that the effect of ultrasound on the reduction of irreducible water saturation is more obvious in the low permeability reservoir with poor physical property.The reason is probably that the sand of the reservoir with relatively high permeability is coarser and there is more fluid in it, and its absorption of ultrasonic energy is more obvious than that of the reservoir with poor physical property. In addition, the fluid in the pore throat of the reservoir with high permeability has a larger relative motion amplitude with the sand skeleton of the reservoir, resulting in greater ultrasonic absorption degree and more obvious energy attenuation.

Influence of treatment time on the irreducible water saturation of the low-permeability reservoir

In the previous study, we explored the effect of ultrasound on heavy oil viscosity and permeability recovery; indeed, ultrasound time has an important effect on the irreducible water saturation of low permeability reservoir. Therefore, we prepared three more natural cores with different permeability, and conducted related experiments here.

To study the influence of ultrasonic treatment time on irreducible water saturation, related experiments were carried out. To enhance its representativeness, three different cores are used, the core 1 permeability K = 22.56 × 10−3µm2, representing the low permeability reservoir; the permeability of core 2 K = 278.93 × 10−3µm2, representing the middle permeability reservoir; the core 3 permeability K = 636.07 × 10−3µm2, representing the high permeability reservoir.

Fig. 8
figure 8

Relationship between irreducible water saturation and treatment time.

Full size image

It was found that in the early stage of ultrasonic action, the irreducible water saturation of the three cores decreased rapidly, showing similar characteristics. The reduction rate of the irreducible water in the high permeability reservoir was significantly higher than that of the low permeability reservoir, indicating that the better the original permeability reservoir, the faster the reduction rate of the irreducible water as shown in Fig. 8.The differences are as follows: the irreducible water saturation of the low permeability core reaches the lowest value after 40 min after ultrasound, then the ultrasound treatment time is increased, the irreducible water saturation remains basically stable; the irreducible water saturation of the medium permeability core reaches the lowest value after 30 min after ultrasound, and then basically stable; for the high permeability core, the treatment time is reduced to 15 min.This shows that for lower permeability core longer ultrasonic excitation time should be used, ultrasonic has an optimal processing time, and the optimal processing time is closely related to the reservoir properties, the reservoir properties the better, the shorter the optimal processing time, too long ultrasonic excitation time may lead to the separation of larger particles2, these particles in the form of bridge back block fluid seepage, affect the reduction of irreducible water saturation.

In this paper, it was found that the optimal treatment time to reduce irreducible water saturation is closely related to reservoir properties21. The low permeability reservoir has small radius and the distance between clay particles and the inner wall is small, which causes the strong adsorption on the irreducible water; moreover, the low permeability reservoir is susceptible to contamination, so the low permeability reservoir needs long treatment time.However, a long time of ultrasonic action will produce strong cavitation at the entrance end of the core, in which the ultrasonic energy applied at the core is reduced, thus leading to more acoustic energy escape.The pressure generated by the cavitation bubble annihilation exerts a back pressure effect on the core displacement system, resulting in the blocked fluid flow, which has a more significant effect on the low permeability core with poor physical properties.

The mechanism of the effect of ultrasonic treatment on fluid flow in the porous media

Minor displacement effect

The ultrasonic wave can cause a small displacement of the fluid particles. Under the action of high intensity ultrasound, this small displacement can produce large displacement, which can cause shear and turbulence of local fluid. In porous media, such shear and turbulence can facilitate fluid flow in the pore and reduce the irreducible water saturation.

Thermal effect

The propagation of the ultrasonic wave in the fluid can cause the vibration and friction of the fluid particles. This vibration and friction will lead to the friction and collision between the fluid molecules, generating heat energy and changing the temperature of the fluid.

Changes in temperature may affect physical parameters such as the viscosity and surface tension of the fluid, thus affecting the flow characteristics of the fluid in a porous medium.

Cavitation effect

Due to the high frequency vibration of the sound wave and the drastic pressure changes, the propagation of the ultrasonic waves in the fluid can cause the formation of the gas molecules in the fluid to form bubbles. Cavitation phenomenon not only affects the physical characteristics of the fluid, but also may cause small changes in the structure of the porous medium22, such as the expansion or blockage of the pores, and then affect the fluid flow of the fluid.

Chemical effect

The chemical effect of ultrasound is an accessory phenomenon dependent on the cavitation effect. The extreme high pressure and high temperature environment produced by the cavitation effect can promote chemical bond breaking and reorganization of molecules in solution, thus triggering chemical reactions.

In porous media, such a chemical reaction may alter the properties of the pore surface, such as increasing or decreasing the hydrophilic or hydrophobicity of the pore surface, and subsequently affecting the fluid flow.

Conclusion

It is found that ultrasonic action can further reduce the irreducible water saturation of low permeability reservoir on the basis of oil flooding. The ultrasound of different frequencies affects the irreducible water saturation of the low permeability reservoir differently, and there is an optimal range of ultrasonic frequency: 17 kHz ~ 125 kHz. Increasing ultrasonic power can effectively reduce the irreducible water saturation, and increasing ultrasonic power can make up for the adverse effects caused by increasing the frequency. Under the condition of the same ultrasonic power, low-frequency ultrasound is more helpful to the removal of irreducible water.With the increase of ultrasonic power, the thickness of water film decreases rapidly at first and then becomes stable gradually, the highest drop could be 67.19%. The effect of ultrasound on the reduction of irreducible water saturation in low permeability reservoir gradually decreases with the increase of temperature. At a higher temperature of 70 ℃, the effect of ultrasound can still reduce the irreducible water saturation. The effect of ultrasound on irreducible water saturation is more intense at low temperature, compared with high temperature, the irreducible water saturation can be reduced by 9.26%, and ultrasound is more suitable for low temperature treatment of low permeability reservoir. The effect of ultrasound on the reduction of irreducible water saturation is more obvious in low permeability reservoirs with poor physical properties.In low permeability reservoirs, higher irreducible water saturation limits effective hydrocarbon flow. Ultrasonic technology can reduce the saturation of irreducible water through physical action, enabling the release of irreducible water molecules, thus increasing the flow channel of oil and gas, and contributing to the recovery of low permeability reservoirs, which is of great significance for the development and production of low-permeability reservoirs.