Investigation on effects of water‑shale interaction on acoustic characteristics of organic‑rich shale in Ordos Basin, China

The water-shale interaction affect the shale structure, leading to wellbore instability and increasing drilling costs. The extent of structural changes within the shale can be determined non-destructively by analyzing its acoustic characteristics. Experiments were conducted to investigate the acoustic properties of shale from the Yanchang Formation in the Ordos Basin before and after exposure to brines of varying types, soaking times, and salinities. The study investigated the effects of brine type, soaking time, and salinity on shale’s acoustic properties, including changes in acoustic wave propagation speed, P/S wave velocity ratio, and both time-domain and frequency-domain amplitudes. The results indicate that although the type of brine has a limited impact on the water-shale interaction, KCl exhibits a significant inhibitory effect. However, the soaking time and the brine salinity have a significant impact on the acoustic properties of shale. As the soaking time increases, the decrease in wave velocity increases, the P/S wave velocity ratio increases, and the decrease in time-domain amplitude increases. The amplitude of the main frequency in the frequency domain signal also decreases with the increase of reaction time, which is consistent with the analysis results of the time domain signal. As the salinity of brine increases, the decrease in wave velocity decreases, the P/S wave velocity ratio decreases, and the decrease in time-domain amplitude decreases. The amplitude of the main frequency in the frequency domain signal also decreases with the increase of brine salinity, which is consistent with the analysis results of the time domain signal. This work establishes the relationship between water-shale interaction and acoustic characteristics, which can quantitatively evaluate the degree of interaction between water and shale without damaging shale. Furthermore, this research provides new insights and guidance for predicting drilling collapse cycles and optimizing drilling fluid compositions.


Introduction
The increasing demand for oil and gas resources has led to growing interest in unconventional resources like shale gas.While the development of shale gas reservoirs in North America has sparked a global energy revolution, the progress in shale gas development in China has not met expectations (Han et al. 2013;Xingang et al. 2013;Sohail et al. 2022;Zhang et al. 2023).In the past decade, wellbore instability is still one of the main factors restricting the development of Shale oil and gas (Lyu et al. 2015;Zhang et al. 2015;Gholami et al. 2018;Yang et al. 2019).The filtrate of water-based drilling fluid enters a formation and interacts with shale, which changes the structure and physical properties of shale itself, and, in turn, will change the strength of shale and lead to wellbore instability (Radwan 2021(Radwan , 2022;;Qadri et al. 2022;Radwan et al. 2022).The degree of interaction between shale and water has only been evaluated from a mechanical perspective, but mechanical experiments can cause irreversible damage to the rock.However, acoustic characteristics can determine the degree of interaction between shale and water without damaging the rock structure, which is of great significance for studying the wellbore stability and drilling fluid optimization during the drilling process.
Wellbore instability is a challenge faced by drilling engineering.Interaction between water and shale is of great significance to the study of wellbore stability.Many scholars have studied the influence of the type of experimental fluid, temperature, pressure and other parameters on the strength of rock.They believe that the strength of the formation will be reduced to varying degrees after the fluid shale interaction, and have observed the change of its microstructure through microscopic experiments (Ewy andStankovich (2002, 2010); Yuan et al.2014;Chen et al.2018;Liu et al.2018;Qiao et al.2018;Liu et al. 2019a, b;Eveline et al.2017;Koteeswaran et al.2018).However, the damage degree of strength has not been quantitatively characterized in these studies.
In order to characterize the degree of interaction between water and shale, Ghasemi et al. (2018) quantitatively characterize the degree of shale water interaction from the perspective of mechanical damage, but this method can damage the shale structure and the experiment is non repeatable.The acoustic characteristics of shale can effectively reflect the internal structural characteristics of shale.Many scholars have studied the influence of rock internal structure, mineral composition and interaction with fluid on the wave velocity and attenuation coefficient of sound wave, and obtained a series of laws.These studies show that the velocity ratio and velocity of sound wave propagating in shale are determined by the pore structure and mineral composition of shale (King 1977;Minear 1982;Tare et al.2000;Tutuncu and Mese (2011;2012); Zhu and Harris 2014;Nooraiepour et al.2017;Xu et al. 2017;Lei et al. 2018;Omovie and Castagna 2019).Therefore, studying the acoustic characteristics of the water-shale interaction has an important reference value for the inversion of internal structural changes in shale.
What is more, the study of acoustic characteristics under an effect of the water-shale interaction is an important step to determine the interaction degree and its associated damage and reproduce changes in internal structure.Many scholars discussed relevant studies.Holt et al. (1996) pointed out that an interaction between a fluid and shale can be determined by a wave velocity by measuring it in shale under different temperature and pressure conditions.Zhou et al. (2013) calculated the dynamic Young's modulus and Poisson's ratio by measuring the acoustic wave velocity of a shale sample interacting with a water-based drilling fluid, which was also used to select a more suitable drilling fluid.Liang et al. (2019) used the time-frequency characteristics of acoustic waves to evaluate the hydration characteristics of shale and mainly discussed the effect of an interaction time on their evaluation.In addition, a new variable of hydration damage was established, and the effect of the hydration damage on shale structure and acoustic characteristics was discussed by using this variable.Liu et al. (2019a, b) established a hydration index and a constitutive model in shale by using the time-frequency characteristics of an acoustic wave, providing a theoretical basis for evaluations of shale hydration damage.However, previous studies only analyzed acoustic characteristics of shale under the water-shale interaction from a certain point of view, and they still lacked consideration of parameters, such as a brine type and salinity, from the perspective of an interaction time.
The acoustic characteristics of water-shale interaction can effectively invert the internal structural changes of shale.Through the change of the acoustic characteristics, the degree of shale water interaction can be evaluated without damaging the original rock structure.This study focuses on the shale in the Ordos Basin of China, and analyzes the changes in acoustic characteristics under different conditions from three perspectives: brine type, salinity, and water-shale interaction time.This work will comprehensively understand the acoustic characteristics changes of shale during the water shale interaction process, which will provide reference guidelines for determining the degree of interaction and related damage, as well as for safe drilling of formations and optimization of drilling fluids.

Shale samples
The Upper Triassic Yanchang Formation in the Ordos Basin developed a set of inland river delta lake facies clastic rock series, as shown in Fig. 1. (Caineng et al. 2010;Jiao et al. 2020).According to the lithological combination, the Yanchang Formation is divided into 10 sections.The Chang 7 section (Fig. 1.) mainly composed of organic shale, which is generally black in color.The rock samples taken from the formation are shown in Fig. 2.
The experimental rock samples are from Chang 7 section in the Ordos Basin, which are organic-rich shale, the TOC content is shown in Fig. 3.The shale belongs to low porosity and low permeability formation (Liu et al. 2019a, b).The studied rock samples are treated as the standard samples (D * L = 2.5 mm * 5.0 mm) and labeled with A-F, as shown in Fig. 4. The mineral and clay compositions of the formation were tested shown in Fig. 5. and Fig. 6., respectively.
Figure 3. shows that the distribution of TOC content in the shale of the experimental section ranges from 3.3% to 12.3%, with an average of 7.1%.It indicates that the Chang 7 shale has a high TOC content, which is beneficial for the generation of hydrocarbon.At the same time, the high TOC content may affect the wettability of the Chang 7 section shale, the higher the TOC content, the easier it is to be lipophilic.Fig. 5. presents that the clay minerals content in this area are relatively high, with an average of 42%, followed by quartz, with an average of 23%. Figure 6.shows that clay minerals are mainly illite, with an average of 54%, followed by chlorite, with an average of 34.7%, and 11.3% of an illite mixed layer.The substantial content of clay minerals establishes the foundation for the water-shale interaction.
To further observe the micro characteristics of the shale in this area, the SEM was tested shown in Figs. 7,8,9,10. Figure 7. and 8. show that the shale microfractures in this area are relatively developed, which provides channels for drilling fluid to enter the formation.The contact between shale and fluid easily leads to a series of interactions between water and shale.Figure 9. depicts that the clay minerals are arranged in order, which is more conducive to an interaction between clay minerals and water.Moreover, Fig. 10.shows that clay minerals are attached  The results of the whole rock analysis and SEM show that the shale in this area has the characteristics of high clay mineral content, mainly illite and an illite mixed layer, development of microcracks, a sequence of clay minerals, and occurrence of clay minerals on rock surfaces and close to pore passages, all of which will promote the water-shale interaction.

Experimental procedures
The research on the acoustic characteristics of shale generally focused on wave velocities.In this work, the change in the waveform before and after the water-shale interaction is discussed, and the change in the attenuation coefficient is calculated by amplitude.In addition, the change in the acoustic wave is analyzed from the perspective of a frequency domain through a Fourier transform.The effects of the water-shale interaction on the acoustic characteristics are systematically studied by comparing the changes in the wave velocities before and after the water-shale interaction, velocity ratio of P − waveandS − wave , amplitude, attenu- ation coefficient, and frequency.This experiment adopts the frequency conversion ultrasonic testing system, which mainly consists of oscilloscopes, probes of different frequencies, etc.

Experimental theory
As common knowledge, the propagation speed of acoustic waves varies in different media.Generally, acoustic waves travel the fastest in solids, followed by liquids and gases.We can effectively judge a medium change by the change in wave velocity.Poisson � sratio( − Formula1) is an elastic constant that reflects the lateral deformation of rock and is one of the important rock mechanics parameters.Poisson's ratio can be calculated by the P/S wave velocity ratio.The amplitude represents the energy of the acoustic wave.In the process of propagation, the acoustic wave will be weakened because of refraction and reflection through changes in its In addition to the analysis of acoustic characteristics before and after the water-shale interaction in a time domain, the acoustic characteristics of shale before and after the water-shale interaction can be analyzed in a frequency domain through Fourier calculations.
This paper will use the following formula for calculating Poisson's ratio σ: where V p and V s are the P-wave and S-wave velocities, respectively.

Experimental samples
The sample numbers and experimental conditions required for different experiments are shown in Table 1.
From Table 1, it can be seen that different samples have been arranged for different experiments.When conducting analysis, the corresponding samples can be selected.

Acoustic experiments of shale with different brine types
Different types of brine will have varying effects on shale and result in different acoustic characteristics.The research on the effect of the brine types on shale acoustic waves after the water-shale interaction can provide a reference for optimizing a drilling fluid.According to the previous research (Liu et al. 2019a, b), the water-shale interaction time was set to 48 h, and the drying temperature was set to 60 °C.NaCl and KCl, which are often added to drilling fluid, are used as experimental fluids.The samples used in this experiment are A、B and C. The specific experimental steps are given as follows: (1) (1) Samples A, B, and C were put into the incubator, the temperature was set to 60 ℃, and the time was set to 48 h; (2) Samples A, B, and C were taken out for cool down, and the P-wave and S-wave at 100 Hz were measured;(3) 5000 mL distilled water, 5000 mg/L NaCl solution, and 5000 mg/L KCl solution were perpared in three beakers; (3) Samples A, B, and C were immersed in three solutions for 48 h, respectively; (4) The cores were taken out,and put into the incubator, the temperature was set to 60 ℃, the time was set to 48 h.(5) Samples A, B, and C were taken out for cool down, and the P-wave and S-wave at 100 Hz were measured; (6) Experimental data was sort out.

Acoustic experiments of shale with different soaking time salinity
The effect of different water-shale interaction times on shale properties also varies.In this experiment, the 5000mg/L NaCl solution is used as the experimental fluid.The samples D, E, and B were used in this experiment.

Acoustic experiments of shale with different brines
The degree of the water-shale interaction also be affected by the different salinity of the solutions.To further study the effect of salinity on the acoustic characteristics of shale after the water-shale interaction, 0-mg/L (as in sample A), 5000-mg/L (as in sample B), and 15,000-mg/L NaCl solutions were used as the experimental fluids.The samples A、B and F were used in this experiment.

Results
The interaction between water and shale changes the internal structure of shale, but the degree of hydration cannot be accurately determined.The internal structure of shale can be reflected by the acoustic characteristics without damaging the original rock structure.In order to determine the changes in the internal structure of shale under different hydration conditions, the water shale interaction was measured from three factors: brine type, soaking time, and brine salinity.The acoustic characteristics of shale are systematically analyzed from the aspects of the change in the wave velocity, the change in the P/S ratio, the change in amplitude in time domain, and the change in amplitude in frequency domain.

Time domain analysis
Velocity The wave velocity is a basic characteristic of the acoustic wave.The propagation velocity of the acoustic wave in different media varies, so the change in the wave velocity can effectively reflect the change in the internal structure of the shale.In this experiment, the P-wave velocity at 100-Hz frequency is taken as the reference.Figure 11.depicts the effect of the type of brine on the wave velocity in shale before and after the water-shale interaction.Figure 11.shows that the P-wave velocity in the original shale is approximately 5000 m/s, distributed between 4000 and 5000 m/s.The wave velocity decreased to varying degrees after interaction with distilled water, NaCl, or KCl solution for 48 h.The acoustic wave velocity decreased the most significantly after interact with distilled water,reaching 777 m/s.While, the minimum decrease in velocity is 290 m/s after the interaction with the KCl solution.This result shows that K + has an effect on inhibiting the interaction between shale and water (Chen et al. 2003).In addition, the effect of NaCl is in the middle, indicating that it has a certain inhibitory effect on shale water interaction, but its effect is not as obvious as that of KCl.The wave velocity of rocks decreases differently after the action of different saline solutions, with distilled water decreasing the most and KCl solution decreasing the least.The experimental results indicate that the presence of cations is a factor promoting shale hydration.The inhibitory effect of K + has also been further validated, which is consistent with previous studies (Chen et al. 2003;Okoro et al.2018).Therefore, this study indicates that appropriate drilling fluids can be selected during the drilling process as needed.
Velocity ratio The P/S wave velocity ratio is an important parameter that relates mechanical parameters and acoustic characteristics of shale.Through the change in the velocity ratio of P-wave and S-wave, we can determine the change in Poisson's ratio of shale and then analyze changes of mechanical characteristics.In this experiment, the P-wave and S-wave velocities at 100 Hz were taken as the experimental parameters to calculate the P/S wave velocity ratio.The experimental results were shown in Fig. 12.
Figure 12. depicts that the P/S wave velocity ratio increases in different degrees after interaction with different fluids.The P/S wave velocity ratio changes the most after interaction with the distilled water, reaching 0.05.The P/S wave velocity ratio does not change much after interaction with the NaCl solution or KCl solution.The results indicate that the change in mechanical properties of shale after interaction with distilled water is greater than that after interaction with other brine.
The P/S wave velocity ratio of shale has been increased to different extents, which corresponds to an increase in Poisson's ratio of shale after the water-shale interaction (Tutuncu 2012).
Amplitude The amplitude represents the magnitude of energy.During the propagation of a medium, sound waves are dampened, which leads to energy attenuation and changes in amplitude.The change in the shale internal structure can also be determined by the changes in amplitude.This experiment is based on amplitude change in the 100-Hz longitudinal wave.The experimental results were depicted by Fig. 13.
It can be seen from Fig. 13. that the amplitude of shale decreases in different degrees after interaction of shale and varying brines, and the most evident decline is after the interaction of shale and distilled water.The initial amplitude Fig. 11 Effect of the types of brine on wave velocity Fig. 12 Effect of the types of brine on the P/S wave velocity ratio of each core is different and thus can only be analyzed by the change in amplitude.The initial amplitude of shale with distilled water is more than 1.7 V, indicating that the attenuation of acoustic energy is the least in the process of shale propagation.The amplitude of shale decreases the most after its interaction with distilled water, indicating that the internal structure of shale has changed evidently during its interaction with distilled water, and the degree of the water-rock interaction is also the most intense.The initial amplitude of shale is close to that of NaCl and KCl brine, and the amplitude after the interaction is also close to that of shale.Among them, the amplitude change of shale after the action of KCl brine is the smallest (Ghasemi et al. 2019), indicating that the change of its internal structure is the least.The inhibition of KCl on the water-rock interaction is also shown.
In terms of a change in amplitude, sample A has the largest change amplitude, whereas samples B and C have a similar change amplitude.This result shows that a change in rock mechanical properties after the action of NaCl and KCl solutions is not particularly evident, but Poisson's ratio of shale after the action of distilled water increases evidently.This case shows that the interaction between distilled water and shale is evident, which corresponds to the analysis result in Sect."Velocity".Poisson's ratio reflects an elastic constant of lateral deformation of rock which is also an important parameter for calculating a formation pressure profile and giving a safe density window.

Frequency domain analysis
Fourier transform is an important means to transform a time domain to a frequency domain.The influence of the water-shale interaction on the dominant frequency can be studied by transforming a time-domain acoustic signal into a frequency-domain signal through the Fourier transform.To better compare the influence of a brine type on acoustic characteristics, frequency-domain curves before and after water-rock interaction are sorted out and analyzed.Figure 14,15,16.show the results.
What we can learn from the Fig. 14, 15, 16. is that the changes in the frequency domain signal before and after shale interacts with different fluids.The results show that after shale interacts with different fluids, its main frequency moves in the direction of decreasing, and its amplitude shows a decreasing trend.Figure 14.presents that the amplitude of the main frequency of the shale acoustic wave affected by distilled water decreases the most, from 0.122 to 0.04 V, down by 0.082 V.Moreover, Fig. 15.shows that the main frequency amplitude of the shale acoustic wave affected by NaCl decreased from 0.116 to 0.043 V, down by 0.073 V. Furthermore, Fig. 16.presents that the amplitude The time-domain acoustic curve shows the strength of energy through its amplitude, and a frequency-domain acoustic curve is displayed through the Fourier transform.The decline in the main frequency shows a change in energy to some extent, as shown in Figs.14, 15, 16.The experimental results show that it exhibits similar characteristics to the time domain.Both time-domain and frequency-domain results indicate that the influence of salt water type on water shale interaction is minimal, and the difference is not significant.The main reason is that KCl has a stronger inhibitory effect on water sensitive clay minerals than NaCl.But the shale clay mineral composition of the target block is mainly illite, with only a small amount of illite mixed layer, so the performance is not very obvious.

Time domain analysis
Velocity According to previous studies, the effect of soaking time on the degree of the water-shale interaction is more evident.To study the effect of soaking time on the acoustic wave, three samples with similar physical properties were selected to interact with the 5000-mg/L NaCl solution for 6, 24, and 48 h, respectively.Then, the acoustic characteristics before and after the interaction with brine were measured and analyzed.The influence of soaking time on wave velocity is studied with 100-Hz frequency P-wave as the reference.Figure 17.depicts the experimental results.
Figure 17.presents that the acoustic wave speed in shale decreases to different degrees after its interaction with brine.After 6 h, the wave speed decreases by 289 m/s, and after 48 h, it decreases by 519 m/s.With an increase in soaking time, the wave velocity decreases more.This case shows that with an increase in soaking time, the degree of the water-shale interaction is also deepened.

Velocity ratio
The P/S wave velocity ratio is an important parameter to connect the acoustic and mechanical properties of rock, which is also an important parameter to calculate Poisson's ratio.The change law is in direct proportion to Poisson's ratio.Figure 18.shows the results of the P/S wave velocity ratio before and after the water-shale interaction.
It can be seen from Fig. 18. is that after immersion, the P/S wave velocity ratio has increased in varying degrees.After soaking for 6 h, the P/S wave velocity ratio increased by 0.043.After 48 h of interaction, the ratio increased by 0.023, showing a trend of decreasing with an increase in action time.Figure 18. also shows that the velocity ratio of the original rock is between 1.4 and 1.9.
Moreover, Poisson's ratio is a bridge connecting the acoustic and mechanical characteristics.Poisson's ratio of rock increases after the water-rock interaction; that is, the elastic constant of shale lateral deformation increases.

Amplitude
The internal structure of the shale will change after the water-shale interaction,, which will cause an expansion of primary fractures and even the formation of new fractures.In this way, acoustic wave propagation in shale will reduce the energy, which can be expressed by the change in amplitude.The degree of the water-rock interaction varies at different times, and the amplitude change is also different.The influence of soaking time on the amplitude of an acoustic wave is obtained by processing the acoustic wave curves with different interaction times.What we can learn from Fig. 19. is that the amplitude of the acoustic wave decreases evidently after the interaction with brine at different times.After soaking for 6 h, the amplitude of the drop was not evident, only 0.02 V.However, after 24 h, the amplitude decreased significantly, from 0.70 to 0.31 V, down by 0.39 V.After 48 h, the amplitude decreased from 0.90 to 0.32 V, and the amplitude decreased as much as 0.58V.Figure 19.also shows that the amplitude decreases with an increase in soaking time.

Frequency domain analysis
The influence of soaking time on the acoustic characteristics of shale has been analyzed from the perspective of a time domain.To make a deeper analysis in a frequency domain, the acoustic wave curves of different action times (6, 24, and 48 h) were analyzed.Through Fourier transform, the curves in the frequency domain were obtained.Figure 20, 21. and 15. show the results of 6, 24, and 48 h, respectively.
Figure 20.shows that the main frequency of the acoustic wave decreases from 0.112 V to 0.064 V, decreased by 0.048 V, after 6 h of interaction with the NaCl solution.Then, Fig. 21.shows that the main frequency of the acoustic wave decreases from 0.116 to 0.048 V, decreased by 0.068 V, after 24 h of interaction with the NaCl solution.Moreover, Fig. 15.shows that the main frequency of the acoustic wave decreases by 0.073 V, after 48 h of interaction with the NaCl solution.These Figs show that the dominant frequency decreases to different degrees after the interaction with the NaCl solution.The extent of the decrease also increased with the increase of the soaking time.

Time domain analysis
Velocity In addition to the brine type and soaking time, the salinity will also affect the water-shale interaction.The main reason is that the salinity of brine will affect the hydration and expansion of clay minerals.In this section, the influence of the brine salinity on the acoustic characteristics of shale was studied by the distilled water, 5000-mg/L solution and 15,000-mg/L NaCl solution.After 24 h of water and shale interaction, Fig. 22. shows the experimental results.
It can be seen from the Fig. 22., that the wave velocity decreases to different degrees after the interaction with brine with different salinities.The lower the salinity is, the more the wave velocity will decrease.The wave velocity of shale decreased by 777 m/s after its interaction with the distilled water, while it is only 187 m/s after its interaction with 15,000-mg/L NaCl solution.This result shows that the higher the salinity of brine, the weaker the interaction between water and shale.High salinity of brine can inhibit the hydration of clay minerals to a certain extent.
Velocity ratio As discussed above, the interaction between water and shale not only affects the acoustic characteristics of shale but also changes the internal structure of shale, resulting in changes in acoustic and mechanical characteristics.Additionally, the P/S wave velocity ratio is an important parameter connecting the acoustic and mechanical characteristics.In this section, the influence of brine salinity on the P/S wave velocity ratio was studied.Different concentra-tions of NaCl solution were used, the results were shown in Fig. 23.
It can be seen from Fig. 23. is that the P/S wave velocity ratio has increased in different degrees, after soaking in brine with different salinities, and the P/S wave velocity ratio of shale after interacting with distilled water is the most evident.The increase in the P/S wave velocity ratio means that Poisson's ratio will increase after shale interacts with water.Moreover, the lower the salinity of brine is, the larger the increase of Poisson's ratio is.Amplitude As one of the important parameters of acoustic characteristics, amplitude represents the magnitude of energy, as mentioned above.Figure 24.shows the change in amplitude after the interaction between shale and brine with different salinities.shows that the amplitude decreases by approximately 1.3 V after the interaction with distilled water, with the largest decrease.The amplitude decreased by approximately 0.6 V after interacts with 5000-mg/L NaCl solution.The amplitude decreased by approximately 0.4 V after interacts with 15,000-mg/L NaCl solution.Figure 20 shows that the amplitude decreases in different degrees after shale interacts with brine with different salinities.Moreover, the amplitude decreases less, with an increase in the brine salinity.It can be seen from the case that the change in the shale's internal structure is less with an increase in salinity.

Frequency domain analysis
To understand the influence of the water-shale interaction on acoustic characteristics systematically, the time-domain signals of acoustic waves were transformed into frequencydomain signals for in-depth analysis.The frequency domain signals of the acoustic wave before and after shale interacts with brine with different salinities were processed.Figures 14,15,and 25. present the results at 0, 5000, and 15,000 mg/L, respectively.
What we can learn from Fig. 14. and 15. is that the amplitude of the main frequency decreases by 0.082V and 0.073 V, after 48 h of interaction with distilled water and 5000-mg/L NaCl solution, respectively.In addition, Fig. 25 depicts that after shale interacts with the 15,000-mg/L NaCl solution, the main frequency decreases from 0.097V to 0.063 V, down by 0.034 V.After comparing these Figs, it can be seen that with an increase in the brine salinity, the amplitude of the main frequency decreases.The variation law of the main frequency amplitude corresponds to the analysis of the time domain signals.

Discussion
Upon interaction with water, the internal structure of shale undergoes alterations, resulting in the development and extension of fractures to varying extents, which in turn elicit distinct acoustic responses.The main results of shale salt water interaction are the decrease in wave velocity and amplitude, the increase of P/S wave velocity ratio and the frequency domain curve exhibits the same properties through Fourier transform.The type of salt water, soaking time, and salt water concentration all have an impact on the interaction between rock and salt water.Different factors have varying impacts.

Effect of brine types
The water-shale interaction, mainly the interaction between clay minerals and water, and K + can inhibit a double electric layer of clay minerals, and, to some extent, inhibit the hydration expansion of clay minerals.Therefore, the change in the shale internal structure is smaller.From the experimental results, a brine type has little influence on the water-shale interaction, and the difference is not evident.The main reason is that KCl has a stronger inhibition on water-sensitive clay minerals than NaCl.However, the shale clay mineral composition in the target block is mainly illite, with only a small amount of the illite mixed layer, so the performance is not very evident.After shale interacts with different types of brine, the wave velocity decreases, the P/S wave velocity ratio increases, and the amplitudes in the time domain and frequency domain decrease.Among them, the shale interacting with distilled water has the most evident characteristics.As a whole, the effect of the KCl solution is slightly stronger than that of the NaCl solution.Therefore, the KCl solution should be properly selected as a treatment agent when selecting the drilling fluid in a shale formation.

Effect of soaking time
The propagation speed of acoustic waves in shale has significantly decreased with the increase of soaking time, indicating that soaking time is an important factor in damaging the internal structure of shale.Therefore, when conducting shale hydration experiments and analyses, special consideration should be given to the factor of soaking time.The P/S wave velocity ratio increases after the water-rock interaction.That is, Poisson's ratio of shale also increases after the water-rock interaction, and as the interaction time increases, the increase is also more.It provides a new possibility to evaluate the degree of a water-rock interaction in shale comprehensively by acoustic and mechanical characteristics.Compared with the types of brine, the soaking time has a greater impact on the amplitude.In other words, with an increase in soaking time, the degree of a water-rock interaction in shale increases, the change in the original rock internal structure is relatively large, and the energy attenuation is also more.When shale contacts with water, the clay minerals in shale will interact with water and undergo hydration expansion.The expansion of clay minerals will lead to a further expansion of the original shale cracks or even the initiation of new cracks.The longer the soaking time is, the more clay minerals will interact with water.The degree of hydration and the change in the internal structure are greater.Therefore, the more immersion time is, the more energy is consumed, and the more evident the amplitude change is.From the experimental results, the interaction time will have a greater impact on the internal structure of shale.The results in the frequency domain are similar to those in the time domain.From the perspective of the frequency domain, compared with the types of brine, the soaking time will have a greater impact on the water-shale interaction.By studying the influence of soaking time on the acoustic characteristics, we can further infer the influence of the soaking time on the water-shale interaction, which provides a new methodology for evaluating the collapse period of drilling and is of great significance for safe and fast drilling.

Effect of the salinity of the brine
The most important factor for the hydration and expansion of clay minerals is an osmotic pressure difference.Therefore, when the salinity of brine increases, the hydration of clay minerals will become weaker.Moreover, the change in the shale's internal structure will also weaken, so the decrease in the acoustic wave velocity will also decrease.In the process of drilling, the selection of the salinity of a working fluid is very important.Studying the influence of salinity on the acoustic properties of shale can infer its impact on the water shale interaction and shale structure.Optimize the salinity of saline water according to on-site requirements, providing strong support for safe and orderly drilling.The selection of working fluid salinity has an important influence on shale formation.On the one hand, the salinity of the fluid may cause sensitivity damage to the reservoir; On the other hand, the salinity of fluids has a significant impact on the hydration and expansion of clay minerals.Poisson's ratio of shale increases to some extent after its interaction with brine.However, with an increase in salinity, the increase in Poisson's ratio declined.
According to the experimental results in this study, the types of brine have the least influence on the water-shale interaction, and the soaking time and salinity have a greater influence.The soaking time has an important reference value for judging the collapse cycle of the wellbore and the design of the drilling cycle.An unreasonable salinity of working fluid can cause sensitivity damage to a reservoir and can also cause the hydration and expansion of clay minerals in the formation, resulting in the instability of the wellbore.Studying the effect of water-shale interactions on acoustic characteristics is of great significance for safe drilling in shale formations.
The research results indicate that acoustic parameters such as amplitude and wave velocity exhibit good correlation in both frequency and time domains.The degree of water-rock interaction can be roughly characterized by amplitude, wave velocity, etc., which provides a new approach for quantitatively characterizing the degree of water rock interaction.However, the characterization of a single parameter has limitations.The next focus of research is to construct a new quantitative characterization method for the degree of water rock interaction by utilizing multiple parameters of sound wave response.

Conclusions
In this work, the acoustic wave characteristics of shale under different conditions from three perspectives: brine type, brine salinity, and soaking time, were investigated.By quantifying the degree of interaction between water and shale, it is possible to better predict wellbore stability issues during drilling, thus reducing the additional costs caused by wellbore collapse and ensuring the protection of wellbore stability during the extraction process.The wave velocity, P/S wave velocity ratio, and amplitude in the time and frequency domains were analyzed, which provided a new methodology for quantitative characterization of the degree of the water-shale interaction.This work can provide theoretical guidance for drilling fluid and engineering design.From the experiments and their analyses in this work, the following understandings have been obtained: (1) The type and concentration of saline water, as well as the interaction time between saline water and shale, all affect the fluid shale interaction, alter the internal structure of shale, reduce the strength of shale, and cause wellbore instability.The interaction time between fluid and shale has the greatest impact.
(2) Shale interacts with different types of brine, different concentrations of brine, or the same type of brine at different times, resulting in a decrease in wave velocity, an increase in P/S wave velocity ratio, a decrease in time-domain and frequency-domain amplitudes, and a decrease in dominant frequency amplitudes in frequency-domain signals.This indicates that the result of water shale interaction is to alter the internal structure of shale, thereby exhibiting different acoustic characteristics.
(3) From the results of the interaction time between shale and water, it can be seen that the initial stage of internal structural failure in shale is relatively obvious, and it tends to flatten out as time increases.A reasonable drilling cycle can be designed based on the wellbore collapse cycle to maintain wellbore stability.(4) The effect of brine type on the interaction between water and shale shows that KCl solution has a good inhibitory effect.For highly water sensitive formations, an appropriate amount of KCl can be added to the drilling fluid to maintain wellbore stability.( 5) The effect of salinity on the interaction between water and shale shows that the higher the salt concentration, the weaker the interaction between water and shale.When designing drilling fluids, a reasonable salinity of the drilling fluid is selected based on the geological conditions.

Fig.
Fig. Standard samples for experiment

Fig.
Fig. Analysis of clay mineral composition

Fig. 13
Fig. 13 Effect of the types of brine on amplitude

Fig. 16
Fig. 16 Frequency domain curve before and after interaction with KCl solution Figure 19.shows the results.

Fig. 19
Fig. 19 Effect of soaking time on amplitude

Fig. 22
Fig. 22 Effect of the salinity of brine on wave velocity in NaCl solution

Fig. 25
Fig. 25 Frequency domain signal of shale before and after interaction with 15000 mg/L NaCl solution

Table 1
Different experiments and required samples