Analysis of mechanical and permeability properties of mudstone interlayers around a strategic petroleum reserve cavern in bedded rock salt

International Journal of Rock Mechanics and Mining Sciences 112 (2018) 1–10

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Analysis of mechanical and permeability properties of mudstone interlayers around a strategic petroleum reserve cavern in bedded rock salt ⁎

T

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Nan Zhanga, Chunhe Yanga,b, Xilin Shib, , Tongtao Wangb, , Hongwu Yinb, J.J.K. Daemenc a

State Key Laboratory of Coal Mine Disaster and Control, Chongqing University, Chongqing 400044, China State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, Hubei, China c Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, NV 89557, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Strategic petroleum reserve (SPR) Mudstone interlayer Brine immersion Crude oil immersion Rock damage Permeability

Using salt caverns for underground strategic petroleum reserve (SPR) is the most popular storage method in the world due to its high security and economy. The rock salt resources in China are mainly bedded salt, containing many mudstone interlayers. In this paper, pH test and total acid number (TAN) tests are carried out to determine the different degradation mechanisms caused by brine-mudstone and oil-mudstone interactions. Uniaxial and triaxial compression tests and permeability tests were carried out to determine the mechanical and permeability parameters of the mudstone interlayers under brine and crude oil immersions. Results show that the thermochemical sulfate reduction (TSR) will not occur during the crude oil storage. The reaction between the anhydrite and the naphthenic acid is the major reason that causes damage to the mudstone interlayers. Both the crude oil and brine can cause damage to the mudstone interlayers, but the damage caused by brine to the mudstone interlayers is much greater than that caused by crude oil because of their different reaction mechanisms. Due to the plugging effect by the asphaltene and resin present in crude oil, the permeability of the samples decreases 67.5% after they have been immersed in crude oil for 30 days, even though the rock damage is still accumulating. This property is beneficial for the SPR salt caverns, to keep good tightness.

1. Introduction With the rapidly increasing fossil energy consumption and higher demand for energy reserves in China, an underground national strategic petroleum reserve (SPR) is being considered seriously.1 Due to the low permeability, damage recovery and stable chemical properties of rock salts, solution-mined caverns constructed in rock salt formations are recognized as the appropriate places for energy storage.2–5 Using salt caverns for SPR is also the most popular way around the world due to its high security and economy. The U.S. stores crude oil in sixty-two caverns located at four different sites in Texas and Louisiana, which accounts for 90% of its oil reserves.6,7 About 42% of Germany's oil is stored in underground salt caverns. Countries such as France, Canada, Mexico and Morocco have also built SPR salt caverns to protect their national energy securities. Overall, salt caverns have an important position in the international energy storage. In recent years, the Chinese government has successfully built many salt caverns for underground gas storage (UGS). However, so far none of the salt caverns have been constructed for SPR. According to the Phase III of China's SPR plan, the government strongly supports the construction of underground storage

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facilities, especially underground SPR salt caverns. Jintan salt mine, located in Jiangsu province, will be the primary construction area. Different from most of the SPR salt caverns in other countries, which are mainly constructed in thick salt domes, salt caverns in China are primarily constructed in bedded rock salt. The formations are usually composed of many thin salt layers and mudstone interlayers. The physical and mechanical properties of these formations are complex.8 Over the past years, a large amount of basic research has been carried out on these kinds of bedded rock salt. Li et al.9 proposed a three-dimensional expanded Cosserat medium constitutive model for bedded rock salt and used it for stability analysis of underground salt caverns. Ślizowski and Lankof10 studied the rheological properties of the mudstone interlayers in bedded salt and indicated that the mudstone with a considerable content of rock salt is a suitable medium for the construction of a radioactive waste repository. Devries et al.11 developed a new stress-based criterion for predicting the onset of damage of bedded rock salt surrounding natural gas storage caverns. Yang et al.12 presented a 3D geomechanical model to evaluate the feasibility of a bedded rock salt cavern using for UGS, and validated its accuracy by the sonar measurement field data. Bruno13 analyzed different parameters that

Corresponding author. E-mail addresses: [email protected] (X. Shi), [email protected] (T. Wang).

https://doi.org/10.1016/j.ijrmms.2018.10.014 Received 14 January 2018; Received in revised form 4 October 2018; Accepted 12 October 2018 1365-1609/ © 2018 Elsevier Ltd. All rights reserved.

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equipment was used to prepare the mudstone samples. It induced little damage to the rock and could ensure the accuracy to less than 0.2 mm. The diameter and height of the cylindrical mudstone samples are about 25 and 50 mm respectively. All the samples meet the requirements of the rock mechanics test standard. An ultrasonic velocity meter was used to determine the longitudinal wave speed of the mudstone samples. To ensure the homogeneity of test samples and the comparability of test results, the samples with a longitudinal wave velocity of about 3000 m/s are selected for mechanical and permeability tests. 2.2. Test and research methodology 2.2.1. TAN test of oil-mudstone interaction The acidity of crude oil is most commonly expressed by its total acid number (TAN). It is the number of milligrams of KOH determined by nonaqueous titration (ASTM D 664–1989) needed to neutralize the acidity in 1 g of oil.17 Generally, the acidic components in crude oils consist of organic acids, inorganic acids, and some other compounds such as esters, phenols, amines, and pyrrole series, which affect the oil acidity.18 Under the crude oil immersion environment, the mineral particles in the mudstone may have a large number of chemical reactions with crude oil, which may cause rock damage. In this test, the degradation mechanism of the oil-mudstone interaction is preliminarily discussed by monitoring the variation of TAN. The density of crude oil used in the tests as measured is 0.823 g/cm3. The viscosity of crude oil is 4.588 mPa s. The contents of wax, resin, asphaltene and water of crude oil are 8.4%, 10.4%, 1.84% and 1.01%, respectively.

Fig. 1. Schematic of the underground SPR salt caverns in bedded rock salt.

affect stability and deformation of bedded salt caverns. Shi et al.14 studied the tensile strength of a mudstone interlayer subjected to brine immersion. Liu et al.15 studied the physical and mechanical properties as well as porosity and permeability of bedded rock salt by laboratory tests and showed that the bedded rock salt is satisfactory for the tightness and stability of caverns. All of this research provides a solid foundation for the first underground SPR salt cavern construction in China. The SPR salt cavern is constructed by injecting water and removing the brine.16 Similarly, oil is removed by displacing it with brine. Fig. 1 shows a schematic of the underground salt cavern group for SPR planned to be built in Jintan. The depth of the SPR salt caverns is about 1000 m. As is shown, during the cavern leaching and oil drainage, the mudstone interlayers are immersed by high-pressure brine. During the storage stage, the mudstone interlayers are immersed by high pressure crude oil. Although crude oil can long-term contact with rock salt steadily, the mechanical and permeability properties of mudstone interlayers in contact with oil or brine are not clear. The contact may have a significant impact on the mudstone interlayers. What is more, these properties are the most crucial factors to ensure the SPR salt cavern long-term safe and stable operation. Thus, it is necessary and urgent to study the mechanical and permeability properties of mudstone interlayers in bedded rock salt under brine or crude oil immersion. However, very little work has been done about the above problems. This paper is a pilot study for the feasibility analysis of underground SPR caverns in bedded rock salt of Jintan, China. The structure of this paper is as follows. In Section 2, a brief description of the experimental scheme is introduced. In Section 3, the degradation mechanism of the mudstone interlayer under brine or crude oil immersion is analyzed through the potential of hydrogen (PH) test and the total acid number (TAN) test. In Section 4, the rock damage evolution rule of the typical mudstone interlayer under brine or crude oil immersion is obtained through uniaxial and triaxial compression tests. In Section 5, the permeability variation of the typical mudstone interlayers during rock damage is analyzed through the permeability tests and scanning electron microscope (SEM) tests. In Section 6, some conclusions and proposals are put forward. The present study provides a theoretical and experimental basis for the feasibility research of underground SPR facilities to be constructed in bedded rock salt, as well as a basic reference for similar engineering practices.

2.2.2. pH test of brine-mudstone interaction The density of the brine sample used in the test is 1.20 g/mL. The main ingredient of the brine sample is NaCl, containing a small amount of SO42-, Ca2+ and a very small amount of Mg2 +. The ions concentration of the Na+, Cl-, SO42-, Ca2+ and Mg2+ are 118.20 g/L, 187.30 g/L, 4.63 g/L, 0.79 g/L and 12.30 × 10−3 g/L, respectively. A pH detector with a measurement accuracy of 0.01 is used to determine the brine pH value. 2.2.3. Mechanical tests To study the mechanical properties of the mudstone interlayer under crude oil immersion and brine immersion conditions, our research group independently designed a liquid phase immersion test device for the tests. It can simulate the underground environment in the SPR caverns based on the in-situ stress and temperature data measured in the field. The pressure of the device ranges from 0 to 50 MPa, and its accuracy is 0.1 MPa. The control temperature of the device ranges from 0 to 110 °C, and its accuracy is 0.1 °C. The temperature in this test was set at 50 °C according to the measured brine temperature at 1000 m depth. The pressure was set as the hydraulic pressure of the oil or brine applied to the mudstone interlayer at 1000 m depth. The hydraulic pressure is calculated as P = Pw + γliquidh, where Pw is the wellhead pressure; γliquid is the unit weight of brine or crude oil; h is the calculated depth. According to the previous research,19 the hydraulic pressure of the brine on the mudstone is about 12 MPa (γwater =12 kPa/m, Pw = 0 MPa). The hydraulic pressure of the crude oil on the mudstone is about 14 MPa (γoil =8.20 kPa/m, Pw = 6 MPa). The immersion time is set as 10, 20 and 30 days. The hydraulic pressure schematic graph of the SPR salt cavern are shown in Fig. 2. To accurately determine the stress-strain relation and mechanical parameters of the mudstone samples, uniaxial and triaxial compression tests were carried out by MTS-815.03 rock mechanics test system. The loading rate is set as 0.001 m/s, until the mudstone samples lose their carrying capacity or reach the residual strength.

2. Experimental scheme 2.1. Sample preparation and selection The mudstone cores were taken from the target formation of Jintan salt mine with depths of approximately from 900 to 1000 m. They are mainly green gray and dark gray, the most typical type of mudstone in bedded rock salt. Wire-cutting by KDXQ-II sample manufacturer

2.2.4. Permeability test The permeability test was carried out by the steady-state method. 2

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Fig. 2. The hydraulic pressure schematic graph of the SPR salt cavern. Fig. 3. The variation curve of TAN respect to reaction time.

Considering that the mudstone easily can react with water, N2 is used as the permeating medium during the permeability test. When using water as the permeating medium, the permeability is generally calculated by the following function:

K∞ =

Q1 μ1 L A (Pin − Pout )

Anhydrite and hydrocarbon are thermodynamically unstable with respect to H2S and oxidised carbon compounds, such as CO2 and carbonate minerals [e.g. Eq. (3)], under almost all diagenetic conditions. Geochemical modelling suggests that direct chemical reaction between the two species should theoretically proceed at temperatures in excess of approximately 25 °C.23,24

(1) 2

where K∞ is the absolute permeability, m ; A is the cross sectional area of the sample, m2; Pin and Pout are upstream and downstream absolute pressures respectively, Pa; μ1 is the dynamic viscosity of water, mPa s; Q1 is the liquid flow, mL/s; L is the length of mudstone sample, m. When using N2 as the permeating medium, a modified differential form of the above equation should be applied considering the compressibility of the gas medium, expressed as

Kg =

Hydrocarbon + CaSO4 → CaCO3 + H2S + S + CO2 + H2O

According to the results of XRD, the mudstone interlayer has 17.51% anhydrite (CaSO4). If the TSR largely occurs in the mudstone interlayers during the storage operation time and severely erodes the surrounding rock, it may adversely affect the stability and tightness of the SPR salt cavern. Thus, it is necessary to study the TSR procedure before crude oil storage. The TSR will produce sulfur compounds and organic acids, which will increase the TAN value. So the reaction of TSR between the crude oil and mudstone can be calibrated through the variation of the TAN value. The test temperatures are set as 50, 75 and 100 °C respectively. The reaction time is set at 10 days. Fig. 3 shows the variation of TAN with respect to reaction time. The initial TAN of crude oil is 0.13 mgKOH/g. The TAN of the crude oil is measured at a given interval. As is shown in Fig. 3, when the temperature is 50 °C and 75 °C, the TAN values range from 0.12 to 0.13 mgKOH/g. The variation of TAN is small, which may be due only to the error of the titration process. It indicates that TSR will not occur when the temperature is below 75 °C. When the temperature is 100 °C, the TAN is also in range of 0.12–0.13 mgKOH/g in the first 4 days. However, when the reaction lasts between 5 and 8 days, the TAN starts to rise to 0.13–0.14 mgKOH/g. At 9 and 10 days, the TAN increases to 0.15 mgKOH/g. This indicates that the TSR between the crude oil and mudstone mineral grains starts to occur. The results indicate there is a threshold of the reaction temperature of the TSR. Due to the temperature in Jintan salt cavern being about 50 °C, it can be concluded that the TSR will not take place. The anhydrite in mudstone will react with the naphthenic acid in crude oil under the crude oil immersion condition. The generated calcium naphthenate will dissolve in the crude oil. The reaction equation is expressed as

2P0 Qg μg L 2 A (Pin2 − Pout )

(3)

(2) 2

where K g is the gas permeability, m ; μg is the dynamic viscosity of nitrogen, μPa·s; P0 is the atmospheric pressure value, Pa; Qg is gas flow, mL/s. It should be noted that when using gas as the permeating medium, the Klinkenberg effect should be considered.20,21 3. Degradation mechanisms of mudstone Under the immersion by crude oil or brine, complex physical and chemical reactions may occur during the liquid permeation into the micro-cracks and intergranular pores of the mudstone. It is also the essential reason that causes the changes of the mechanical and permeability properties of the mudstone interlayers. Therefore, it is necessary to study the reaction mechanism of crude oil and brine on the mudstone interlayers before the mechanical and permeability tests. Actually, the oil-mudstone interaction and the brine-mudstone interaction are both long and slow processes in the real underground environment. In order to obtain the test results more rapidly, the mudstone samples were ground into powder and mixed well with the crude oil and brine respectively. The main mineral components of the mudstone interlayers in bedded rock salt have been determined by XRay diffraction (XRD) analysis. According to the test result, the mudstone is composed by halite (4.62%), quartz (10.32%), analcime-c (13.31%), anhydrite (17.51%), ankerite (28.28%), kaolinite (3.23%), montmorillonite (10.86%) and illite (11.87%), respectively.

CnH2n-1COO- + Ca2+ → (CnH2n-1COO)2Ca

(4)

This reaction will not change the TAN of crude oil. The degree of reaction is related to the naphthenic acid content of the crude oil. Therefore, chemical reaction may take place between the naphthenic acid and mudstone if the crude oil in the SPR salt cavern has a higher content of naphthenic acid. This may cause secondary pore development and degrade the mechanical properties of the mudstone. In the petroleum industry, if the TAN number of a crude oil is higher than 0.5 mg KOH/g, the crude oil is considered as acidic crude oil.18 The acid crude oil has a strong corrosive effect on pipelines during the crude oil storage.25,26 Thus, the crude oil stored in the underground

3.1. Oil-mudstone interaction mechanism Relevant research in the petrochemical area shows that thermochemical sulfate reduction (TSR) is a complex organic-inorganic interaction in reservoirs. The acid composition produced by TSR has been demonstrated to play an important role in the reformation of reserves on the basis of detailed geological observations.22 3

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rate of the aluminum silicate minerals to temperature is relatively low. With increasing reaction time, when the pH starts to decrease, the reaction rate and the decrease of the pH value at 50 °C are obviously greater than those at 25 °C. This indicates that the temperature has a catalytic effect on the hydrolysis of quartz. The water swelling and disintegration of the clay minerals are also significant during the brine-mudstone interaction, which are also important factors causing degradation of the mechanical properties of mudstone. Previous studies29,30 have demonstrated that the interaction between illite and water can increase their original volumes by 50–60%. The reaction equation is expressed as

SPR salt cavern is usually low acid crude oil or normal crude oil (TAN＜ 0.5 mgKOH/g). However, the limited amount of naphthenic acid can also have an effect on the mechanical properties and permeability of mudstone interlayers. It is an important prerequisite for the feasibility analysis of underground SPR salt cavern to study the mechanical and permeability properties of mudstone under crude oil immersion. 3.2. Brine-mudstone interaction mechanism The interaction between brine and mudstone interlayers is extremely complicated.27 Dissolution, surface chemical reaction and diffusion migration will occur on the mineral particles and the cement in the mudstone. The composition, size and shape of mudstone mineral particles will also change with increasing immersion time. Even the chemical reaction types and reaction degrees of the brine-mudstone interaction at different times are different. The changes of the internal mineral particle composition and its microstructure are the essential reasons that lead to the degradation of the mudstone interlayers. In this test, the pH value of the brine-mudstone interaction is monitored with the increase of reaction time. The samples are divided into two groups and the temperatures are set as 25 °C and 50 °C respectively. The pH value is measured every 10 min in the initial reaction stage due to the brine-mudstone interaction being much stronger at first. After the reaction becomes relatively stable, the pH value is measured every 1000 min. The initial pH value of the brine sample is 8.04, which is weakly alkaline. The test result is shown in Fig. 4. At the beginning of the brine-mudstone interaction (see in the detail insert view), the pH value of both the brine samples fluctuate significantly and start to increase. This is due to the ion exchange, hydrolysis reaction of the aluminum silicate minerals such as analcime, montmorillonite, kaolinite and illite. The brine samples will be alkalinized because the ions such as K+, Na+, Mg2+ and Ca2+ which are displaced through the ion exchange and the hydrolysis reaction combine with the OH- in the solution.26,28 Its general chemical equation is

K0.9Al2·9Si3·1O10(OH)2 + nH2O → K0.9Al2·9Si3·1O10(OH)2·nH2O

According to the result of XRD analysis, the total content of kaolinite, montmorillonite and illite in the mudstone is nearly about 26%. Due to the small particles of clay mineral and its extremely hydrophilic property, the water molecules form polarized water layers between adjacent clay minerals particles, causing the expansion of the mudstone. The expansion and disintegration will exist in the whole process of the brine-mudstone interaction. In addition, a large number of microcracks will be generated owing to uneven stress within the mudstone caused by the water swelling. These cracks destroy the structural system inside the sample, which leads to serious degradation of the physical and mechanical properties of the mudstone. 4. Mechanical properties of mudstone 4.1. Uniaxial compression test 4.1.1. Analysis of the stress - strain curve and test result To study the variation in the mechanical response under different conditions of the uniaxial compression tests, the mudstone interlayer samples are divided into three groups. Three samples are tested in the natural condition, three samples after crude oil immersion and three samples after brine immersion, as shown in Fig. 5. To ensure the comparability of the test results, the mudstone samples in natural conditions are also placed into the high-pressure environment. The samples are wrapped by heat-shrinkable tubing and the ends of the samples are sealed with silicone rubber to prevent liquid permeation, as shown in Fig. 5(a). The uniaxial compression test results are shown in Table 1. The uniaxial compressive strengths of all the samples in natural conditions are around 45 MPa, showing a good uniformity. When the samples are immersed in crude oil or brine, the uniaxial compressive strengths of the samples show a certain degree of degradation. This indicates the immersion in brine has more significant effects on decreasing the mudstone strength than immersion in crude oil. The stress-strain curves of uniaxial compressive tests under different immersion conditions are shown in Fig. 6. All the samples in natural condition have similar brittle deformation characteristics and their stress strain curves have experienced the following four stages, i.e.

MSiAlOn + H+(OH)- → M+(OH)- + [Si(OH)0–4]n + [Al(OH)6]n3(5) +

+

2+

2+

and other cationic ions. where M stands for K , Na , Mg , Ca With the increase of the immersion time, the pH of the solution starts to decrease and tends to stabilize. That is due to the quartz hydrolysis in alkaline condition depleting the OH- in the brine. With the decrease of the OH-, the hydrolysis of quartz decreases as well and leads to a constant PH value at last. The chemical equation is expressed as SiO2 + 2OH- → SiO32- + H2O

(7)

(6)

The sensitivity to temperature of the brine-mudstone interaction is different in different reaction stages. During the initial reaction stage, the rising rates of the pH value at different temperatures show little difference. This indicates that the sensitivity of the hydrolysis reaction

(1) Crack closure stage: Pre-existing micro-cracks and pores close and decrease when the load is applied.31 The stress-strain curve has a concave shape and its slope keeps increasing with increasing loads. (2) Linear elastic stage: the micro-cracks and pores have closed to the limit and the axial load is not large enough to generate new cracks. The stress-strain curve shows a good linear relationship, which means the elastic modulus is a constant value. (3) Nonlinear deformation stage: the micro-cracks inside the mudstone samples begin to develop, connect and expand with increasing axial load. Unstable cracks grow and irrecoverable deformations are produced at this stage.32 This stage continues until the uniaxial compressive strength is reached. (4) Failure stage: when the axial load reaches the stress limit, due to the

Fig. 4. PH test results of the brine samples at different temperatures. 4

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Fig. 5. The mudstone samples under different conditions.

crude oil or brine. The immersion environment causes a degree of initial damage to the mudstone. When it comes to the linear elastic stage, the stress-drop phenomenon starts to appear gradually. Under the crude oil immersion condition, the stress-drop phenomenon is not evident, the first stress-drop appeared after the sample is immersed for 30 days. While under the brine immersion, the stress-drop phenomenon obviously increases over immersion time. The stress-drop phenomenon is almost accompanied by the entire deformation process when the immersion time reaches 30 days, which can be seen in the partial magnification plot in Fig. 6. Each stress-drop means a new stress redistribution of the sample. It shows that the pores inside the samples may develop rapidly under the brine immersion, and the degree of the degradation is far more serious than that under the crude oil immersion. With increasing immersion time in crude oil or brine, the region of the nonlinear deformation stage of the stress-strain curve becomes larger and the region of the linear elastic stage becomes smaller. The peak strengths also decrease significantly. The deformation characteristics are not typical brittle any more. It can be inferred that the fluid filling the pore system of the sample may cause the whole system to obey a viscoelastic law and the damage process of the samples immersed in crude oil or brine may also cause the whole system to obey an elastoplastic law. To identify how much impact the fluid filling the pore system had on the mechanical response of the samples, uniaxial compression tests have been carried out under different immersion conditions with different degrees of fluid saturation. The detailed experimental conditions, processes and results are given in the Supplemental Information file. The test results indicate that the fluid inside the samples has little influence on the mechanical response during the uniaxial compression tests. Thus, it is supposed that the damage of the samples immersed in crude oil or in brine causes the rock to obey an elastoplasticity law. Fig. 7 presents the elastic modulus of the mudstone with respect to immersion time under different immersion conditions. The elastic modulus in natural conditions shows little change after high-pressure sealing for 10, 20 and 30 days. Its mean value is about 6.04 GPa with a good uniformity. By contrast, the elastic modulus after immersion in crude oil or brine decreases markedly. The modulus decrease of the samples immersed in brine is larger than that of samples immersed in crude oil. The modulus decrease rate of the samples immersed in crude oil gradually decreases while that of the samples immersed in brine increases with time. Fig. 8 presents the Poisson's ratios of the mudstone with respect to time under different immersion conditions. The Poisson's ratio of the

Table 1 Results of the uniaxial compression tests. Test conditions

Peak stress (MPa)

Elastic modulus (GPa)

Poisson's ratio

Natural condition (10 d) Natural condition (20 d) Natural condition (30 d) Crude oil immersion (10 d) Crude oil immersion (20 d) Crude oil immersion (30 d) Brine immersion (10 d) Brine immersion (20 d) Brine immersion (30 d)

44.20 46.40 46.89 42.51

6.11 5.97 6.03 5.35

0.26 0.24 0.25 0.25

40.26

4.62

0.26

37.93

4.47

0.23

32.02 28.17 20.16

4.86 3.74 1.91

0.28 0.31 0.35

Fig. 6. The stress-strain curves of uniaxial compression test under different conditions.

damage accumulation and the micro-cracks propagation, macro fractures start to appear and make the mudstone samples fail instantly. Stress decreases rapidly as well. Compared to the stress-strain curve of the mudstone samples in natural condition above, the stress-strain curves after the samples have been immersed in crude oil or in brine fluctuate dramatically. During the crack closure stage, the concavity of the stress-strain curves increases notably, indicating that the pores and natural micro-cracks of the mudstone samples have developed significantly after immersion in 5

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Eo (t ) = 4.47+

1.57 1+100.16667(t −11.30137)

(9)

Brine immersion:

Eb (t ) = 9.5155 − 3.51902e 0.02558t

(10)

Combining with Eq. (8) and Eq. (9), as well as Eq. (8) and Eq. (10), we obtain the damage evolution equation of the mudstone under crude oil immersion [Do (t )] and the damage evolution equation of the mudstone under brine immersion [Db (t )] respectively, expressed as

Do (t ) = 0.25993−

0.25993 1+100.16667(t −11.30137)

Db (t ) = 0.58262e 0.02558t −0.57541 Fig. 7. Elastic modulus of the mudstone with respect to reaction time under different conditions.

(12)

The damage rate evolution equation of the mudstone under crude oil immersion [Do (̇ t )] and the damage rate evolution equation of the mudstone under brine immersion [Db (̇ t )] can be obtained by taking the derivative with respect to Eq. (11) and Eq. (12), respectively.

Do (̇ t ) =

0.099754×100.16667(t −11.30137) [1+100.16667(t −11.30137) ]2

Db (̇ t ) = 0.014903e 0.02558t

(13) (14)

According to the above equations, we can obtain the evolution laws of damage and damage rate variation with time of the mudstone under crude oil immersion and brine immersion, which are shown in Fig. 9 and Fig. 10 respectively. As is shown in Fig. 9, the damage rate of the mudstone under crude oil immersion can be divided into two stages: accelerated damage stage and decelerated damage stage. During the deceleration stage, the damage rate of the mudstone decreases gradually and finally turns to zero, which also leads the rock damage to a constant value (0.25). However, as shown in Fig. 10, the damage and damage rate of the mudstone immersed in brine increase exponentially. Comparing the above two figures, the brine's caused damage and damage rate are much larger than those caused by crude oil. That is because of the different degradation mechanisms of oil and brine on the mudstone. The change of the microstructure inside the mudstone is the essential reason that influences its macro-mechanical property degradation. During the oil-mudstone interaction, the naphthenic acid in the crude oil reacts with the sulfate on the surface of the mudstone sample. As the crude oil gradually permeates into the inside, the contact area between the crude oil and the mudstone becomes larger, causing the damage rate of the mudstone to increase rapidly. As the sample gradually becomes saturated, the contact area between crude oil and mudstone remains constant. The oil-mudstone interaction rate starts to decrease due to the decrease of naphthenic acid caused by the reaction. It indicates that the oil-mudstone interaction would be terminated once

Fig. 8. Poisson's ratio of the mudstone with respect to reaction time under different conditions.

mudstone samples immersed in crude oil changes slightly with time, and is basically equal to that of the samples in natural conditions. It indicates the damage caused by crude oil is isotropic. However, the Poisson's ratio of the mudstone samples immersed in brine clearly increases with time. This may have been caused by the different swelling rates of clay minerals in the mudstone.33 Uneven stress will form inside the samples and result in more micro-cracks after the water swelling and disintegration of the clay minerals. It enlarges the lateral deformation of the mudstone and leads to the increase of the Poisson's ratio. 4.1.2. Damage evolution analysis The degradation of the mechanical properties of rock during the water-rock interaction or the oil-rock interaction is called rock damage.34 According to the previous analysis, it can be concluded that the immersion time has a significant effect on the damage of mudstone in different immersion conditions. We define the damage of mudstone as D (t ) , expressed as

D (t ) = 1−Et / Ei

(11)

(8)

where t is the immersion time, days. Et is the elastic modulus of a sample after immersion for t days, GPa. Ei is the initial elastic modulus, GPa. D = 0 means no rock damage, D = 1 means complete rock damage. The elastic modulus Ei is set as 6.04 GPa, the mean elastic modulus of the mudstone samples in natural conditions. Fitting is conducted on the elastic modulus results. The elastic modulus variation curve of the samples under crude oil immersion is defined as Eo (t ) and that of the samples under brine immersion is defined as Eb (t ) . The relational equations are expressed as Crude oil immersion:

Fig. 9. Damage and damage rate evolution laws of the mudstone under crude oil immersion. 6

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Table 2 Triaxial compression test results under different conditions. Condition

Confining pressure (MPa)

Peak stress (MPa)

cohesion (MPa)

internal friction angle (°)

Natural condition

0 5 10 20 0 5 10 20 0 5 10 20

46.89 68.87 91.71 130.54 37.93 54.93 75.42 114.12 20.16 32.02 45.26 68.89

10.53

42.53

9.44

35.94

6.45

24.83

Crude oil immersion

Brine immersion

Fig. 10. Damage and damage rate evolution laws of the mudstone under brine immersion.

the samples shows different degrees of degradation under crude oil or brine immersion. In the crude oil immersion, when the confining pressure is 0, 5, 10 and 20 MPa, the triaxial compressive strengths of the mudstone samples decrease 19.1%, 20.2%, 17.8% and 2.6% respectively, compared to those under natural conditions. The triaxial compressive strengths of the mudstone samples under brine immersion decrease 57.0%, 53.5%, 50.6% and 47.2% respectively, much larger than those under crude oil immersion, confirming that brine has a much greater impact than crude oil on the mechanical properties of the mudstone. The triaxial compression test results are listed in Table 2. Both crude oil and brine cause degradation of the mechanical properties of mudstone, but the degradation of brine immersion on the mudstone is much more significant than that of crude oil immersion, which is consistent with the uniaxial compression test results. It can be also concluded that although the confining pressure increases the strength and limits the crack propagation of the mudstone samples, the degradation trends of the mudstone under crude oil and brine immersion are not changed.

the naphthenic acid is completely consumed. The brine-mudstone interaction is extremely complicated and rapid. As the brine permeates the mudstone sample, the dissolution and diffusion migration effects occur on the mineral particles and the cement in the mudstone. Meanwhile, new micro-cracks in the mudstone are generated due to the expansion of the clay minerals, which further increases the contact area between the brine and mudstone mineral particles. All of these factors make the brine's caused damage and damage rate increase exponentially. It can be assumed that the damage of the mudstone will accumulate further until the strength is completely lost if the immersion time is long enough. 4.2. Triaxial compression tests The triaxial compression tests of the mudstone samples under different immersion conditions are divided into three groups. There are four samples for natural condition (high-pressure sealing), four samples for crude oil immersion and four samples for brine immersion. Based on the in-situ stress measurement results of Jintan salt mine, the in-situ stress field approximately approaches a hydrostatic state when the depth exceeds 432 m due to the excellent rheological property of rock salt.19,35,36 The in-situ stress of the mudstone at 1000 m depth in Jintan is about 23 MPa, calculated by the stress gradient of the overlying rock mass36. Thus, the confining pressures are set as 0, 5, 10 and 20 MPa, respectively. The immersion times are set as 30 days. The stress-strain curves for triaxial compression tests after different immersion conditions are shown in Fig. 11. The triaxial compressive strengths of the mudstone samples increase significantly with the confining pressure. This is due to the confining pressure limiting the crack propagation of rock. By comparing the stress-strain curves of triaxial compression results, we see that the degradation effect is still significant. The triaxial compressive strength of

5. Permeability of mudstone 5.1. Permeability analysis during rock damage The permeability tests of the mudstone samples are divided into four groups. There is one sample for initial permeability testing, three samples for natural condition (high pressure sealing), three samples for crude oil immersion and three samples for brine immersion. The immersion time is 10, 20 and 30 days. Before the permeability test, all the samples are dried for 24 h. Due to the permeability of mudstone being low, the Klinkenberg effect should be considered during the permeability test analysis. Klinkenberg showed experimentally that in low-permeability porous

Fig. 11. The stress-strain curves of triaxial compression test under different conditions. 7

International Journal of Rock Mechanics and Mining Sciences 112 (2018) 1–10

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media the intrinsic permeability to gases is significantly higher than the permeability to liquids.37 The Klinkenberg effect on gas permeability can be written as20,21,38:

b b ⎞ = K∞ (1+f ), K g = K∞ ⎛1+ ⎞≈K∞ ⎛1+ ̅ P P + Pout ⎠ ⎝ ⎠ in ⎝ ⎜

⎟

f=

b Pin + Pout

(15)

where K g is the gas permeability, m2; K∞ is the absolute permeability, m2; P̅ is the mean gas pressure, Pa; b is the Klinkenberg coefficient, which depends on the pore structure of the porous medium and the mean free path of the given gas molecules, and generally decreases with increasing permeability.39 The Klinkenberg coefficient is given by the equation:40

b=

16cμg w̅

2RT πM

(16)

Fig. 12. Relationship between rock damage evolution and permeability variation with respect to reaction time.

where c is a constant (typically taken as 0.9); μg is the dynamic viscosity of the gas (the dynamic viscosity of N2 is 17.81 μPa s), μPa s; M is the molecular weight of the gas (the molecular weight of N2 is 28); w̅ is the mean pore diameter of the sample, nm; R is the universal gas constant (8.3143 J mol−1 K−1) and T is the absolute temperature, K. The mean pore diameter (w̅ ) of the mudstone samples under different immersion conditions was determined by static nitrogen adsorption experiment. Table 3 presents the specific test parameters of each sample during the permeability test. Combined with the damage evolution characteristics of mudstone under different immersion conditions as previously analyzed, the relationship between the rock damage evolution and the permeability variation with time can be obtained, as shown in Fig. 12. As shown in Fig. 12, the permeability and the mean pore diameter (w̅ ) of mudstone samples in natural conditions decrease slightly with time. This is due to the high confining pressure causing compaction of micro-cracks of the mudstone samples. The compaction does not reduce the particle spacing of the mudstone sample. Due to the deformation of micro-cracks not having recovered to the initial level, the permeability of the samples is lower than the initial value. The damage of the mudstone samples increases slowly with time as well as the mean pore diameter (w̅ ) during the crude oil immersion. However, the permeability of the samples decreases with time. The reason may be that the asphaltene and resin contents of crude oil are attached to the surface of the mudstone mineral particles and block their pore spaces. With the increase of immersion time, more crude oil permeates the sample and more asphaltene and resin are attached to the pore throats around the mineral particles. This impedes the permeating medium to pass through. Thus, the permeability of the mudstone gradually decreases even though the rock damage is rising. So it can be concluded that compared to the blocking action caused by the asphaltene and resin contents of crude oil, the rock damage caused by the crude oil immersion has a smaller influence on the permeability. This property is beneficial for the SPR salt caverns long-term safe operation and to keep good tightness. During the brine immersion, the damage of the mudstone samples

increases exponentially over time. The mean pore diameter (w̅ ) of the mudstone after contact with brine for 10, 20 and 30 days increase significantly. The permeability of the mudstone samples also increases with time. This indicates that the brine severely erodes the pore structure inside the sample and causes the pore diameter to increase significantly, which leads to the increase of permeability. This is beneficial to accelerate the collapse of the mudstone interlayer during the cavern leaching. However, when the salt cavern is already completed, the high pressure brine will penetrate into the mudstone interlayer and cause the mechanical and permeability properties of the mudstone to degrade, which may induce hidden trouble for cavern stability and tightness. Therefore, we propose that during the construction of the SPR facilities in bedded rock salt, crude oil should be injected into the underground cavern as soon as possible so as to minimize the degradation of the mudstone interlayers by brine. 5.2. Microscopic analysis of permeability variation According to the above permeability test results, it can be concluded that the permeability of the mudstone under brine immersion will significantly increase with time, but the permeability of the mudstone under crude oil immersion will gradually reduce. This conclusion can also be explained from the microscopic perspective. The microstructure of the mudstone samples under natural conditions, crude oil immersion conditions and brine immersion conditions have been investigated by scanning electron microscope (SEM). The SEM results are shown in Fig. 13. As shown in Fig. 13(a), under natural conditions, the clay minerals and crystal grains are embedded closely in each other and just a few micro-cracks can be found on the scanning plane of the mudstone. It shows a good tightness of the mudstone. When the mudstone is immersed by crude oil, shown in Fig. 13(b), the oil-mudstone interaction does not obviously change the pore structure. What is more, the scanning plane of the mudstone is obviously covered by some white thick

Table 3 Specific parameters during the permeability test. Condition

L (cm)

A(cm2)

Pin(Psi)

Pout(Psi)

Qg (mL/s)

w̅ (nm)

f

K∞(m2)

Initial Natural(10 d) Natural(20 d) Natural(30 d) Brine (10 d) Brine (20 d) Brine (30 d) Crude oil (10 d) Crude oil (20 d) Crude oil (30 d)

4.71 4.68 5.06 5.02 4.94 4.81 5.06 4.97 4.99 5.02

4.88 4.87 4.90 4.68 4.79 4.60 4.91 4.75 4.64 4.68

50.546 52.031 64.701 60.121 20.385 18.568 18.364 62.601 63.721 62.791

14.761 14.761 14.761 14.761 14.761 14.761 14.761 14.761 14.761 14.761

0.768 0.702 0.808 0.653 0.835 0.878 0.943 0.500 0.462 0.267

12.51 12.37 13.27 12.68 18.37 24.69 28.41 13.54 13.96 14.57

10.36 10.24 8.02 8.91 13.11 10.28 8.99 8.08 7.72 7.49

2.11E− 16 1.82E− 16 1.76E− 16 1.57E− 16 2.33E− 15 4.85E− 15 6.16E− 15 1.18E− 16 1.12E− 16 6.85E− 17

8

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Fig. 13. Scanning Electron Microscope (SEM) analysis of the mudstone under different conditions.

Research Team (Grant No. 51621006), Youth Innovation Promotion Association CAS (Grant No. 2016296), and Natural Science Foundation for Innovation Group of Hubei Province, China (Grant No. 2016CFA014).

substance. That is due to the asphaltene and resin of crude oil attaching to the surface of the mineral particles and blocking their pore spaces. It is also the reason that the permeability gradually decreases under the crude oil immersion. Fig. 13(c) shows the SEM result of the mudstone after immersion in brine, a number of micro-cracks appear on the scanning plane of the mudstone, due to the brine-mudstone interaction, as well as the water swelling and disintegration effects of the clay minerals. Many crystal grains and clay minerals are dissolved by the brine and migrate, leading the cementation between them to become loose. The micro pore structure of the mudstone has been changed significantly. This also explains why the permeability of mudstone will evidently increase under brine immersion from a microscopic perspective.

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6. Summary and conclusions The thermochemical sulfate reduction (TSR) will not occur at the SPR operating temperature. The reaction between the anhydrite in the mudstone interlayers and the naphthenic acid in the crude oil is the major mechanism by which crude oil damages the mudstone interlayers. The interaction between brine and mudstone interlayers is extremely complicated. All of the factors such as dissolutioning, surface chemical reaction, diffusion migration, as well as the water swelling and disintegration effects of clay minerals cause the degradation of the mechanical and permeability properties of mudstone interlayers. Both crude oil and brine can damage the mudstone interlayers, but the degradation due to brine is far greater than that due to crude oil, because of the different degradation mechanisms. Due to the asphaltene and resin in crude oil becoming attached to the surface of the mineral particles and blocking their pore spaces, the permeability of the mudstone interlayers immersed in crude oil will evidently decrease even though the rock damage is still increasing. This property is beneficial for the SPR salt caverns long-term safe operation, and to keep good tightness. Considering that the brine has a strong damaging action on the mudstone interlayers in bedded rock salt, it is proposed that crude oil should be injected into the SPR salt cavern as soon as possible after the underground facilities are completely constructed in case brine contact brings hidden risk for the cavern stability and tightness. Acknowledgments The authors wish to acknowledge the financial supports of National Natural Science Foundation of China (Grant Nos. 51774266, 41502296, 51404241), National Natural Science Foundation of China Innovative 9

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