Experimental investigation on the long-term interactions of anhydrite rock, crude oil, and water in a mine-out space for crude-oil storage

Journal Pre-proof Experimental investigation on the long-term interactions of anhydrite rock, crude oil, and water in a mine-out space for crudeoil storage

Hanxun Wang, Bin Zhang, Lei Wang, Xiong Yu, Lei Shi, Dong Fu PII:

S0013-7952(18)31378-4

DOI:

https://doi.org/10.1016/j.enggeo.2019.105414

Reference:

ENGEO 105414

To appear in:

Engineering Geology

Received date:

13 August 2018

Revised date:

6 November 2019

Accepted date:

11 November 2019

Please cite this article as: H. Wang, B. Zhang, L. Wang, et al., Experimental investigation on the long-term interactions of anhydrite rock, crude oil, and water in a mine-out space for crude-oil storage, Engineering Geology (2019), https://doi.org/10.1016/ j.enggeo.2019.105414

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© 2019 Published by Elsevier.

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Experimental investigation on the long-term interactions of anhydrite rock, crude oil, and water in a mine-out space for crude-oil storage Hanxun Wang

a,b

, Bin Zhang

a,b*

c

d

, Lei Wang , Xiong (Bill) Yu , Lei Shi

a,b

, Dong Fu

a,b

a.School of Engineering & Technology, China University of Geosciences (Beijing) 100083, China

b.Key Laboratory of Deep Geodrilling Technology, Ministry of Natural Resources, Beijing 100083, China

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c.School of Engineering and Applied Sciences, University of the District of Columbia, Washington DC 20008, United States

d.Department of Civil Engineering, Case Western Reserve University, Cleveland, OH 44106, United States

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Corresponding author E-mail: [email protected]

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Abstract: To evaluate the effectiveness of the usage of anhydrite mine-out spaces for crude-oil storage, this study

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conducted a long-term experimental investigation on the interaction of anhydrite rock with crude oil and water with varying pressure using pressurized reactors designed in this work. The physical and mechanical properties of

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anhydrite-rock samples for different test periods were studied to evaluate the effect of crude oil and water on the anhydrite-rock properties and reveal the action mechanism. The crude-oil property was tested after the experiments

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were completed to evaluate the effect of anhydrite on the crude oil. The results demonstrated that under long-term contact with varying pressure, the crude oil did not affect the physical and mechanical properties of anhydrite, and the anhydrite did not affect the crude-oil properties. However, the dissolution and recrystallization effects of water on the anhydrite induced reduction in the strength and stiffness of the anhydrite rock to a certain extent. Thus, water can potentially reduce the stability of an anhydrite mine-out space, which should be focused on when an anhydrite mine-out space is used for underground crude-oil storage. Key words: Anhydrite; Mine-out space; Crude-oil storage; Interaction of anhydrite with crude oil and water; Properties of anhydrite rock

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1 Introduction The utilization of abandoned underground mine-out spaces, including derelict salt caverns and closed underground coal mines, has a long history in civilization. Since the 1960s, thousands of salt caverns induced by mining activities all over the world have been reused to store liquid hydrocarbons, gaseous hydrocarbons, and associated products (Bays 1963; Thoms and Gehle, 2000). In recent years, an increasing number of such mine-out

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spaces have been reused as storage spaces for power sources, such as oil, gas, and compressed air, owing to their low permeability, self-recovery capability from damage, and high reserve (Kim et al., 2012; Wang et al., 2013; Niu et al.,

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2015). For example, the abandoned coal-mine space located in Jiangsu, China, has been proposed for use as a

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compressed-air energy storage or pumped storage power station (He et al., 2013). Additionally, the Prosper–Haniel

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coal mine that has been used for approximately 150 years and is located in North Rhine-Westphalia, Germany will be reused as a pumped storage power station to store surplus wind and solar power. It will become the world’s first

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abandoned coal mine that is converted into a storage power station (China Storage Network, 2017). The utilization of abandoned mine-out space reduces the adverse influence of mining activities on the local environment to a large

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extent, thus aiding in the sustainable development of the national economy. Many countries around the world have engaged in this type of utilization (Langer, 1993; Bérest and Brouard, 2003; Karin et al., 2014). As a non-metallic type of material, gypsum or anhydrite is exploited and applied in various fields, including medical and engineering applications. Moreover, as a sedimentary-rock type, anhydrite-rock masses are often encountered in mine engineering (Kaufmann et al., 2017), tunnel engineering (Butscher et al., 2015), underground storage engineering (Hemme et al., 2017), slope engineering (Chuvashov, 1965), etc. In recent years, because of the exploitation of gypsum and anhydrite, many mine-out spaces have been used around the world. Re-exploitation of anhydrite mine-out spaces has also been proposed. Hangx et al. (2010) discussed the feasibility of using anhydrite

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mine-out spaces as an underground CO2 storage space. Indeed, a portion of anhydrite rock possesses high engineering strength and stiffness (Liu et al., 2000; Guo et al., 2010). Some anhydrite mine-out spaces possess good stability and airtightness characteristics (Wang et al., 2018), which means that these mine-out spaces can potentially be reused as underground storage spaces. According to these anhydrite properties, some Chinese scholars have proposed a novel idea, i.e., reuse abandoned anhydrite cavern groups as underground storage space for crude oil (Wang et al., 2018).

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Currently, the usual measures for storage of crude oil include underground water-sealed oil-storage depots, which are usually constructed in granitic rock mass (Morfeldt 1983; Hoshino 1993; Lu 1998; Mawire 2013; Wang et al., 2015a;

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Shi et al., 2018; Zhang et al., 2019), earth-surface steel-tank storage for oil (Shi et al., 2014), or salt-cavern storage for

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oil (Thoms et al., 2000). Compared with these methods, the storage of crude oil in anhydrite mine-out spaces offers

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the advantages of lesser land occupation and lower investment costs than the traditional underground storage. Previous research indicated that the anhydrite mine-out space in the Anhui Hengtai anhydrite mine, located in

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Anhui Province, China, has good stability and airtightness characteristics and offers potential for use as an underground oil storage space (Wang et al., 2018). However, the theory that abandoned anhydrite mine-out spaces are

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used as crude-oil storage spaces is still young. In addition, many key scientific problems need to be solved, including the determination of whether the properties of an anhydrite rock or rock mass would vary, that is, whether the anhydrite mine-out space has good stability or the anhydrite rock would affect the properties of crude oil under long-term crude-oil storage. Moreover, in oil storage caverns, the water contained in oil usually separates out and accumulates at the bottom of the cavern, generating a water cushion. Therefore, avoiding contact between the anhydrite rock and water is difficult. Considering these problems, the present study conducts a long-term experimental investigation on the interaction of anhydrite rock with crude oil and water. By studying the variations in the physical and mechanical properties of anhydrite under different soaking periods, the tests reveal the interaction

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trend of anhydrite with crude oil and water under long-term contact, which solves the technical problems and provides theoretical support to this field. The current research provides useful references for other engineering fields that deal with anhydrite-rock masses.

2. Test system and method 2.1 Overview of an anhydrite mine-out space

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The anhydrite samples were obtained from the working face of a mine-out space in Anhui Hengtai Anhydrite Mine located in Anhui Province, China. The anhydrite was excavated for more than 10 years and formed a large-scale

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underground mine-out space that included more than 350 caverns and tunnels whose volume is more than 2.0 × 106

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m3 . Its burial depth is more than 400 m.

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According to the field investigation, the strata in the mining region are divided into four main layers from top to bottom, namely, diluvian soil, gravel rock and silty sandstone, limestone, and anhydrite (Fig. 1). Three main gentle

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folds are developed in the mine area, and some small local folds are developed in the anhydrite ore body. Four main

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faults cut the anhydrite ore body, forming a fault depression zone where the anhydrite mine-out spaces are located.

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Fig. 1 Stratigraphic section The underground water mainly contains pore water in the diluvian soil, fracture water in the gravel rock and silty

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sandstone, and karst water in the limestone. The underground water level is shallow. The main chemical elements of

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anhydrite rock are Ca, S, and O (Fig. 2), that is, the chemical formula of the mineral anhydrite is CaSO 4 . The

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anhydrite exhibits a close-grained crystal structure (Fig. 3) with low porosity and permeability. Field investigation reveals that no large-scale joints or faults are present in the anhydrite-rock mass, whereas a small number of cracks or fracture can be observed, which are filled with calcium or gypsum. The anhydrite ore body is more than 200 m thick, and the thickness of the anhydrite overlying strata over the mine-out space is more than 50 m. Although the underground water level is shallow, the anhydrite-rock mass does not contain groundwater. In addition, no water-leakage points are present on the cavern surface. Therefore, the anhydrite can be regarded as a water-resistant layer. The hydrogeological conditions of the anhydrite mine area are relatively simple, and groundwater has little influence on the stability of the anhydrite mine-out space. In summary, the anhydrite mine-out space features good engineering geological characteristics, which offers the 5

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potential for reuse as an underground storage space.

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Fig. 2 Main chemical elements in anhydrite

Fig. 3 Crystal and micro-structure diagram of anhydrite rock

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2.2 Sample preparation 2.2.1 Sample type Ten groups of anhydrite cylindrical samples with a diameter of 50 mm and a height of 100 mm (denoted as #I sample) are chosen for the uniaxial and triaxial tests of anhydrite soaked in crude oil and water for different periods. Their basic physical parameters are listed in Table A1 in the Appendix. Ten groups of anhydrite cylindrical samples

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with a diameter of 50 mm and a height of 25 or 30 mm (defined as #II samples) are chosen to obtain the tensile strength of anhydrite soaked in crude oil and water for different periods. Their basic physical parameters are listed in

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Table A2 in the Appendix.

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2.2.2 Basic physical and mechanical properties of anhydrite sample s

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The mechanical parameters of anhydrite rock in its natural state have been tested and provided by Wang et al. (2018), and the test data are listed in Tables 1 and 2. Through curve fitting (Fig. 4), the Hoek-Brown (H-B) failure

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criteria of the anhydrite rock located in Anhui Hengtai Anhydrite Mine were obtained. In summary, in its natural state, the Young’s modulus of the anhydrite rock is 38.77 GPa, Poisson’s ratio is 0.15, uniaxial compressive strength

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(UCS) is 69.03 MPa, and H-B failure-criteria parameter mi is 19.32. Table 1 Physical and mechanical parameters of the anhydrite rock in its natural state Confining

Failure principal

Elastic

Poisson’s

pressure

stress

modulus

ratio

σ3 (M Pa)

σ1 (M Pa)

E (GPa)

1*-1

0

50.2

28.70

0.18

1*-2

4

119.0

-

-

8

115.1

-

-

1*-4

12

140.1

-

-

1*-5

16

175.2

-

-

2*-1

0

105.5

43.70

0.14

4

132.2

-

-

2*-3

8

145.7

-

-

2*-4

10

152.8

-

-

Specimen number

1*-3

Group

1*

2*-2 2*

7

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12

150.9

-

-

2*-6

16

319.5

-

-

3*-1

0

90.9

46.12

0.13

3*-2

4

132.9

-

-

8

160.7

-

-

3*-4

10

174.2

-

-

3*-5

12

169.1

-

-

3*-6

16

159.0

-

-

4*-1

0

61.3

36.57

0.16

4*-2

4

99.3

-

-

8

192.5

-

-

4*-4

10

121.2

-

-

4*-5

12

171.8

-

-

4*-6

16

167.8

-

-

5*-1

0

59.9

-

-

5*-2

8

106.0

-

-

108.8

-

-

120.4

-

-

156.3

-

-

-

38.77

0.15

3*-3 3*

4*-3

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5*

10 12

5*-5

16

Average value

-

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5*-4

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5*-3

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4*

Specimen number

6*-1

6*-2

6*-3

6*-4

Average value

3.39

3.68

5.94

6.93

4.99

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Tensile strength (M Pa)

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Table 2 Tensile strength of the anhydrite rock in its natural state

Fig. 4 Test results of the anhydrite rock located in Anhui Hengtai Anhydrite Mine in its natural state

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2.3 Test equipment and experimental setup In general, the underground crude-oil storage space will be used for decades or even longer than 100 years. Moreover, many cycles for injection and extraction of crude oil can be performed. Completing such long-term tests under laboratory conditions is difficult. With regard to the tests performed in this study, to accelerate the interaction of anhydrite with crude oil and water, including the potential chemical reaction and seepage, approximately 4-5 MPa

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of pressure provided by an argon bottle (nitrogen was avoided because it generates a low-temperature environment) was loaded into the reactors designed in this work (Komilov et al., 2017; Lu et al., 2017). In the selection process of

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the samples, loading and unloading cycles were used to simulate the injection and extraction of crude oil. In the mine

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cavern, the range of temperature varied from 19 to 22 ℃. However, the injection and extraction process of crude oil

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could possibly temporarily change the temperature environment of the cavern. Considering this adverse circumstance, we selected room temperature as the test temperature, which was convenient to meet the field condition. The reactor

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could perform the following roles. 1) More than 5 MPa of pressure can be applied, and the reactor could still maintain a sufficient safety factor. 2) The pressure and temperature in the reactor could be monitored. 3) The device possesses

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sufficient cubage to store the samples and liquid (crude oil and water). 4) The entire structure could easily be disassembled and installed (Fig. 5).

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Fig. 5 Test device In this experiment, the mechanical parameters of the anhydrite rock soaked in crude oil and water for different periods were obtained and compared with the original parameters provided by Wang et al. (2018) to evaluate the variation in the anhydrite properties. Moreover, the physical properties of crude oil in the experimental process were tested. Five groups of I# samples and five groups of II# samples were soaked in the reactors with crude oil, whereas 10

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the remaining five groups of I# samples and five groups of II# samples were soaked in the reactors with water under 4-5 MPa of pressure and at room temperature. The samples were taken out after soaking periods of 69, 136, 271, 451, and 549 d. Through testing of the mass, size, and transverse- and longitudinal-wave velocities before and after soaking, the effects of crude oil and water on the physical properties of the anhydrite rock were evaluated. According to the mechanical parameters obtained from the Brazilian disk-splitting tests and uniaxial and triaxial compressive

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tests, the effects of crude oil and water on the mechanical properties of the anhydrite rock, including the Young’s

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modulus, Poisson’s ratio, H-B failure criterion parameter mi , UCS, and tensile strength, were determined and

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compared with those in the natural state. The test program is listed in Table 3.

Soaking period (d)

Group of I# samples

Group of II# samples

1

1

2

2

3

3

451

4

4

549

5

5

69

6

6

136

7

7

271

8

8

451

9

9

549

10

10

69 136 271

3 Test results

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Soaked in oil

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Soaked in water

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Test status

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Table 3 Test program

Determining the solubleness and chemical stability of anhydrite (formula CaSO4 ) in crude oil and water could reveal the interaction trend of anhydrite rock with crude oil and water. On the basis of the ion concentration in the crude oil and water and the test conditions, the solubleness and chemical stability of anhydrite was determined. Moreover, according to the tests of the mass, density, size, and transverse- and longitudinal-wave velocities of the anhydrite-rock samples for different experimental periods, the effects of crude oil and water on the physical properties of the anhydrite rock were evaluated. The variation in mass and density reflects the solubleness of 11

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anhydrite in crude oil and water, whereas that in the wave velocities implies a variation in the internal structure of the anhydrite rock. According to the variation in the mechanical parameters of the anhydrite rock, including the Young’s modulus, Poisson’s ratio, H-B failure criteria parameter mi , UCS, and tensile strength, the effect of crude oil and water on the mechanical properties of the anhydrite rock were determined. The physical and mechanical effect trends of the crude oil and water on anhydrite were revealed. Furthermore, the stability of the anhydrite mine-out space was

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qualitatively analyzed. Moreover, in the test process, the internal pressure and temperature of the reactors were monitored, as shown in

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Fig. 6, which shows that the pressure was gradually reduced during the test period because of the slight dissolution of

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argon in the crude oil or water and could be held at 4-5 MPa. The temperature, namely, room temperature, varied with

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the change in the weather and was maintained at 10-30 ℃.

Fig. 6 Internal pressure and temperature of the reactors during the test period

3.1 Effect of crude oil on the anhydrite rock 3.1.1 Solubleness and chemical stability of anhydrite in crude oil The ion concentration in the crude oil after the experiment is listed in Table 4, indicating that the solubleness of

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anhydrite in crude oil is 23.73 mg/L, which is approximately 0.25 mmol/L. It is almost insoluble in crude oil (in crude oil, the anhydrite dissolves in emulsified water drops, and the solubility is related to the water content). Moreover, Fig. 7 shows that the anhydrite possesses good chemical stability, and the crude oil does not corrode the anhydrite rock or seep into the anhydrite under long-term contact between the anhydrite and crude oil at 4-5 MPa of pressure. Table 4 Ion concentration in the crude oil after the experiment Na+ , K +

Ca2+

M g2+

Cl-

SO42-

HCO 3-

mmol/L

0.10

0.62

0.12

0.49

0.25

0.60

mg/L

2.58

24.75

3.00

23.73

36.42

Types of ion

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Concentration

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17.53

Fig. 7 Anhydrite samples soaked in crude oil and their fracture 3.1.2 Effect of crude oil on the anhydrite physical properties Figure 8 shows the variation in the physical properties of the anhydrite samples soaked in crude oil for different periods. In the experimental process, the variation values of the mass and density of the anhydrite samples are almost zero, and their maximum variation rates are 1.26% and 2.81%, respectively. The variation rate of the transverse-wave velocity is mainly in the range from -4% to +8%, whereas the longitudinal-wave velocity fluctuates in a variation rate

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range of ±4%. Considering the variation in the physical properties, it could be concluded that crude oil does not affect the anhydrite physical properties, which agrees with the previous analysis that anhydrite remains undissolved in crude

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oil.

Fig. 8 Variations in the physical properties of the anhydrite samples soaked in crude oil for different periods 3.1.3 Effect of crude oil on the anhydrite mechanical properties Figure 9 shows the mechanical parameters and their variation of the anhydrite samples soaked in crude oil for different periods, which shows that the Young’s modulus changes in the range of 35-45 GPa with a small variation ranging from -10% to +15%. The Poisson’s ratio range is 0.11-0.17, i.e., the variation is within the range from -32% 14

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to +14%. H-B failure criterion parameter mi is in the range of 14-22 with a variation rate from -25% to +11%, which shows a small decreasing trend. UCS is approximately 62-92 MPa and has a variation rate from -10% to +33%. It also shows an increasing trend. The tensile strength is 5-9 MPa with a variation rate of less than +54% and an obvious increasing trend. In summary, under long-term contact of the anhydrite with crude oil at 4-5-MPa pressure, the tensile strength of anhydrite exhibits an obvious increasing trend. UCS demonstrates a small increasing trend, whereas mi

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exhibits a small decreasing trend. Considering the variation in the physical properties and mechanical parameters and the self-dispersion of the

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anhydrite samples, crude oil does not affect the anhydrite mechanical properties. The obvious increasing trend of the

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tensile strength is due to the smaller initial value.

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Fig. 9 Variation in the mechanical properties of the anhydrite samples soaked in crude oil for different periods

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3.2 Effect of water on the anhydrite rock 3.2.1 Solubleness and chemical stability of anhydrite in water +

+

-

2-

The ion concentration in water after the experiment is listed in Table 5. The test ions include Na , K , Cl , SO4 , 2-

and HCO3 . However, because of rusting of the device that contained water, the Mg

2+

2+

and Ca

ions in the water

could not be tested. Therefore, in this experiment, the solubility of anhydrite in water was determined according to the 2-

2-

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SO4 concentration. Table 5 indicates that the solubility of SO4 in water is 237.27 mg/L and that of anhydrite is approximately 2.47 mmol/L; thus, anhydrite is slightly soluble in water.

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Table 5 Ion concentration in water after the experiment

mmol/L

5.38

mg/L

2.58

M g2+

Cl-

SO 42-

HCO 3-

-

-

0.25

2.47

0.19

-

-

8.77

237.27

11.65

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Concentration

Ca2+

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Na+ ,K+

Types of ion

Because of the slight solubleness of anhydrite and the hydration reactions, water could cause two types of

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corrosion effects: 1) Dissolution effect, which occurred at the interface of the anhydrite and water and generated dissolution scars and dissolution holes [Fig. 10(a)] accompanied by a large amount of exfoliated powder. This

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phenomenon occurred at the surface of the sample at the initial stage of the experiment and lasted until the end of the experiment. 2) Recrystallization effect, which occurred along the existing rock cracks [Fig. 10(b)]. The anhydrite recrystallized (Fig. 11) and generated areatus needle-like crystals whose chemical formula is 2H2 O·CaSO4 [Eq. (1)]. The crystals displayed columnar, colorless, and transparent characteristics and contained sections of regular triangles and hexagons. The phenomenon began to occur at a slightly later stage (in this experiment, 136 d). In addition, as the experiment continued, the recrystallization effect became increasingly destructive. Moreover, the recrystallization induced expansion, resulting in the samples being cracked or destroyed and even lost strength [Fig. 10(b)]. In this test, when the experiment lasted up to 549 d, two I# and four II# samples were destroyed (Fig. 10) (the density and wave

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velocity of the destroyed samples are not presented). (1)

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CaSO4 + 2·H2 O = 2H2 O·CaSO4

Fig. 10 Two types of corrosion effects of water on the anhydrite

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Fig. 11 X-ray diffraction test of the crystals To explain the recrystallization effect, hydrogeochemical modeling was performed using PHREEQC, which is

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widely used to simulate the aqueous geochemical calculations (Charlton and Parkhurst, 2011; Parkhurst and Wissmeier, 2015). The effect of temperature on the anhydrite solubility was first considered. Figure 12 shows the

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solubility variation of anhydrite and gypsum (formula 2H2 O·CaSO4 ) with the variation in the temperature. It can be concluded that in this test (the temperature cyclically varied from 10 to 30 °C), the solubility of anhydrite first increased and then decreased as the temperature increased, and the maximum anhydrite solubility occurred at 20 °C. Meanwhile, the solubility of gypsum increased as the temperature increased. In the preliminary period of the test, only the anhydrite existed. In the middle and later periods, anhydrite and gypsum coexisted. In summary, variation in temperature affected the solubility of anhydrite and gypsum and induced dissolution and precipitation. Therefore, the anhydrite recrystallized. Moreover, pressure had a negligible effect on the solubility of solids, which was not considered in this test.

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Fig. 12 variation in the solubility of anhydrite and gypsum with temperature 3.2.2 Effect of water on the anhydrite physical properties

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Figure 13 shows the variation in the physical properties of the anhydrite samples soaked in water for different

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periods. In the experimental process, the mass of the samples exhibited a downward trend as the test period increased.

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The maximum mass reduction was 18 g with a maximum variation rate of -5.05%. The sample density decreased, and the maximum reduction was -0.15 g/cm3 with a maximum variation rate of −4.99%. The dissolution of anhydrite generated many dissolution scars and holes, inducing reduction in the mass and density of the anhydrite. The variation in the transverse-wave velocity fluctuated within the range of ±10.00%, whereas the longitudinal-wave velocity fluctuated within the range of ±7.50%. The wave velocity exhibited irregular variations, and the variation rate changed to almost zero. This result indicates that the internal structure of the anhydrite sample did not significantly change. Therefore, the dissolution effect occurred at the surface of the sample.

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Fig. 13 Variation in the physical properties of the anhydrite samples soaked in water for different periods 3.2.3 Effect of water on the anhydrite mechanical properties Figure 14 shows the mechanical parameters and their variation of the anhydrite samples soaked in water for different periods. Figure 14 shows that the Young’s modulus changed in the range of 20-56 GPa with the variation ranging within ±50%. The Young’s modulus demonstrated a large variation rate and a decreasing trend as the experiment continued. The Poisson’s ratio range was 0.12-0.17, which was within ±25%. H–B failure criterion parameter mi was in the range of 11-23, showing a variation rate from -41% to +16% and a decreasing trend as the experiment progressed. UCS was approximately 63-86 MPa with a variation rate from -8% to +24%. The reduction in the Young’s modulus and mi was caused by the dissolution erosion of water in the anhydrite rock, which agreed with 21

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the previous analysis. However, the tensile strength was 5-9 MPa with a variation rate from -4% to +57% and demonstrated an increasing trend to small extent. According to the previous analysis, we consider that this result is induced by the self-dispersion of the anhydrite samples and smaller initial value. In summary, water weakens the

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strength and stiffness of the anhydrite rock.

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Fig. 14 Variation in the mechanical properties of the anhydrite samples soaked in water for different periods

3.3 Effect of anhydrite on the crude-oil properties Table 6 lists the original parameters of crude oil, and Fig. 15 shows the variation in the crude-oil properties after 23

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the experiments, including the density, kinematic viscosity, solidifying point, acid value, sulfur content, and initial boiling point. Fig. 15 shows that the variation rates of the density, acid value, and sulfur content are in the range of ±8.00%. The kinematic viscosity increases, and the variation rate is 22.85%. The solidifying point decreases, and the variation rate is -10.00%. The initial boiling point increases, and the variation rate is 32.33%. Anhydrite is insoluble and has good chemical stability in crude oil. Therefore, the variation rates of the density, acid value, and sulfur

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content are small. Because during the long-term tests the anhydrite-rock samples were removed many times, which caused the crude oil to be exposed to air for a long time, the partial light component became volatile. Thus, the

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kinematic viscosity, solidifying point, and initial boiling point varied to larger extent than the other properties.

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However, the variation did not affect the petroleum refinement. Moreover, in actual engineering, the crude oil would

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be sealed in an underground storage space. It can be concluded that anhydrite does not affect the crude-oil quality.

al

Table 6 Original physical parameters of the crude-oil sample Initial

Crude-oil

Density

Kinematic viscosity/40 °C

Solidifying point

Acid value

Sulfur content

parameters

(kg/m3)

(mm2/s)

(°C)

(mg KOH/g)

(%)

Value after test

899.8

rn

883.0

point (°C)

33.0

-20.0

1.02

1.05

66.50

40.5

-18.0

1.10

1.01

88.00

Jo u

Value before test

boiling

Fig. 15 Variation in the crude-oil properties before and after the experiment 24

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4 Discussion The reuse of abandoned anhydrite mine-out spaces for underground crude-oil storage is a challenging work. This study reveals the interaction trend of anhydrite rock with crude oil and water under long-term contact with varying pressure, which could provide guidance to the study of anhydrite mine-out space crude-oil storage and other engineering cases that deal with anhydrite stratum.

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Anhydrite intact rock is tight. Therefore, crude oil and water have difficulty permeating through the intact anhydrite rock. However, because of the different solubleness and chemical stability of anhydrite (Fig. 16), crude oil

pr

and water could induce different effects on the anhydrite intact rock. Anhydrite is insoluble and has good chemical

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stability in crude oil, and crude oil does not seep into the intact anhydrite rock. Therefore, crude oil does not affect

Pr

the physical and mechanical properties of the anhydrite rock and stability of anhydrite mine-out spaces. Moreover, anhydrite does not affect the crude-oil properties. However, anhydrite is slightly soluble in water, and its solubleness

rn

al

is affected by temperature and pressure. Thus, it can corrode the anhydrite-rock surface and internal structure along existing cracks, weaken the strength and stiffness of anhydrite intact rock under long-term contact with varying

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pressure, and even induce a loss in strength of anhydrite-rock samples, thus possibly affecting the stability of anhydrite mine-out spaces.

In summary, the Anhui Hengtai Anhydrite mine-out space can potentially be used as an underground oil storage space. Nevertheless, necessary engineering measures should be applied to avoid long-term contact between the anhydrite rock and water at the bottom of the cavern. For example, waterproofer can be used or a water-absorption device can be installed at the bottom of the anhydrite cavern.

25

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Fig. 16 Ion concentration of water and crude oil after the tests

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Additionally, an underground crude-oil storage space can usually be used for decades or even longer than 100 years. Thus, the long-term deformation or stability of anhydrite mine-out spaces should be considered. For example, during its period of operation, the Tersanne gas-storage cavern located in France (Boucly and Legreneur, 1981) and the Eminence gas-storage cavern located in America (Baar, 1977) underwent large creep deformation, which reduced by 35%-40% the storage volume. Additionally, the storage caverns were out of operation. Therefore, focusing on the creep behavior of anhydrite rock is necessary. Moreover, this work studied the anhydrite intact rock, rather than the rock mass. In situ scale, fracture distributions in the anhydrite-rock masses, and seepage problem of crude oil and water are complex. Therefore, field investigations and experimental work should also be conducted to check the airtightness of anhydrite mine-out spaces and determine the seepage trend of crude oil and water. Thus, substantial 26

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work remains to be done to scientifically reuse the anhydrite mine-out spaces.

5 Conclusions Previous tests and analysis revealed the interaction pattern of anhydrite rock with crude oil and water and evaluated the effects of crude oil and water on the anhydrite rock. The influence of anhydrite rock on the crude-oil quality was determined. According to the test results, the following conclusions were drawn:

oo

f

1. Anhydrite (formula CaSO4 ) is insoluble and exhibits good chemical stability in crude oil. Thus, crude oil does not affect the physical and mechanical properties of anhydrite rock and the stability of anhydrite mine-out spaces. In

pr

addition, anhydrite does not affect the crude-oil quality.

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2. Anhydrite is slightly soluble in water and can recrystallize as 2H2 O·CaSO4 . Therefore, water can induce two

Pr

types of erosion effects on the anhydrite rock, namely, dissolution and recrystallization effects, under long-time contact at 4-5 MPa pressure. The erosion effects can cause a decrease or even loss of strength and stiffness of the

rn

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anhydrite rock. Thus, water can potentially reduce the stability of anhydrite mine-out spaces. 3. Therefore, the anhydrite mine-out space can potentially be used as an underground crude-oil storage space.

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However, because of the harmful effects of water on the surrounding rock of the anhydrite mine-out spaces, necessary engineering measures need to be adopted to avoid long-term contact of anhydrite with water. For example, waterproofer or a water-absorption device can be used.

Acknowledgements This work is funded by the National Natural Science Foundation of China (Nos. 41572301, 41972300 and 61427802) and the Fundamental Research Funds for the Central Universities of China (No. 2652018108).

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Journal Pre-proof Appendix Table A1 Basic parameters of anhydrite cylindrical samples with 50 mm diameter and 100 mm height (I# samples) Group

NO.

m/g

D/mm

H/mm

ρ/g·cm-3

NO.

m/g

D/mm

H/mm

ρ/g·cm-3

1-1

566

49.30

100.50

2.95

2-1

533

48.16

99.06

1-2

570

49.54

100.50

2.94

2-2

536

48.14

1-3

553

50.80

101.68

2.68

2-3

535

48.22

Group

NO.

m/g

D/mm

H/mm

ρ/g·cm-3

2.96

3-1

570

49.50

100.46

2.95

99.90

2.95

3-2

566

49.20

100.52

2.96

99.50

2.95

3-3

567

49.30

100.50

2.96

3-4

562

49.30

100.36

2.94

3-5

568

49.42

100.62

2.94

3-6

568

49.50

100.28

2.94

Group

1 2

3

1-4

574

50.54

100.40

2.85

2-4

535

48.10

99.42

2.96

-

-

-

-

-

-

2-5

534

48.18

99.50

2.95

-

-

-

-

-

-

2-6

531

48.10

99.14

2.95

4-1

572

49.34

100.32

2.98

5-1

573

49.42

100.40

2.98

f o

6-1

562

49.10

100.00

2.97

4-2

573

49.50

100.70

2.96

5-2

570

49.50

100.38

2.95

6-2

568

49.44

99.82

2.97

4-3

571

49.68

100.32

2.94

5-3

570

49.18

100.46

2.99

6-3

571

49.60

100.40

2.94

4

5 4-4

572

49.44

100.90

2.95

5-4

568

4-5

570

49.52

100.42

2.95

5-5

571

4-6

573

49.58

100.40

2.96

5-6

568

7-1

570

49.62

100.54

2.93

8-1

527

7-2

567

49.32

100.50

2.95

8-2

7-3

572

49.64

100.40

2.95

8-3

7

o J

r P

p e

6

49.44

100.74

2.94

6-4

570

49.50

100.56

2.95

49.58

100.44

2.95

6-5

566

49.30

100.46

2.95

49.26

100.5

2.97

6-6

569

49.40

100.40

2.96

48.10

99.42

2.92

9-1

569

49.58

100.34

2.94

541

48.50

99.58

2.94

9-2

567

49.42

100.30

2.95

526

48.14

99.38

2.91

9-3

574

49.46

100.32

2.98

al

n r u

ro

8

9

7-4

573

49.62

100.62

2.95

8-4

538

48.10

99.50

2.98

9-4

565

49.36

100.26

2.95

7-5

570

49.44

100.40

2.96

8-5

532

48.38

98.84

2.93

9-5

568

49.44

100.98

2.93

7-6

570

49.46

100.44

2.96

8-6

536

48.12

99.44

2.97

9-6

571

49.52

100.54

2.95

10-1

570

49.48

100.94

2.94

10-2

566

49.22

101.00

2.95

10-3

572

49.54

100.42

2.96

10-4

574

49.62

100.64

2.95

10-5

567

49.50

100.52

2.93

10-6

564

49.24

100.90

2.94

10

31

Journal Pre-proof Appendix Table A2 Basic parameters of anhydrite cylindrical samples with 50 mm diameter and 25 or 30 mm height (II# samples) Group

1

4

-

7

NO.

m/g

D/mm

H/mm

ρ/g·cm-3

NO.

m/g

D/mm

H/mm

ρ/g·cm-3

NO.

m/g

D/mm

H/mm

ρ/g·cm-3

1-1

171

49.10

30.64

2.95

2-1

171

49.36

30.94

2.89

3-1

129

48.10

24.42

2.91

1-2

171

49.30

30.60

2.93

2-2

172

49.46

30.58

2.93

3-2

135

48.10

25.50

2.91

1-3

173

49.52

30.80

2.92

2-3

172

49.30

30.60

2.95

3-3

132

47.96

24.92

2.93

1-4

172

49.42

30.46

2.95

2-4

172

49.24

30.92

2.92

3-4

48.14

24.80

2.88

2-5

171

49.26

30.30

2.96

3-5

134

48.24

25.06

2.93

2.89

5-1

168

49.30

30.36

2.90

6-1

173

54.82

25.60

2.86

25.50

2.87

5-2

173

49.50

30.90

2.91

f o

130

1-5

173

49.42

30.46

2.96

4-1

162

52.90

25.50

4-2

171

54.52

6-2

169

54.30

25.28

2.89

4-3

150

51.48

24.62

2.93

5-3

172

49.36

6-3

169

54.78

25.10

2.86

Group

2

5

4-4

145

54.34

22.08

2.83

5-4

171

4-5

162

54.10

23.94

2.95

5-5

169

-

-

-

-

-

-

-

7-1

172

49.34

30.42

2.96

8-1

171

7-2

170

49.26

30.10

2.96

8-2

172

7-3

171

49.32

30.30

-

n r u

2.96

10

2.97

6

30.40

2.95

6-4

160

53.74

24.40

2.89

30.32

2.92

6-5

171

54.40

25.20

2.92

-

-

6-6

159

53.12

25.14

2.86

49.30

30.20

2.97

9-1

138

48.32

25.74

2.93

49.52

30.30

2.95

9-2

136

48.40

25.86

2.86

r P 49.32 -

9

8-3

171

49.50

30.20

2.94

9-3

133

48.20

24.94

2.92

2.98

8-4

172

49.50

30.46

2.94

9-4

132

48.14

25.40

2.86

o J

9-5

133

48.14

24.94

2.93

-

-

-

-

-

8

-

al

3

ro

p e

49.32

30.32

Group

7-4

171

49.26

30.16

7-5

172

49.58

30.62

2.91

8-5

170

49.14

30.10

2.98

-

-

-

-

-

8-6

171

49.40

30.16

2.96

10-1

129

48.20

24.22

2.92

10-2

134

48.20

24.80

2.96

10-3

136

48.14

26.00

2.88

10-4

131

48.14

24.00

3.00

10-5

131

48.10

24.36

2.96

32

-

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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Highlights 1. Partial anhydrite mine-out has the potential to be a suitable underground storage space for crude oil. 2. The time-dependent relationship and mechanism for interactions of anhydrite with crude oil and water was revealed.

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3. There is no interaction effect between anhydrite rock and crude oil.

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4. By dissolution and recrystallization effects of water, strength and stiffness of anhydrite rock decreased.

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