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Engineering characterization of hydraulic properties in a pilot rock cavern for underground LNG storage
Engineering Geology 84 (2006) 229 – 243 www.elsevier.com/locate/enggeo
Engineering characterization of hydraulic properties in a pilot rock cavern for underground LNG storage Sung-Soo Cha a , Jin-Yong Lee b,⁎, Dae-Hyuck Lee c , Eric Amantini d , Kang-Kun Lee a a School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, Republic of Korea GeoGreen21 Co., Ltd., SEK Building, 1687-22, Bongchon6-dong, Gwanak-gu, Seoul 151-812, Republic of Korea SK Engineering and Construction Co., Ltd., 192-18, Kwanhun-dong, Jongro-gu, Seoul 110-300, Republic of Korea d Geostock, 7, Rue E. et A. Peugeot 92563 Rueil-Malmaison Cedex, France
b c
Received 15 September 2005; received in revised form 11 January 2006; accepted 14 February 2006 Available online 23 March 2006
Abstract Feasibility of storing LNG in a lined rock cavern was evaluated using a pilot cryogenic rock cavern constructed in Daejeon, Korea. The pilot program included hydrogeological and engineering characterization of the rock mass around the cavern, design and construction of a drainage system, and pilot operation of the cryogenic cavern. An appropriate drainage system is most important to protect the containment system of LNG from thermal shocks due to ice lenses and hydrostatic pressure of groundwater. As a part of the pilot program, this study focused on the evaluation of hydraulic and engineering properties of the rock mass around the cavern. For this purpose, engineering logging of the rock cores, single and cross-hole hydraulic tests, and recharge/drainage tests were performed using seven drilled holes with different trends and plunges. Three main joint sets were found from the logging of the rock cores, acoustic borehole televiewer, and window mapping. The orientations of the three major joint sets were 60/209, 40/171, and 29/331, which can provide the main groundwater flow paths. Mean RQD values ranged from 56 to 88, which were classified as fair and good, although varying with depth along single boreholes. Hydraulic conductivity from the single and cross-hole hydraulic tests estimated in the order of 10−6 or 10−7 m/s and corresponding transmissivity ranged between 10−5 and 10−6 m2/s. Permeable intervals identified from the hydraulic tests were mostly located above the cavern roof. Below the roof, the permeable zone was difficult to observe. According to the hydraulic communication tests performed for some designated intervals, hydraulic connection between boreholes was highly varied with depth or location, which indicated a very different distribution of water conducting joint sets along the boreholes. When water was injected at R1 with constant or varying flow rates, monotonous and stable seepage was observed at observation boreholes. From this, some stable drainage was expected even in relatively heavy rainfalls. When designing the drainage system of the cavern, the drainage holes should be orientated to maximize frequency of encountering the major joint sets and the permeable intervals identified from this study. © 2006 Elsevier B.V. All rights reserved. Keywords: LNG underground storage; Lined rock cavern; Ice ring; Drainage system; Lugeon test; Korea
1. Introduction
⁎ Corresponding author. Tel.: +82 2 875 9491; fax: +82 2 875 9498. E-mail address: [email protected] (J.-Y. Lee). 0013-7952/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2006.02.001
In the aftermath of the oil embargo in 1979, a plan to stockpile emergency crude oil was established in Korea as a strategic reserve program. For the strategic reserve,
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underground storage was considered due to safety and economics. Since the first underground storage project started in 1981, a number of underground caverns were constructed for oil and gas storage in the country. Five underground gas storages using hydrostatic pressure of groundwater were built by the government and private companies (Kim et al., 2000). In order to store gas in the caverns, it may be liquefied to increase storage capacity. The liquefied gases stored in the caverns are propane and butane, so-called LPG (Liquefied Petroleum Gas). The pressure needed for liquefaction of the gases is less than 10 bars at room temperature. Thus, LPG, which does not necessarily require refrigerated, can be stored a hundred meters below water table under liquefied condition. But in the case of methane storage, very high pressure for liquefaction is required at room temperature. Since methane, the so-called natural gas,
is imported as liquid phase (LNG: Liquefied Natural Gas) by ships into the country, the refrigerated storage of LNG is more economical. Underground storage of LPG, petrochemicals, and other petroleum products may be accomplished in a number of different ways worldwide (PB-KBB, 1998). Most commonly these products are held in inventory underground under pressure in three types of facilities (ECE, 1999). The one type is reuse of depleted reservoirs in oil and/or gas fields, the other is in natural aquifers, and the third type is in salt cavern formations (Bérest et al., 2001). Each storage type has its own physical characteristics and economics, which govern its suitability to particular applications (Thomson et al., 2003). Most existing gas storage is in depleted natural gas or oil fields that are close to consumption centers and are the most commonly used underground storage
Fig. 1. Location map of the study site showing the pilot cavern for LNG storage.
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sites, because of their wide availability (EIA, 2004). Natural aquifers have been converted to gas storage reservoirs and an aquifer is suitable for gas storage if the water bearing sedimentary rock formation is overlaid with an impermeable cap rock. Salt caverns provide very high withdrawal and injection rates relative to their working gas capacity (EIA, 2004). There have been efforts to use abandoned mines to store natural gas and the potential for commercial use of hard-rock cavern storage is currently undergoing testing. However, few of these are commercially operational as natural gas storage sites at the present time (Dahlström and Evans, 2002; EIA, 2004; Glamheden and Curtis, 2006).
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Conventional storage is generally in the form of gas. But all the storages of natural gas have the condition of LNG (liquid phase) in Korea. Therefore, a different storage concept is required for the country's imported gas, in the condition of LNG from ships. Above-ground and in-ground tanks are the general methods for LNG storage. These methods have a big boil-off ratio due to radiant heat from sun. Underground storage has been studied from the viewpoint of thermal advantage of rock and safety from an air raid or terrorism. Many attempts were made in the past to store LNG in unlined rock caverns but they were not successful (Anderson, 1989; Dahlström and Evans, 2002). The failures were due to thermal stresses generating cracks in the host rock mass.
Fig. 2. Layout of the cavern. (a) Ground profile. (b) Relative boreholes locations around the cavern. The upper figure was modified from Synn et al. (1999).
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Table 1 Specification of the investigation boreholes I.D.
Length (m)
Hole size (mm)
Trend a
Plunge a
Casing length (m)
R1 P1 P2 D1 D2 D3 D4
14 25 35 38 35 38 38
76 76 76 76 76 76 76
70 – – 218 218 225 232
70 90 90 − 20 − 18 − 18 20
4 3 2 – – – –
a
Starting point (m) N
E
Z
33.34 − 0.43 25.81 20.74 20.19 18.23 17.12
67.93 99.10 75.96 50.90 52.48 54.85 53.98
23.06 23.07 23.97 2.44 2.65 2.80 0.67
Units are in degrees.
The thermal cracks induced by extremely low temperatures in the rock mass contributed to deteriorate the operational efficiency of the cavern. Gas leakage and increased heat flux between ground and storage occurred (Dahlström, 1992; Glamheden, 2001; Glamheden and Lindblom, 2002). The way to prevent a hard rock mass from cracking at LNG boiling temperatures (−162 °C) would be to locate the storage caverns deep enough below ground level so that the geostatic stresses counterbalance the tensile
stresses caused by the cooling. The necessary depth varies with rock type from 500 to 1000 m which makes this unlined cavern storage concept very expensive. On the other hand, in-ground insulated concrete tanks have been developed as reliable, though expensive, LNG storage techniques (Amantini and Chanfreau, 2004; Amantini et al., 2005). To provide a safer and cost effective solution, a new concept of storing LNG in a mined cavern has been developed (Poten and Partners, 2004; Amantini, in press). The concept consists of
Fig. 3. Schemes of hydraulic tests.
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protecting the host rock against the extreme low temperature and of providing a liquid and gas tight liner. Moreover, the moderated and controlled frost development in the surrounding rock mass contributes to create an ice ring, acting as a secondary barrier (Amantini et al., 2005; Cha et al., in press). A dedicated water drainage system installed around the cavern allows control of the ice formation in the rock mass during the cooling process (Chanfreau and Amantini, 2004). A pilot cavern was constructed in order to demonstrate feasibility of the new concept of LNG storage in a lined rock cavern. The main purpose of this pilot cavern was to validate numerical modeling and calculations, evaluate the global performance of the concept, and improve the future projects by feed-back experience (Lee et al., 2003). The pilot program included hydrogeological characterization, design and construction of the drainage system, efficiency evaluation of the drainage system, and pilot operation of the cryogenic cavern using LN2 (liquid nitrogen), surrogate to LNG due to safety and practical reasons (Amantini et al., 2005). The pilot plant consists of a horseshoe shaped cavern with geometrical working volume of about 110 m3. This paper includes some of the results obtained from the pilot programs. The objective of the study was to evaluate the engineering geology of the site and hydraulic properties of rocks around and within the pilot cavern, which were mainly used to
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design the drainage system of the cryogenic cavern. The hydrogeological investigations were performed to define rock mass characteristics, which are important in the dewatering process. This involves geology, structural features such as joints, hydraulic parameters and hydraulic communication. 2. Methods and materials 2.1. Pilot cavern The pilot cavern is located in Daejeon, about 200 km south of Seoul, Korea (Fig. 1). In 1995, a research facility was built at KIGAM (Korea Institute for Geology And Mineral resources), Daejeon, for the purpose of cold and chilled storage study. The former research facility consists of two caverns for cold and chilled rooms. The minimum temperatures of cold and chilled rooms were −25 °C and 0 °C, respectively. The thermal study of the research facility mainly focused on thermo-mechanical monitoring of rock mass around the pilot plant and underground food storage at low temperature (Park et al., 1999; Synn et al., 1999; Choi et al., 2000). Hydrogeology was not included in the previous study and the main concern was the mechanical stability of the cavern at low temperatures. This research program was completed in 1998. The new pilot study of cryogenic storage was planned for the reuse of this research facility. The
Table 2 Mean rock tunneling quality index (Q) of rock cores obtained from the drilled boreholes I.D.
Interval
No. of section
RQD a
Jn b
Jr c
Ja d
Jw e
SRF f
Q
Description
R1
4–14 m
3
P1
3–25 m
5
P2
2–35 m
6
D1
0–38 m
8
D2
0–35 m
7
D3
0–38 m
8
D4
0–38 m
8
10–60 (57) g 10–82 (56) 10–100 (78) 45–100 (88) 52–100 (86) 27–100 (79) 22–100 (87)
6.0–20.0 (10.5) 3.0–15.0 (7.8) 2.0–20.0 (6.2) 3.0–15.0 (6.3) 3.0–9.0 (4.9) 2.0–15.0 (6.8) 1.0–9.0 (4.5)
0.5–3.0 (2.5) 2.0–3.0 (2.6) 0.5–3.0 (2.3) 1.0–2.0 (1.3) 2.0–3.0 (2.3) 0.5–4.0 (2.6) 1.5–3.0 (2.4)
3.0–4.0 (3.0) 2.0–3.0 (2.4) 1.0–4.0 (2.2) 1.0–4.0 (2.4) 2.0–4.0 (2.6) 1.0–8.0 (3.1) 1.0–4.0 (2.0)
0.66 (0.66) 0.66 (0.66) 0.66 (0.66) 0.66 (0.66) 0.66 (0.66) 0.66 (0.66) 0.66 (0.66)
2.5 (2.5) 2.5 (2.5) 2.5 (2.5) 2.5 (2.5) 2.5 (2.5) 2.5 (2.5) 2.5 (2.5)
0.02–2.6 (1.6) 0.2–10.8 (5.2) 0.02–26.4 (12.2) 0.4–8.8 (3.7) 0.8–13.2 (6.5) 0.1–26.4 (6.9) 1.9–19.8 (10.7)
Ext. poor–fair (fair) Very poor–good (fair) Ext. poor–good (good) Very poor–fair (poor) Very poor–good (fair) Very poor–good (fair) Poor–good (good)
a b c d e f g
Rock quality designation. Joint set number. Joint roughness number. Joint alteration number. Joint water reduction. Stress reduction factor. Mean of range.
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cryogenic pilot cavern was built by enlarging the cold and chilled rooms, which were also used as a drilling chamber for some drainage holes. The cold room was used as a cryogenic pilot cavern. Dimensions of the designed cavern are 4.52 m × 4.02 m × 10.64 m with a net storage volume of liquefied gas of 110 m3. Access to the pilot cavern is provided through an existing horizontal tunnel and chamber known as the laboratory room (Fig. 2a). The pilot cavern roof lies at depths of 22–26 m below the ground surface. The entrance to the cryogenic cavern was blocked by a concrete wall, the plug at the front of cryogenic room. After the pilot cavern was commissioned, the inward flow of LN2 and outward flow of gaseous N2 were possible through pipes from the outside. These were transported through the
access tunnel and going inside the cavern over the plug, through a special part of the roof. 2.2. Boreholes and hydraulic tests Seven boreholes for the geological and hydrogeological investigation were drilled in and around the cavern (Fig. 2b). The holes were drilled and tested between June and September 2002. The holes are grouped into three types designated as P, R, and D series (Table 1). R series borehole is a short inclined hole, which was drilled from surface and dedicated to water recharge for all the tests. R1 is a recharge borehole for the water reinjection system. This borehole is located on the axis of the gallery (45 m away from
Fig. 4. (a) Stereonet of joints. (b) Water level fluctuation at three selected wells.
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were evaluated using injection tests, which essentially adopted the Lugeon method. The single packer method is recommended when the rock mass is weak or intensively jointed and there is a possibility of the hole collapsing. In a completed borehole, straddle packers can be used to isolate the required section of the hole from the remainder. In the cross-hole test, pressure and flow response were monitored in neighboring boreholes and hydraulic communication among the boreholes for each test interval was evaluated. The single-hole tests were conducted on all the boreholes using single packers during the drilling, and straddle packer at various intervals after drilling was completed. In addition, to examine the possibility of drainage through the D series boreholes, recharge/drainage tests were conducted by injecting water into R1 with constant and variable rates. Seepage was measured at the observation boreholes.
the cavern) and the bottom extends to a point of five meters above the top of the cavern. The casing depth is 4 m and a total of 10 m is used for injection screen. P series boreholes were used for piezometers. The holes were drilled vertically from the surface and were dedicated to monitoring. D series boreholes were used in both investigation and drainage tests. The holes were drilled downwards and upwards with inclination of about 20°. The starting point of the holes was at the access tunnel. To evaluate hydraulic properties of the rock mass around the cavern, various hydraulic tests were conducted including single packer tests, straddle packer tests and hydraulic communication tests at specific intervals. The hydraulic tests can be grouped into two types. The first, carried out mainly for estimating hydraulic parameters of the rock mass, was a singlehole test, in which all the tests were implemented. The other was a cross-hole test, which evaluated hydraulic communication among the boreholes. These tests were performed using single and straddle packers. The singlehole test gives information of the hydraulic conductivity and transmissivity for each complete borehole. The resulting hydraulic conductivity and transmissivity represent the total influence from all joints that intersect the borehole. The single-packer test performed in a single-hole defines the hydraulic properties of the entire borehole or mostly a 10 m interval of borehole (Fig. 3). The distribution of hydraulic parameters along a borehole was obtained through straddle packer tests. The hydraulic characteristics at intervals of mostly 1 m
3. Results and discussion 3.1. General geology and engineering geology The geology and structural features were examined by core logging and by mapping the exposed rock surface in the pilot cavern. The base rock consists of Jurassic biotitic granite intruding into Pre-Cambrian gneiss. Based on the photomicrographs, the main mineral compositions of granite in the study area are quartz, K-feldspar (microcline, perthite), plagioclase, and biotite. The hydrological study on the pilot
Table 3 Hydraulic properties obtained from the single packer tests I.D. R1 P1 P2
D1 D2 D3
D4 a b
Interval a 10–14 m NA b 9–19 m 18–29 m 28–35 m 27–35 m 8.5–35 m 15–25 m 25–35 m 15–25 m 25–35 m 15–25 m 25–33.5 m 28–38 m 15–25 m 25–38 m
Lugeon value 20.8 NA 4.1 5.1 4.1 – – 2.2 2.0 2.0 14.7 14.9 8.1 11.2 5.9 – 0.6
Distance from the borehole starting point. Not available.
K (m/s) −6
2.6 × 10 NA 6.1 × 10−7 7.6 × 10−7 5.6 × 10−7 2.6 × 10−7 4.0 × 10−7 4.1 × 10−7 2.9 × 10−7 2.9 × 10−7 2.2 × 10−6 2.2 × 10−6 1.2 × 10−6 1.6 × 10−6 8.7 × 10−7 No flow 9.0 × 10−8
T (m2/s) −5
1.0 × 10 NA 6.1 × 10−6 8.4 × 10−6 3.9 × 10−6 2.1 × 10−6 3.2 × 10−6 1.1 × 10−5 2.9 × 10−6 2.9 × 10−6 2.2 × 10−5 2.2 × 10−5 1.2 × 10−5 8.8 × 10−6 8.7 × 10−6 – 1.2 × 10−6
Flow
Method
Turbulent NA Turbulent Turbulent Turbulent – – Turbulent Dilation Dilation Turbulent Turbulent Void filling Dilation Dilation – Void filling
Lugeon NA Lugeon Lugeon Lugeon Injection Fall-off Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon – Lugeon
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Table 4 Hydraulic properties obtained from the straddle packer tests I.D.
Interval a
R1 P1
b
NA 5.5–8.5 m 8–11 m 10.5–13.5 m 13–16 m 16–20 m 20–25 m
P2
7.5–9 m 9.5–12.5 m 12.5–15.5 m 15.5–18.5 m 18.5–20 m 20–23 m 23–26 m 26–29 m 15–18 m 18–19 m 19–24 m 24–25 m 25–34 m 34–35 m 35–36 m 36–37 m 37–38 m 32–38 m 15–16 m 16–17 m 17–20 m 20–21 m 21–23 m 23–24 m 24–25 m 25–26 m 26–27 m 27–28 m 28–30 m 30–31 m 31–32 m 32–34 m 34–35 m 15–19 m 19–20 m 20–23 m 23–24 m 24–29 m 29–30 m 30–31 m 31–32 m 32–33 m 33–34 m 34–35 m 35–36 m 36–37 m 37–38 m
D1
D2
D2
D3
Lugeon value
K (m/s)
T (m2/s)
Flow
Method
NA 27.4 26.3 25.0 6.2 – – – 23.0 12.4 6.3 11.0 – 12.6 – 5.2 – 18.0 – 19.6 – 131.4 15.6 1.4 32.8 19.7 53 121 – 149 – 166 172 – 97 70 – 60 35 – 13 – 123 – 171 – 60 61 127 129 69 68 89 33 6
NA 3.2 × 10−6 3.1 × 10−6 2.9 × 10−6 6.8 × 10−7 No flow 5.7 × 10−7 6.4 × 10−7 2.3 × 10−6 1.4 × 10−6 4.2 × 10−7 1.3 × 10−6 No flow 1.5 × 10−6 No flow 5.3 × 10−7 No flow 1.5 × 10−6 No flow 1.8 × 10−6 No flow 1.1 × 10−5 1.4 × 10−6 8.2 × 10−8 2.8 × 10−6 1.1 × 10−6 4.6 × 10−6 1.0 × 10−5 No flow 1.3 × 10−5 No flow 1.4 × 10−5 1.5 × 10−5 No flow 8.4 × 10−6 6.0 × 10−6 No flow 5.2 × 10−6 3.1 × 10−6 No flow 1.2 × 10−6 No flow 1.1 × 10−5 No flow 1.5 × 10−5 No flow 5.2 × 10−6 5.3 × 10−6 1.1 × 10−5 1.1 × 10−5 5.9 × 10−6 5.9 × 10−6 7.7 × 10−6 2.8 × 10−6 5.7 × 10−7
NA 9.6 × 10−6 9.3 × 10−6 8.7 × 10−6 2.0 × 10−6 – 2.9 × 10−6 3.2 × 10−6 3.5 × 10−6 4.2 × 10−6 1.3 × 10−6 3.9 × 10−6 – 4.5 × 10−6 – 1.6 × 10−6 – 1.5 × 10−6 – 1.8 × 10−6 – 1.1 × 10−5 1.4 × 10−6 8.2 × 10−8 2.8 × 10−6 6.6 × 10−6 4.6 × 10−6 1.0 × 10−5 – 1.3 × 10−5 – 1.4 × 10−5 1.5 × 10−5 – 8.4 × 10−6 6.0 × 10−6 – 5.2 × 10−6 3.1 × 10−6 – 1.2 × 10−6 – 1.1 × 10−5 – 1.5 × 10−5 – 5.2 × 10−6 5.3 × 10−6 1.1 × 10−5 1.1 × 10−5 5.9 × 10−6 5.9 × 10−6 7.7 × 10−6 2.8 × 10−6 5.7 × 10−7
NA Turbulent Turbulent Turbulent Wash-out – – – Turbulent Wash-out Dilation Turbulent – Turbulent – Dilation – Dilation – Turbulent – Turbulent Wash-out Dilation Dilation Dilation Turbulent Turbulent – Turbulent – Turbulent Turbulent – Turbulent Turbulent – Turbulent Turbulent – Turbulent – Turbulent – Turbulent – Turbulent Turbulent Turbulent Turbulent Turbulent Turbulent Turbulent Wash-out Void-filling
NA Lugeon Lugeon Lugeon Lugeon – Injection Fall-off Lugeon Lugeon Lugeon Lugeon – Lugeon – Lugeon – Lugeon – Lugeon – Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon – Lugeon – Lugeon Lugeon – Lugeon Lugeon – Lugeon Lugeon – Lugeon – Lugeon – Lugeon – Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon
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Table 4 (continued) I.D.
Interval a
Lugeon value
K (m/s)
T (m2/s)
Flow
Method
D4
15–31 m 31–32 m 31–34 m 32–38 m
– 7 1 –
No flow 6.6 × 10−7 1.4 × 10−7 No flow
– 6.6 × 10−7 4.2 × 10−7 –
– Turbulent Dilation –
– Lugeon Lugeon –
a b
Distance from the borehole starting point. Not available.
cavern mainly focused on the heavy rain season. Torrential rains will induce large inflows into the cavern and the seepage may provide serious problems to the membrane installation due to thermal expansion of ice lenses. Engineering design of the cavern requires perfect drying of the lining surface. In the study area, annual precipitation for the last 10 years averaged 1487 mm and the heavy rain tends to occur between June and September, in which period over 65% of the total precipitation occurs (Lee and Lee, 2000). In the rock cores, engineering and geological properties including joint frequency, RQD (rock quality designation) related to fracture characteristics, were examined (Table 2). The host rock mass of the cavern mainly consists of fissured biotitic granite. Except for the weathered upper part of the rock (extremely poor– very poor), the geomechanical quality of rock is in average fair. The average spacing of the rock mass fissures is 1 m. Three main joint sets (J1–J3) obtained were identified from acoustic borehole televiewers of the boreholes and window mapping, forming angles ranging from 20 to 40° with respect to the pilot cavern axis (Fig. 4a). The orientations of the three major joint sets are 60/209, 40/171, and 29/331 (Cha et al., in press). However, the orientations of major joints within the pilot cavern showed a slight difference from the acoustic borehole televiewer data. Joint set J1 was mainly observed in the vertical surface boreholes P1 and P2 and in the wall of the pilot cavern. This joint set did not appear in the downward, underground borehole D4. Joint set J2 contains finer fractures than other joint sets. Joint set J3 is largely observed in borehole P2 and the pilot cavern wall, but not in P1 and D4. The permeability of the granitic rock mass is moderate to high (discussed below in detail). The weathered upper part of the rock mass shows permeability values of 10−4 m/s. At the cavern depth, the mean permeability of the fresh granite is of 10−7 m/s. The fissured granite was initially water saturated, and the water level was located at 10 m above the cavern ceiling. Fluctuations of water levels at three selected boreholes
during the heavy rain season are shown in Fig. 4b. Water levels of R1 most sensitively and instantaneously responded to site rainfall without a time lag (r = 0.81) but those of P1 (r = 0.40) and P2 (r = 0.29) showed less sensitive and delayed response (lag time = l day). The amplitudes of fluctuation (maximum–minimum water levels) were very large (R1 = 4.19 m, P1 = 11.97 m, P2 = 21.17 m), which were mainly due to the very small storativity of the rock mass compared to porous media (Lee et al., 2005). 3.2. Single and straddle packer tests The single packer tests were performed during borehole drilling. The very weak interval in each borehole was excluded from the test. Table 3 shows results of the single packer tests, in which it is assumed that hydraulic conductivity is uniformly distributed along the test section. Values of storage coefficients were omitted because of uncertainty of this parameter in Table 5 Comparison of transmissivity values obtained from different tests I.D.
Test interval
T of complete interval (m2/s)
Sum of T of each interval (∑T, m2/s)
Methods
R1 P1 P2
10–14 m 5.5–25 m 9–35 m
D1
15–35 m
D2
15–35 m
D3
15–33.5 m
D4
25–38 m
1.0 × 10−5 2.9 × 10−5 1.1 × 10−5 – – – 1.5 × 10−5 – – 3.6 × 10–5 – – 2.8 × 10−5 – – 1.3 × 10−6
1.0 × 10−5 2.9 × 10−5 2.3 × 10−5 1.7 × 10−5 5.8 × 10−6 2.0 × 10−5 – 4.4 × 10−5 8.1 × 10−5 – 2.5 × 10−5 8.1 × 10−5 – 1.2 × 10−6 6.6 × 10−7 –
Single packer test Straddle packer test Single packer test Straddle packer test Single packer test Straddle packer test Active borehole a Single packer test Straddle packer test Active borehole Single packer test Straddle packer test Active hole Single packer test Straddle packer test Active borehole
a
Injection borehole used in the interference test.
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a single-hole test. From the pressure response measured during the tests, flow patterns such as laminar, turbulent, dilation, washing-out or void-filling were also identified (Houlsby, 1976). Test intervals were mostly 10 m (range = 4–26.5 m). Estimated hydraulic conductivities ranged between 9.0 × 10−8 and 2.6 × 10−6 m/s with a geometric mean of 6.61 × 10−7 m/s. The corresponding transmissivity values were between 2.9 × 10−6 and 2.2 × 10−5 m2/s (for the 10 m interval data). Flow types were mostly turbulent, while laminar flow was not observed. The turbulent flow indicated an inverse relationship between injection pressures and Lugeon values (Houlsby, 1976). That is a symmetrical distribution with the lowest value occurring at the highest pressure. The turbulent flow (rough, eddying, turbulent motion) usually indicates fast flow in wider fractures or cracks. The pattern of values of dilation is somewhat the
reverse of the previous one. The dilation pattern was mainly observed at D1 and D3, which were upwardly inclined boreholes. Results of hydraulic tests for D and P series boreholes using the straddle packers are presented in Table 4. In these tests, the intervals were mostly within 3 m (86%), with a few exceptions. Estimates of hydraulic conductivity from all the boreholes ranged between 8.2 × 10−8 and 1.5 × 10−5 m/s with a geometric mean of 2.6 × 10−6 m/s (except for practically no flow data). Corresponding transmissivity values were between 8.2 × 10−8 and 1.5 × 10−5 m2/s (only for 1 m interval data). Decrease of permeability for any one borehole was rather large, from practically impermeable (no flow in Table 4) to a value of 10−6 or 10−5 m/s. This meant that structural features like joints and fractures controlling groundwater flow were irregularly
Fig. 5. Results of cross-hole hydraulic tests. Injection boreholes. (a) D1, (b) D2, (c) D3, (d) D4, and (e) P2.
S.-S. Cha et al. / Engineering Geology 84 (2006) 229–243 Table 6 Hydraulic parameters estimated from the cross-hole interference tests Active hole
Observation hole
Distance (m)
T (m2/s)
S (−)
D1 (15–38 m) a
D2 D3 D4 D1 D3 D4 D1 D2 D4 D1 D2 D3 D1 D2 D3
6.5 8.0 15.0 6.5 1.5 17.0 8.0 1.5 18.0 15.0 17.0 18.0 11.7 24.3 30.0
4.8 × 10−5 2.0 × 10−5 – 5.7 × 10−5 2.7 × 10−5 – 2.2 × 10−5 3.9 × 10−5 – 2.6 × 10−6 – – 2.8 × 10−5 4.3 × 10−5 3.8 × 10−5
3.9 × 10−6 4.2 × 10−5 – 3.0 × 10−6 1.1 × 10−4 – 7.6 × 10−5 6.5 × 10−5 – 3.1 × 10−5 – – 7.4 × 10−5 8.7 × 10−6 1.2 × 10−5
D2 (15–35 m)
D3 (15–38 m)
D4 (15–38 m)
P2 (15–35 m)
a
Interval into which water was injected.
developed along the borehole. Meanwhile the turbulent flow type was also predominant (70%) in the single packer tests for which test intervals were relatively wider. Table 5 shows a comparison of transmissivity values obtained from different hydraulic tests. By summing up the transmissivity value for each test interval, the
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values for the complete interval were obtained. Theoretically, transmissivity of the whole interval should equal the sum of transmissivities for each interval. The single and straddle packer tests showed a very similar sum of transmissivity values within one order of magnitude (∑Tsingle /∑Tstraddle = 0.3–1.8). In addition, results for all the borehole tests were not significantly different. 3.3. Cross-hole interference tests Hydraulic connection or communication between boreholes is very important in designing the drainage system for this site. The cross-hole tests in this study are divided into two types including interference tests and recharge/drainage tests. The objectives of the interferences tests were to determine which holes did hydraulically communicate and to estimate hydraulic properties of the rock mass between the test holes. For estimation of the hydraulic parameters, water was injected into an active borehole at a certain depth interval, and pressure variations were monitored at other observation boreholes around the active hole. For the hydraulic communication test, water was injected into the active borehole at a 1 m interval, then pressure responses were monitored at the observation boreholes, which was repeated several
Table 7 Results of hydraulic communication tests Response (pressure buildup, kgf/cm2)
Active (injection) borehole I.D.
Interval
Mean flow rate (L/min)
D1
D2
D3
D4
D1
18–19 m 19–20 m 25–26 m 34–35 m 35–36 m 15–16 m 16–17 m 20–21 m 23–24 m 24–25 m 26–27 m 30–31 m 31–32 m 19–20 m 22–23 m 23–24 m 29–30 m 30–31 m 31–32 m 32–33 m 33–34 m 34–35 m 35–36 m
16.2 3.0 3.5 16.1 16.2 9.6 22.1 25.6 27.7 23.9 6.8 6.5 8.9 30.4 27.1 29.3 7.9 12.4 13.2 6.9 11.9 16.3 14.6
2 3 3 2.5 2.5 0.78 0.86 – – – – – – 0.8 0.95 0.77 – – – – 0.22 – –
0.61 0.75 – – – 3 3 2.5 3 3 3 3 3 1.2 0.7 – 0.62 0.61 0.69 0.60 0.62 0.63 0.61
– – – – – 0.84 1 1 0.9 0.77 0.8 0.75 0.78 3 3 3 3 3 3 3 3 3 3
– – – – 0.1 – – – – – – – – – – – – – – – – – –
D2
D3
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times. In the recharge/drainage tests, water was injected at R1 and groundwater flow (seepage) into the observation boreholes was measured. Results of cross-hole hydraulic tests are shown in Fig. 5. When water was injected into D1, D2, D3 or P2 boreholes, respectively, obvious pressure responses were observed at other observation boreholes except for D4, which is the only downwardly inclined borehole. Meanwhile, when water was injected into D4, pressure buildup was detected only at D1. Using the pressure responses, hydraulic parameters of the rock mass between the test holes were estimated (Table 6). Estimates of transmissivity ranged from 2.6 × 10−6 to 5.7 × 10 − 5 m 2 /s with a geometric mean of 2.6 × 10−5 m2/s. Values of storage coefficient were between 3.0 × 10−6 and 1.1 × 10−4. Compared with the transmissivity values from the single well tests, those from the interference tests are within the same order of magnitude, which implies relatively homogeneous distribution of joints or fractures with a horizontal direction, over the tested spatial magnitude.
Table 7 shows results of hydraulic communication tests for D series wells. Specifically 1 m interval was adopted in these tests. Unlike the cross-well hydraulic tests in which the injection interval was relatively large, the pressure responses at the observations were very different. For the whole injection interval, all the observation wells (except for D4) responded excellently. For the specific 1 m interval injection, however, each observation borehole showed very selective response. It means that hydraulic communication between boreholes varies greatly with depth or spatial vertical location. These results should be seriously considered in the design involving the effective drainage system of the cavern. Permeable intervals identified from the single and cross-hole tests are presented in Fig. 6. The boreholes were installed with different trends and plunges (see Table 1) and absolute spatial coordinates of the test points were not available. Direct or simple connection among the permeable intervals or zones appeared not to be possible. Considering only the plunges of the
Fig. 6. Permeable intervals identified from (a) the single hole tests and (b) the cross-hole interference tests.
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Fig. 7. Responses of water injection at R1 with (a) 1.0 bar and (b) 1.3 bar.
boreholes, some general interpretations are possible. When approaching the ground surface from the cavern roof or cavern wall (D1, D2, D3: upward plunges), permeable intervals become more frequent, which is to be expected considering the hydrogeological layers at this site (see Fig. 2a). But at D4 (upward plunge), permeable intervals were not nearly observable with depth, which appeared also reasonable. Fig. 7 shows flow rates measured at observation boreholes with response to water injection into R1. Natural seepage was measured at D1–D3 boreholes but overflow was measured at D4. In average, D1 showed the least seepage while D2 had the largest seepage flow. Weak hydraulic connection was indicated between D1 and R1 boreholes. Interestingly in spite of irregular or fluctuated injection at R1 (very large amplitude of variation), relatively stable increasing flow rates were shown at observation boreholes, which means that modulation or attenuation of the hydraulic stresses occurs during water flow through the joints or fractures
of the rock mass. Therefore, flow channeling was not expected in the rock mass (Neretnieks, 1993). Based on this test, it was concluded that an appropriate drainage system would safely drain the surrounding water even if relatively heavy rainstorms occur. 3.4. Implications for drainage system As previously described, the main objective of this study was to evaluate important hydrogeologic conditions used to design the drainage system for the cavern. The information on hydraulic properties, major joint sets and hydraulic communication between boreholes should be incorporated to determine the optimum location and orientation of the planned drainage holes. Orientation of the drainage holes would be placed to penetrate major water carrying joints. However, due to limitations of drilling accomplished from inherent cavern geometry, the drainage holes are difficult to penetrate every joint set in a perpendicular direction (Cha et al., in press).
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Even if the perpendicular penetration of joint sets would maximize the frequency for encountering discontinuities in the drainage boreholes, inclined crossing would also work well for the drainage. Relatively high hydraulic conductivity of the rock mass above the cavern roof is expected to facilitate water drainage around the cavern. Design of the drainage system based on this study and results of efficiency tests for the system will be reported in a companion paper (Cha et al., in press).
This was related to different distributions of water conducting joint sets along the boreholes. When water was injected at R1 with constant or varying flow rates, it showed monotonous and stable seepage at observation boreholes. In the design of the drainage system of the cavern, the drainage holes should be orientated to encounter the major joint sets and the permeable intervals identified from this study. Acknowledgements
4. Summary and conclusions To evaluate the feasibility of storing LNG in a lined rock cavern, a pilot cryogenic rock cavern was prepared in Daejeon, Korea, by reconstructing an existing research facility. The pilot program included hydrogeological and engineering characterization of the rock mass around the cavern, design and construction of the drainage system, and pilot operation of the cryogenic cavern. To protect the containment system of LNG from thermal shocks due to ice lenses surrounding the cavern and hydrostatic pressure of groundwater, an appropriate drainage system was considered most important. As a part of the pilot program, this study focused on the evaluation of hydraulic and engineering properties of the rock mass in and around the cavern. For this purpose, engineering logging of the rock cores, single and cross-hole hydraulic tests, and recharge/drainage tests were performed using seven drilled holes with different trends and plunges. From findings of this study, the following main conclusions were reached. Three main joint sets were found from the logging of rock cores, acoustic borehole televiewer and window mapping. The orientations of the three major joint sets were 60/209, 40/171, and 29/331, which likely shows the main groundwater flow paths. Mean RQD values ranged between 56 and 88, which classified the rock as fair and good. But the values varied greatly along single borehole with depth. Estimates of hydraulic conductivity from the single and cross-hole hydraulic tests ranged between 10−6 to 10−7 m/s and corresponding transmissivity ranged between 10−5 and 10−6 m2/s. Storage coefficients were estimated as values near 10−5. Permeable intervals (zones) identified from the hydraulic tests were mostly located between surface and the cavern roof. Below the roof, the permeable zones were not observed. According to the hydraulic communication tests performed for some designated intervals, hydraulic connection between boreholes or intervals varied much more with depth or vertical location, and indicated very different hydraulic behavior over a short interval.
Constructive comments from Professor E.C. Kalkani at National Technical University of Athens, Greece and an anonymous reviewer are greatly appreciated. Many relevant staffs of KIGAM (Korea Institute of Geoscience and Mineral Resources) are also appreciated. References Amantini, E., in press. LNG storage in lined rock caverns. GASTECH 2006 (22nd International Conference and Exhibition for the LNG, LPG and Natural Gas Industries) (2006, 4–7 December, Abu Dhabi). Amantini, E., Chanfreau, E., 2004. Development and construction of pilot lined cavern for LNG underground storage. LNG 14 (14th International Conference and Exhibition on Liquefied Natural Gas), PO-33 (2004, 21–24 March, Doha). Amantini, E., Chanfreau, E., Kim, H.Y., 2005. The LNG storage in lined rock cavern: pilot cavern project in Daejeon, South Korea. GASTECH 2005 (21st International Conference and Exhibition for the LNG, LPG and Natural Gas Industries), (2005, 14–17 March, Bilbao), pp. 1–16. Anderson, U.H., 1989. Steel lined rock caverns. In: Nilsen, Olsen (Eds.), Storage of Gases in Rock Caverns. Balkema, Rotterdam, pp. 1–10. Bérest, P., Bergues, J., Brouard, B., Durup, J.G., Guerber, B., 2001. A salt cavern abandonment test. Int. J. Rock Mech. Min. Sci. 38, 357–368. Cha, S.S., Lee, K.K., Lee, D.H., Bodin, J.L., in press. Efficiency test for evaluation of drainage system around lined pilot cavern for underground cryogenic LNG storage. Tunn. Undergr. Space Technol. Chanfreau, E., Amantini, E.M., 2004. Development and construction of lined cavern LNG storage pilot. 2004 Spring Meeting of AIChE (American Institute of Chemical Engineers), 76b, (2004, 25–29 April, New Orleans). Choi, S.O., Park, H.D., Park, Y.J., Kim, H.Y., Jang, H.D., 2000. Test running of an underground food storage cavern in Korea. Tunn. Undergr. Space Technol. 15, 91–95. Dahlström, L.O., 1992. Rock Mechanical Consequences of Refrigeration—A Study Based on a Pilot Scale Rock Cavern. PhD thesis, Chalmers University of Technology, Gothenburg, Sweden. Dahlström, L.O., Evans, J., 2002. Underground storage of petroleum and natural gases. 17th WPC, pp. 128–129 (2002, 1–5 September, Rio). ECE (Economic Commission for Europe), 1999. Underground Storage in Europe and Central Asia. Survey 1996–1999. UN/ECE, Geneva.
S.-S. Cha et al. / Engineering Geology 84 (2006) 229–243 EIA (Energy Information Administration), 2004. The Basics of Underground Natural Gas Storage. Energy Information Administration, Washington, DC. Glamheden, R., 2001. Thermo-mechanical Behavior of Refrigerated Caverns in Hard Rock. PhD thesis, Chalmers University of Technology, Göteborg, Sweden. Glamheden, R., Curtis, P., 2006. Excavation of a cavern for highpressure storage of natural gas. Tunn. Undergr. Space Technol. 21, 56–67. Glamheden, R., Lindblom, U., 2002. Thermal and mechanical behavior of refrigerated caverns in hard rock. Tunn. Undergr. Space Technol. 17, 341–353. Houlsby, A.C., 1976. Routine interpretation of the Lugeon water-test. Q. J. Eng. Geol. Hydrogeol. 9, 303–313. Kim, T., Lee, K.K., Ko, K.S., Chang, H.W., 2000. Groundwater flow system inferred from hydraulic stresses and heads at an underground LPG storage cavern site. J. Hydrol. 236, 165–184. Lee, D.H., Kim, H.Y., Gatelier, N., Amantini, E., 2003. Numerical study on the estimation of the temperature profile and thermomechanical behavior of the rock around Daejon LNG pilot cavern. International Symposium on the Fusion Technology of Geosystem Engineering, Rock Engineering, and Geophysical Exploration, pp. 233–237 (2003, 18–19 November, Seoul). Lee, J.Y., Lee, K.K., 2000. Use of hydrologic time series data for identification of recharge mechanism in a fractured bedrock aquifer system. J. Hydrol. 229, 190–201.
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Lee, J.Y., Yi, M.J., Hwang, D., 2005. Dependency of hydrologic responses and recharge estimates on water-level monitoring locations within a small catchment. Geosci. J. 9, 333–342. Neretnieks, I., 1993. Solute transport in fractured rock-applications to radionuclide waste repositories. In: Lee, C.H., Farmer, I. (Eds.), Flow and Contaminant Transport in Fractured Rock. Academic Press, San Diego, pp. 39–127. Park, H.D., Synn, J.H., Park, Y.J., Kim, H.Y., 1999. A pilot study on the design of an underground food storage cavern in Korea. Tunn. Undergr. Space Technol. 14, 67–73. PB-KBB, Inc., 1998. Advanced Underground Gas Storage Concepts: Refrigerated-Mined Cavern Storage. DOE DE-AC26-97FT3434901. PB-KBB, Inc., Houston, TX. Poten, Partners, 2004. Korea evaluates rock cavern storage for LNG. LNG in World Markets. October 2004. Synn, J.H., Park, C., Park, Y., Kim, H.Y., 1999. Analysis of heat transfer and heat load on underground cold storage cavern. 9th International Congress on Rock Mechanics, International Society for Rock Mechanics, (1999, 25–28 August, Paris, France), pp. 57–60. Thomson, M., Davison, M., Rasmussen, H., 2003. Natural Gas Storage Valuation and Optimization: A Real Options Application. University of Western Ontario, Ontario, Canada.
Engineering characterization of hydraulic properties in a pilot rock cavern for underground LNG storage Sung-Soo Cha a , Jin-Yong Lee b,⁎, Dae-Hyuck Lee c , Eric Amantini d , Kang-Kun Lee a a School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, Republic of Korea GeoGreen21 Co., Ltd., SEK Building, 1687-22, Bongchon6-dong, Gwanak-gu, Seoul 151-812, Republic of Korea SK Engineering and Construction Co., Ltd., 192-18, Kwanhun-dong, Jongro-gu, Seoul 110-300, Republic of Korea d Geostock, 7, Rue E. et A. Peugeot 92563 Rueil-Malmaison Cedex, France
b c
Received 15 September 2005; received in revised form 11 January 2006; accepted 14 February 2006 Available online 23 March 2006
Abstract Feasibility of storing LNG in a lined rock cavern was evaluated using a pilot cryogenic rock cavern constructed in Daejeon, Korea. The pilot program included hydrogeological and engineering characterization of the rock mass around the cavern, design and construction of a drainage system, and pilot operation of the cryogenic cavern. An appropriate drainage system is most important to protect the containment system of LNG from thermal shocks due to ice lenses and hydrostatic pressure of groundwater. As a part of the pilot program, this study focused on the evaluation of hydraulic and engineering properties of the rock mass around the cavern. For this purpose, engineering logging of the rock cores, single and cross-hole hydraulic tests, and recharge/drainage tests were performed using seven drilled holes with different trends and plunges. Three main joint sets were found from the logging of the rock cores, acoustic borehole televiewer, and window mapping. The orientations of the three major joint sets were 60/209, 40/171, and 29/331, which can provide the main groundwater flow paths. Mean RQD values ranged from 56 to 88, which were classified as fair and good, although varying with depth along single boreholes. Hydraulic conductivity from the single and cross-hole hydraulic tests estimated in the order of 10−6 or 10−7 m/s and corresponding transmissivity ranged between 10−5 and 10−6 m2/s. Permeable intervals identified from the hydraulic tests were mostly located above the cavern roof. Below the roof, the permeable zone was difficult to observe. According to the hydraulic communication tests performed for some designated intervals, hydraulic connection between boreholes was highly varied with depth or location, which indicated a very different distribution of water conducting joint sets along the boreholes. When water was injected at R1 with constant or varying flow rates, monotonous and stable seepage was observed at observation boreholes. From this, some stable drainage was expected even in relatively heavy rainfalls. When designing the drainage system of the cavern, the drainage holes should be orientated to maximize frequency of encountering the major joint sets and the permeable intervals identified from this study. © 2006 Elsevier B.V. All rights reserved. Keywords: LNG underground storage; Lined rock cavern; Ice ring; Drainage system; Lugeon test; Korea
1. Introduction
⁎ Corresponding author. Tel.: +82 2 875 9491; fax: +82 2 875 9498. E-mail address: [email protected] (J.-Y. Lee). 0013-7952/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2006.02.001
In the aftermath of the oil embargo in 1979, a plan to stockpile emergency crude oil was established in Korea as a strategic reserve program. For the strategic reserve,
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underground storage was considered due to safety and economics. Since the first underground storage project started in 1981, a number of underground caverns were constructed for oil and gas storage in the country. Five underground gas storages using hydrostatic pressure of groundwater were built by the government and private companies (Kim et al., 2000). In order to store gas in the caverns, it may be liquefied to increase storage capacity. The liquefied gases stored in the caverns are propane and butane, so-called LPG (Liquefied Petroleum Gas). The pressure needed for liquefaction of the gases is less than 10 bars at room temperature. Thus, LPG, which does not necessarily require refrigerated, can be stored a hundred meters below water table under liquefied condition. But in the case of methane storage, very high pressure for liquefaction is required at room temperature. Since methane, the so-called natural gas,
is imported as liquid phase (LNG: Liquefied Natural Gas) by ships into the country, the refrigerated storage of LNG is more economical. Underground storage of LPG, petrochemicals, and other petroleum products may be accomplished in a number of different ways worldwide (PB-KBB, 1998). Most commonly these products are held in inventory underground under pressure in three types of facilities (ECE, 1999). The one type is reuse of depleted reservoirs in oil and/or gas fields, the other is in natural aquifers, and the third type is in salt cavern formations (Bérest et al., 2001). Each storage type has its own physical characteristics and economics, which govern its suitability to particular applications (Thomson et al., 2003). Most existing gas storage is in depleted natural gas or oil fields that are close to consumption centers and are the most commonly used underground storage
Fig. 1. Location map of the study site showing the pilot cavern for LNG storage.
S.-S. Cha et al. / Engineering Geology 84 (2006) 229–243
sites, because of their wide availability (EIA, 2004). Natural aquifers have been converted to gas storage reservoirs and an aquifer is suitable for gas storage if the water bearing sedimentary rock formation is overlaid with an impermeable cap rock. Salt caverns provide very high withdrawal and injection rates relative to their working gas capacity (EIA, 2004). There have been efforts to use abandoned mines to store natural gas and the potential for commercial use of hard-rock cavern storage is currently undergoing testing. However, few of these are commercially operational as natural gas storage sites at the present time (Dahlström and Evans, 2002; EIA, 2004; Glamheden and Curtis, 2006).
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Conventional storage is generally in the form of gas. But all the storages of natural gas have the condition of LNG (liquid phase) in Korea. Therefore, a different storage concept is required for the country's imported gas, in the condition of LNG from ships. Above-ground and in-ground tanks are the general methods for LNG storage. These methods have a big boil-off ratio due to radiant heat from sun. Underground storage has been studied from the viewpoint of thermal advantage of rock and safety from an air raid or terrorism. Many attempts were made in the past to store LNG in unlined rock caverns but they were not successful (Anderson, 1989; Dahlström and Evans, 2002). The failures were due to thermal stresses generating cracks in the host rock mass.
Fig. 2. Layout of the cavern. (a) Ground profile. (b) Relative boreholes locations around the cavern. The upper figure was modified from Synn et al. (1999).
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Table 1 Specification of the investigation boreholes I.D.
Length (m)
Hole size (mm)
Trend a
Plunge a
Casing length (m)
R1 P1 P2 D1 D2 D3 D4
14 25 35 38 35 38 38
76 76 76 76 76 76 76
70 – – 218 218 225 232
70 90 90 − 20 − 18 − 18 20
4 3 2 – – – –
a
Starting point (m) N
E
Z
33.34 − 0.43 25.81 20.74 20.19 18.23 17.12
67.93 99.10 75.96 50.90 52.48 54.85 53.98
23.06 23.07 23.97 2.44 2.65 2.80 0.67
Units are in degrees.
The thermal cracks induced by extremely low temperatures in the rock mass contributed to deteriorate the operational efficiency of the cavern. Gas leakage and increased heat flux between ground and storage occurred (Dahlström, 1992; Glamheden, 2001; Glamheden and Lindblom, 2002). The way to prevent a hard rock mass from cracking at LNG boiling temperatures (−162 °C) would be to locate the storage caverns deep enough below ground level so that the geostatic stresses counterbalance the tensile
stresses caused by the cooling. The necessary depth varies with rock type from 500 to 1000 m which makes this unlined cavern storage concept very expensive. On the other hand, in-ground insulated concrete tanks have been developed as reliable, though expensive, LNG storage techniques (Amantini and Chanfreau, 2004; Amantini et al., 2005). To provide a safer and cost effective solution, a new concept of storing LNG in a mined cavern has been developed (Poten and Partners, 2004; Amantini, in press). The concept consists of
Fig. 3. Schemes of hydraulic tests.
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protecting the host rock against the extreme low temperature and of providing a liquid and gas tight liner. Moreover, the moderated and controlled frost development in the surrounding rock mass contributes to create an ice ring, acting as a secondary barrier (Amantini et al., 2005; Cha et al., in press). A dedicated water drainage system installed around the cavern allows control of the ice formation in the rock mass during the cooling process (Chanfreau and Amantini, 2004). A pilot cavern was constructed in order to demonstrate feasibility of the new concept of LNG storage in a lined rock cavern. The main purpose of this pilot cavern was to validate numerical modeling and calculations, evaluate the global performance of the concept, and improve the future projects by feed-back experience (Lee et al., 2003). The pilot program included hydrogeological characterization, design and construction of the drainage system, efficiency evaluation of the drainage system, and pilot operation of the cryogenic cavern using LN2 (liquid nitrogen), surrogate to LNG due to safety and practical reasons (Amantini et al., 2005). The pilot plant consists of a horseshoe shaped cavern with geometrical working volume of about 110 m3. This paper includes some of the results obtained from the pilot programs. The objective of the study was to evaluate the engineering geology of the site and hydraulic properties of rocks around and within the pilot cavern, which were mainly used to
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design the drainage system of the cryogenic cavern. The hydrogeological investigations were performed to define rock mass characteristics, which are important in the dewatering process. This involves geology, structural features such as joints, hydraulic parameters and hydraulic communication. 2. Methods and materials 2.1. Pilot cavern The pilot cavern is located in Daejeon, about 200 km south of Seoul, Korea (Fig. 1). In 1995, a research facility was built at KIGAM (Korea Institute for Geology And Mineral resources), Daejeon, for the purpose of cold and chilled storage study. The former research facility consists of two caverns for cold and chilled rooms. The minimum temperatures of cold and chilled rooms were −25 °C and 0 °C, respectively. The thermal study of the research facility mainly focused on thermo-mechanical monitoring of rock mass around the pilot plant and underground food storage at low temperature (Park et al., 1999; Synn et al., 1999; Choi et al., 2000). Hydrogeology was not included in the previous study and the main concern was the mechanical stability of the cavern at low temperatures. This research program was completed in 1998. The new pilot study of cryogenic storage was planned for the reuse of this research facility. The
Table 2 Mean rock tunneling quality index (Q) of rock cores obtained from the drilled boreholes I.D.
Interval
No. of section
RQD a
Jn b
Jr c
Ja d
Jw e
SRF f
Q
Description
R1
4–14 m
3
P1
3–25 m
5
P2
2–35 m
6
D1
0–38 m
8
D2
0–35 m
7
D3
0–38 m
8
D4
0–38 m
8
10–60 (57) g 10–82 (56) 10–100 (78) 45–100 (88) 52–100 (86) 27–100 (79) 22–100 (87)
6.0–20.0 (10.5) 3.0–15.0 (7.8) 2.0–20.0 (6.2) 3.0–15.0 (6.3) 3.0–9.0 (4.9) 2.0–15.0 (6.8) 1.0–9.0 (4.5)
0.5–3.0 (2.5) 2.0–3.0 (2.6) 0.5–3.0 (2.3) 1.0–2.0 (1.3) 2.0–3.0 (2.3) 0.5–4.0 (2.6) 1.5–3.0 (2.4)
3.0–4.0 (3.0) 2.0–3.0 (2.4) 1.0–4.0 (2.2) 1.0–4.0 (2.4) 2.0–4.0 (2.6) 1.0–8.0 (3.1) 1.0–4.0 (2.0)
0.66 (0.66) 0.66 (0.66) 0.66 (0.66) 0.66 (0.66) 0.66 (0.66) 0.66 (0.66) 0.66 (0.66)
2.5 (2.5) 2.5 (2.5) 2.5 (2.5) 2.5 (2.5) 2.5 (2.5) 2.5 (2.5) 2.5 (2.5)
0.02–2.6 (1.6) 0.2–10.8 (5.2) 0.02–26.4 (12.2) 0.4–8.8 (3.7) 0.8–13.2 (6.5) 0.1–26.4 (6.9) 1.9–19.8 (10.7)
Ext. poor–fair (fair) Very poor–good (fair) Ext. poor–good (good) Very poor–fair (poor) Very poor–good (fair) Very poor–good (fair) Poor–good (good)
a b c d e f g
Rock quality designation. Joint set number. Joint roughness number. Joint alteration number. Joint water reduction. Stress reduction factor. Mean of range.
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cryogenic pilot cavern was built by enlarging the cold and chilled rooms, which were also used as a drilling chamber for some drainage holes. The cold room was used as a cryogenic pilot cavern. Dimensions of the designed cavern are 4.52 m × 4.02 m × 10.64 m with a net storage volume of liquefied gas of 110 m3. Access to the pilot cavern is provided through an existing horizontal tunnel and chamber known as the laboratory room (Fig. 2a). The pilot cavern roof lies at depths of 22–26 m below the ground surface. The entrance to the cryogenic cavern was blocked by a concrete wall, the plug at the front of cryogenic room. After the pilot cavern was commissioned, the inward flow of LN2 and outward flow of gaseous N2 were possible through pipes from the outside. These were transported through the
access tunnel and going inside the cavern over the plug, through a special part of the roof. 2.2. Boreholes and hydraulic tests Seven boreholes for the geological and hydrogeological investigation were drilled in and around the cavern (Fig. 2b). The holes were drilled and tested between June and September 2002. The holes are grouped into three types designated as P, R, and D series (Table 1). R series borehole is a short inclined hole, which was drilled from surface and dedicated to water recharge for all the tests. R1 is a recharge borehole for the water reinjection system. This borehole is located on the axis of the gallery (45 m away from
Fig. 4. (a) Stereonet of joints. (b) Water level fluctuation at three selected wells.
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were evaluated using injection tests, which essentially adopted the Lugeon method. The single packer method is recommended when the rock mass is weak or intensively jointed and there is a possibility of the hole collapsing. In a completed borehole, straddle packers can be used to isolate the required section of the hole from the remainder. In the cross-hole test, pressure and flow response were monitored in neighboring boreholes and hydraulic communication among the boreholes for each test interval was evaluated. The single-hole tests were conducted on all the boreholes using single packers during the drilling, and straddle packer at various intervals after drilling was completed. In addition, to examine the possibility of drainage through the D series boreholes, recharge/drainage tests were conducted by injecting water into R1 with constant and variable rates. Seepage was measured at the observation boreholes.
the cavern) and the bottom extends to a point of five meters above the top of the cavern. The casing depth is 4 m and a total of 10 m is used for injection screen. P series boreholes were used for piezometers. The holes were drilled vertically from the surface and were dedicated to monitoring. D series boreholes were used in both investigation and drainage tests. The holes were drilled downwards and upwards with inclination of about 20°. The starting point of the holes was at the access tunnel. To evaluate hydraulic properties of the rock mass around the cavern, various hydraulic tests were conducted including single packer tests, straddle packer tests and hydraulic communication tests at specific intervals. The hydraulic tests can be grouped into two types. The first, carried out mainly for estimating hydraulic parameters of the rock mass, was a singlehole test, in which all the tests were implemented. The other was a cross-hole test, which evaluated hydraulic communication among the boreholes. These tests were performed using single and straddle packers. The singlehole test gives information of the hydraulic conductivity and transmissivity for each complete borehole. The resulting hydraulic conductivity and transmissivity represent the total influence from all joints that intersect the borehole. The single-packer test performed in a single-hole defines the hydraulic properties of the entire borehole or mostly a 10 m interval of borehole (Fig. 3). The distribution of hydraulic parameters along a borehole was obtained through straddle packer tests. The hydraulic characteristics at intervals of mostly 1 m
3. Results and discussion 3.1. General geology and engineering geology The geology and structural features were examined by core logging and by mapping the exposed rock surface in the pilot cavern. The base rock consists of Jurassic biotitic granite intruding into Pre-Cambrian gneiss. Based on the photomicrographs, the main mineral compositions of granite in the study area are quartz, K-feldspar (microcline, perthite), plagioclase, and biotite. The hydrological study on the pilot
Table 3 Hydraulic properties obtained from the single packer tests I.D. R1 P1 P2
D1 D2 D3
D4 a b
Interval a 10–14 m NA b 9–19 m 18–29 m 28–35 m 27–35 m 8.5–35 m 15–25 m 25–35 m 15–25 m 25–35 m 15–25 m 25–33.5 m 28–38 m 15–25 m 25–38 m
Lugeon value 20.8 NA 4.1 5.1 4.1 – – 2.2 2.0 2.0 14.7 14.9 8.1 11.2 5.9 – 0.6
Distance from the borehole starting point. Not available.
K (m/s) −6
2.6 × 10 NA 6.1 × 10−7 7.6 × 10−7 5.6 × 10−7 2.6 × 10−7 4.0 × 10−7 4.1 × 10−7 2.9 × 10−7 2.9 × 10−7 2.2 × 10−6 2.2 × 10−6 1.2 × 10−6 1.6 × 10−6 8.7 × 10−7 No flow 9.0 × 10−8
T (m2/s) −5
1.0 × 10 NA 6.1 × 10−6 8.4 × 10−6 3.9 × 10−6 2.1 × 10−6 3.2 × 10−6 1.1 × 10−5 2.9 × 10−6 2.9 × 10−6 2.2 × 10−5 2.2 × 10−5 1.2 × 10−5 8.8 × 10−6 8.7 × 10−6 – 1.2 × 10−6
Flow
Method
Turbulent NA Turbulent Turbulent Turbulent – – Turbulent Dilation Dilation Turbulent Turbulent Void filling Dilation Dilation – Void filling
Lugeon NA Lugeon Lugeon Lugeon Injection Fall-off Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon – Lugeon
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Table 4 Hydraulic properties obtained from the straddle packer tests I.D.
Interval a
R1 P1
b
NA 5.5–8.5 m 8–11 m 10.5–13.5 m 13–16 m 16–20 m 20–25 m
P2
7.5–9 m 9.5–12.5 m 12.5–15.5 m 15.5–18.5 m 18.5–20 m 20–23 m 23–26 m 26–29 m 15–18 m 18–19 m 19–24 m 24–25 m 25–34 m 34–35 m 35–36 m 36–37 m 37–38 m 32–38 m 15–16 m 16–17 m 17–20 m 20–21 m 21–23 m 23–24 m 24–25 m 25–26 m 26–27 m 27–28 m 28–30 m 30–31 m 31–32 m 32–34 m 34–35 m 15–19 m 19–20 m 20–23 m 23–24 m 24–29 m 29–30 m 30–31 m 31–32 m 32–33 m 33–34 m 34–35 m 35–36 m 36–37 m 37–38 m
D1
D2
D2
D3
Lugeon value
K (m/s)
T (m2/s)
Flow
Method
NA 27.4 26.3 25.0 6.2 – – – 23.0 12.4 6.3 11.0 – 12.6 – 5.2 – 18.0 – 19.6 – 131.4 15.6 1.4 32.8 19.7 53 121 – 149 – 166 172 – 97 70 – 60 35 – 13 – 123 – 171 – 60 61 127 129 69 68 89 33 6
NA 3.2 × 10−6 3.1 × 10−6 2.9 × 10−6 6.8 × 10−7 No flow 5.7 × 10−7 6.4 × 10−7 2.3 × 10−6 1.4 × 10−6 4.2 × 10−7 1.3 × 10−6 No flow 1.5 × 10−6 No flow 5.3 × 10−7 No flow 1.5 × 10−6 No flow 1.8 × 10−6 No flow 1.1 × 10−5 1.4 × 10−6 8.2 × 10−8 2.8 × 10−6 1.1 × 10−6 4.6 × 10−6 1.0 × 10−5 No flow 1.3 × 10−5 No flow 1.4 × 10−5 1.5 × 10−5 No flow 8.4 × 10−6 6.0 × 10−6 No flow 5.2 × 10−6 3.1 × 10−6 No flow 1.2 × 10−6 No flow 1.1 × 10−5 No flow 1.5 × 10−5 No flow 5.2 × 10−6 5.3 × 10−6 1.1 × 10−5 1.1 × 10−5 5.9 × 10−6 5.9 × 10−6 7.7 × 10−6 2.8 × 10−6 5.7 × 10−7
NA 9.6 × 10−6 9.3 × 10−6 8.7 × 10−6 2.0 × 10−6 – 2.9 × 10−6 3.2 × 10−6 3.5 × 10−6 4.2 × 10−6 1.3 × 10−6 3.9 × 10−6 – 4.5 × 10−6 – 1.6 × 10−6 – 1.5 × 10−6 – 1.8 × 10−6 – 1.1 × 10−5 1.4 × 10−6 8.2 × 10−8 2.8 × 10−6 6.6 × 10−6 4.6 × 10−6 1.0 × 10−5 – 1.3 × 10−5 – 1.4 × 10−5 1.5 × 10−5 – 8.4 × 10−6 6.0 × 10−6 – 5.2 × 10−6 3.1 × 10−6 – 1.2 × 10−6 – 1.1 × 10−5 – 1.5 × 10−5 – 5.2 × 10−6 5.3 × 10−6 1.1 × 10−5 1.1 × 10−5 5.9 × 10−6 5.9 × 10−6 7.7 × 10−6 2.8 × 10−6 5.7 × 10−7
NA Turbulent Turbulent Turbulent Wash-out – – – Turbulent Wash-out Dilation Turbulent – Turbulent – Dilation – Dilation – Turbulent – Turbulent Wash-out Dilation Dilation Dilation Turbulent Turbulent – Turbulent – Turbulent Turbulent – Turbulent Turbulent – Turbulent Turbulent – Turbulent – Turbulent – Turbulent – Turbulent Turbulent Turbulent Turbulent Turbulent Turbulent Turbulent Wash-out Void-filling
NA Lugeon Lugeon Lugeon Lugeon – Injection Fall-off Lugeon Lugeon Lugeon Lugeon – Lugeon – Lugeon – Lugeon – Lugeon – Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon – Lugeon – Lugeon Lugeon – Lugeon Lugeon – Lugeon Lugeon – Lugeon – Lugeon – Lugeon – Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon Lugeon
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Table 4 (continued) I.D.
Interval a
Lugeon value
K (m/s)
T (m2/s)
Flow
Method
D4
15–31 m 31–32 m 31–34 m 32–38 m
– 7 1 –
No flow 6.6 × 10−7 1.4 × 10−7 No flow
– 6.6 × 10−7 4.2 × 10−7 –
– Turbulent Dilation –
– Lugeon Lugeon –
a b
Distance from the borehole starting point. Not available.
cavern mainly focused on the heavy rain season. Torrential rains will induce large inflows into the cavern and the seepage may provide serious problems to the membrane installation due to thermal expansion of ice lenses. Engineering design of the cavern requires perfect drying of the lining surface. In the study area, annual precipitation for the last 10 years averaged 1487 mm and the heavy rain tends to occur between June and September, in which period over 65% of the total precipitation occurs (Lee and Lee, 2000). In the rock cores, engineering and geological properties including joint frequency, RQD (rock quality designation) related to fracture characteristics, were examined (Table 2). The host rock mass of the cavern mainly consists of fissured biotitic granite. Except for the weathered upper part of the rock (extremely poor– very poor), the geomechanical quality of rock is in average fair. The average spacing of the rock mass fissures is 1 m. Three main joint sets (J1–J3) obtained were identified from acoustic borehole televiewers of the boreholes and window mapping, forming angles ranging from 20 to 40° with respect to the pilot cavern axis (Fig. 4a). The orientations of the three major joint sets are 60/209, 40/171, and 29/331 (Cha et al., in press). However, the orientations of major joints within the pilot cavern showed a slight difference from the acoustic borehole televiewer data. Joint set J1 was mainly observed in the vertical surface boreholes P1 and P2 and in the wall of the pilot cavern. This joint set did not appear in the downward, underground borehole D4. Joint set J2 contains finer fractures than other joint sets. Joint set J3 is largely observed in borehole P2 and the pilot cavern wall, but not in P1 and D4. The permeability of the granitic rock mass is moderate to high (discussed below in detail). The weathered upper part of the rock mass shows permeability values of 10−4 m/s. At the cavern depth, the mean permeability of the fresh granite is of 10−7 m/s. The fissured granite was initially water saturated, and the water level was located at 10 m above the cavern ceiling. Fluctuations of water levels at three selected boreholes
during the heavy rain season are shown in Fig. 4b. Water levels of R1 most sensitively and instantaneously responded to site rainfall without a time lag (r = 0.81) but those of P1 (r = 0.40) and P2 (r = 0.29) showed less sensitive and delayed response (lag time = l day). The amplitudes of fluctuation (maximum–minimum water levels) were very large (R1 = 4.19 m, P1 = 11.97 m, P2 = 21.17 m), which were mainly due to the very small storativity of the rock mass compared to porous media (Lee et al., 2005). 3.2. Single and straddle packer tests The single packer tests were performed during borehole drilling. The very weak interval in each borehole was excluded from the test. Table 3 shows results of the single packer tests, in which it is assumed that hydraulic conductivity is uniformly distributed along the test section. Values of storage coefficients were omitted because of uncertainty of this parameter in Table 5 Comparison of transmissivity values obtained from different tests I.D.
Test interval
T of complete interval (m2/s)
Sum of T of each interval (∑T, m2/s)
Methods
R1 P1 P2
10–14 m 5.5–25 m 9–35 m
D1
15–35 m
D2
15–35 m
D3
15–33.5 m
D4
25–38 m
1.0 × 10−5 2.9 × 10−5 1.1 × 10−5 – – – 1.5 × 10−5 – – 3.6 × 10–5 – – 2.8 × 10−5 – – 1.3 × 10−6
1.0 × 10−5 2.9 × 10−5 2.3 × 10−5 1.7 × 10−5 5.8 × 10−6 2.0 × 10−5 – 4.4 × 10−5 8.1 × 10−5 – 2.5 × 10−5 8.1 × 10−5 – 1.2 × 10−6 6.6 × 10−7 –
Single packer test Straddle packer test Single packer test Straddle packer test Single packer test Straddle packer test Active borehole a Single packer test Straddle packer test Active borehole Single packer test Straddle packer test Active hole Single packer test Straddle packer test Active borehole
a
Injection borehole used in the interference test.
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a single-hole test. From the pressure response measured during the tests, flow patterns such as laminar, turbulent, dilation, washing-out or void-filling were also identified (Houlsby, 1976). Test intervals were mostly 10 m (range = 4–26.5 m). Estimated hydraulic conductivities ranged between 9.0 × 10−8 and 2.6 × 10−6 m/s with a geometric mean of 6.61 × 10−7 m/s. The corresponding transmissivity values were between 2.9 × 10−6 and 2.2 × 10−5 m2/s (for the 10 m interval data). Flow types were mostly turbulent, while laminar flow was not observed. The turbulent flow indicated an inverse relationship between injection pressures and Lugeon values (Houlsby, 1976). That is a symmetrical distribution with the lowest value occurring at the highest pressure. The turbulent flow (rough, eddying, turbulent motion) usually indicates fast flow in wider fractures or cracks. The pattern of values of dilation is somewhat the
reverse of the previous one. The dilation pattern was mainly observed at D1 and D3, which were upwardly inclined boreholes. Results of hydraulic tests for D and P series boreholes using the straddle packers are presented in Table 4. In these tests, the intervals were mostly within 3 m (86%), with a few exceptions. Estimates of hydraulic conductivity from all the boreholes ranged between 8.2 × 10−8 and 1.5 × 10−5 m/s with a geometric mean of 2.6 × 10−6 m/s (except for practically no flow data). Corresponding transmissivity values were between 8.2 × 10−8 and 1.5 × 10−5 m2/s (only for 1 m interval data). Decrease of permeability for any one borehole was rather large, from practically impermeable (no flow in Table 4) to a value of 10−6 or 10−5 m/s. This meant that structural features like joints and fractures controlling groundwater flow were irregularly
Fig. 5. Results of cross-hole hydraulic tests. Injection boreholes. (a) D1, (b) D2, (c) D3, (d) D4, and (e) P2.
S.-S. Cha et al. / Engineering Geology 84 (2006) 229–243 Table 6 Hydraulic parameters estimated from the cross-hole interference tests Active hole
Observation hole
Distance (m)
T (m2/s)
S (−)
D1 (15–38 m) a
D2 D3 D4 D1 D3 D4 D1 D2 D4 D1 D2 D3 D1 D2 D3
6.5 8.0 15.0 6.5 1.5 17.0 8.0 1.5 18.0 15.0 17.0 18.0 11.7 24.3 30.0
4.8 × 10−5 2.0 × 10−5 – 5.7 × 10−5 2.7 × 10−5 – 2.2 × 10−5 3.9 × 10−5 – 2.6 × 10−6 – – 2.8 × 10−5 4.3 × 10−5 3.8 × 10−5
3.9 × 10−6 4.2 × 10−5 – 3.0 × 10−6 1.1 × 10−4 – 7.6 × 10−5 6.5 × 10−5 – 3.1 × 10−5 – – 7.4 × 10−5 8.7 × 10−6 1.2 × 10−5
D2 (15–35 m)
D3 (15–38 m)
D4 (15–38 m)
P2 (15–35 m)
a
Interval into which water was injected.
developed along the borehole. Meanwhile the turbulent flow type was also predominant (70%) in the single packer tests for which test intervals were relatively wider. Table 5 shows a comparison of transmissivity values obtained from different hydraulic tests. By summing up the transmissivity value for each test interval, the
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values for the complete interval were obtained. Theoretically, transmissivity of the whole interval should equal the sum of transmissivities for each interval. The single and straddle packer tests showed a very similar sum of transmissivity values within one order of magnitude (∑Tsingle /∑Tstraddle = 0.3–1.8). In addition, results for all the borehole tests were not significantly different. 3.3. Cross-hole interference tests Hydraulic connection or communication between boreholes is very important in designing the drainage system for this site. The cross-hole tests in this study are divided into two types including interference tests and recharge/drainage tests. The objectives of the interferences tests were to determine which holes did hydraulically communicate and to estimate hydraulic properties of the rock mass between the test holes. For estimation of the hydraulic parameters, water was injected into an active borehole at a certain depth interval, and pressure variations were monitored at other observation boreholes around the active hole. For the hydraulic communication test, water was injected into the active borehole at a 1 m interval, then pressure responses were monitored at the observation boreholes, which was repeated several
Table 7 Results of hydraulic communication tests Response (pressure buildup, kgf/cm2)
Active (injection) borehole I.D.
Interval
Mean flow rate (L/min)
D1
D2
D3
D4
D1
18–19 m 19–20 m 25–26 m 34–35 m 35–36 m 15–16 m 16–17 m 20–21 m 23–24 m 24–25 m 26–27 m 30–31 m 31–32 m 19–20 m 22–23 m 23–24 m 29–30 m 30–31 m 31–32 m 32–33 m 33–34 m 34–35 m 35–36 m
16.2 3.0 3.5 16.1 16.2 9.6 22.1 25.6 27.7 23.9 6.8 6.5 8.9 30.4 27.1 29.3 7.9 12.4 13.2 6.9 11.9 16.3 14.6
2 3 3 2.5 2.5 0.78 0.86 – – – – – – 0.8 0.95 0.77 – – – – 0.22 – –
0.61 0.75 – – – 3 3 2.5 3 3 3 3 3 1.2 0.7 – 0.62 0.61 0.69 0.60 0.62 0.63 0.61
– – – – – 0.84 1 1 0.9 0.77 0.8 0.75 0.78 3 3 3 3 3 3 3 3 3 3
– – – – 0.1 – – – – – – – – – – – – – – – – – –
D2
D3
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times. In the recharge/drainage tests, water was injected at R1 and groundwater flow (seepage) into the observation boreholes was measured. Results of cross-hole hydraulic tests are shown in Fig. 5. When water was injected into D1, D2, D3 or P2 boreholes, respectively, obvious pressure responses were observed at other observation boreholes except for D4, which is the only downwardly inclined borehole. Meanwhile, when water was injected into D4, pressure buildup was detected only at D1. Using the pressure responses, hydraulic parameters of the rock mass between the test holes were estimated (Table 6). Estimates of transmissivity ranged from 2.6 × 10−6 to 5.7 × 10 − 5 m 2 /s with a geometric mean of 2.6 × 10−5 m2/s. Values of storage coefficient were between 3.0 × 10−6 and 1.1 × 10−4. Compared with the transmissivity values from the single well tests, those from the interference tests are within the same order of magnitude, which implies relatively homogeneous distribution of joints or fractures with a horizontal direction, over the tested spatial magnitude.
Table 7 shows results of hydraulic communication tests for D series wells. Specifically 1 m interval was adopted in these tests. Unlike the cross-well hydraulic tests in which the injection interval was relatively large, the pressure responses at the observations were very different. For the whole injection interval, all the observation wells (except for D4) responded excellently. For the specific 1 m interval injection, however, each observation borehole showed very selective response. It means that hydraulic communication between boreholes varies greatly with depth or spatial vertical location. These results should be seriously considered in the design involving the effective drainage system of the cavern. Permeable intervals identified from the single and cross-hole tests are presented in Fig. 6. The boreholes were installed with different trends and plunges (see Table 1) and absolute spatial coordinates of the test points were not available. Direct or simple connection among the permeable intervals or zones appeared not to be possible. Considering only the plunges of the
Fig. 6. Permeable intervals identified from (a) the single hole tests and (b) the cross-hole interference tests.
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Fig. 7. Responses of water injection at R1 with (a) 1.0 bar and (b) 1.3 bar.
boreholes, some general interpretations are possible. When approaching the ground surface from the cavern roof or cavern wall (D1, D2, D3: upward plunges), permeable intervals become more frequent, which is to be expected considering the hydrogeological layers at this site (see Fig. 2a). But at D4 (upward plunge), permeable intervals were not nearly observable with depth, which appeared also reasonable. Fig. 7 shows flow rates measured at observation boreholes with response to water injection into R1. Natural seepage was measured at D1–D3 boreholes but overflow was measured at D4. In average, D1 showed the least seepage while D2 had the largest seepage flow. Weak hydraulic connection was indicated between D1 and R1 boreholes. Interestingly in spite of irregular or fluctuated injection at R1 (very large amplitude of variation), relatively stable increasing flow rates were shown at observation boreholes, which means that modulation or attenuation of the hydraulic stresses occurs during water flow through the joints or fractures
of the rock mass. Therefore, flow channeling was not expected in the rock mass (Neretnieks, 1993). Based on this test, it was concluded that an appropriate drainage system would safely drain the surrounding water even if relatively heavy rainstorms occur. 3.4. Implications for drainage system As previously described, the main objective of this study was to evaluate important hydrogeologic conditions used to design the drainage system for the cavern. The information on hydraulic properties, major joint sets and hydraulic communication between boreholes should be incorporated to determine the optimum location and orientation of the planned drainage holes. Orientation of the drainage holes would be placed to penetrate major water carrying joints. However, due to limitations of drilling accomplished from inherent cavern geometry, the drainage holes are difficult to penetrate every joint set in a perpendicular direction (Cha et al., in press).
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Even if the perpendicular penetration of joint sets would maximize the frequency for encountering discontinuities in the drainage boreholes, inclined crossing would also work well for the drainage. Relatively high hydraulic conductivity of the rock mass above the cavern roof is expected to facilitate water drainage around the cavern. Design of the drainage system based on this study and results of efficiency tests for the system will be reported in a companion paper (Cha et al., in press).
This was related to different distributions of water conducting joint sets along the boreholes. When water was injected at R1 with constant or varying flow rates, it showed monotonous and stable seepage at observation boreholes. In the design of the drainage system of the cavern, the drainage holes should be orientated to encounter the major joint sets and the permeable intervals identified from this study. Acknowledgements
4. Summary and conclusions To evaluate the feasibility of storing LNG in a lined rock cavern, a pilot cryogenic rock cavern was prepared in Daejeon, Korea, by reconstructing an existing research facility. The pilot program included hydrogeological and engineering characterization of the rock mass around the cavern, design and construction of the drainage system, and pilot operation of the cryogenic cavern. To protect the containment system of LNG from thermal shocks due to ice lenses surrounding the cavern and hydrostatic pressure of groundwater, an appropriate drainage system was considered most important. As a part of the pilot program, this study focused on the evaluation of hydraulic and engineering properties of the rock mass in and around the cavern. For this purpose, engineering logging of the rock cores, single and cross-hole hydraulic tests, and recharge/drainage tests were performed using seven drilled holes with different trends and plunges. From findings of this study, the following main conclusions were reached. Three main joint sets were found from the logging of rock cores, acoustic borehole televiewer and window mapping. The orientations of the three major joint sets were 60/209, 40/171, and 29/331, which likely shows the main groundwater flow paths. Mean RQD values ranged between 56 and 88, which classified the rock as fair and good. But the values varied greatly along single borehole with depth. Estimates of hydraulic conductivity from the single and cross-hole hydraulic tests ranged between 10−6 to 10−7 m/s and corresponding transmissivity ranged between 10−5 and 10−6 m2/s. Storage coefficients were estimated as values near 10−5. Permeable intervals (zones) identified from the hydraulic tests were mostly located between surface and the cavern roof. Below the roof, the permeable zones were not observed. According to the hydraulic communication tests performed for some designated intervals, hydraulic connection between boreholes or intervals varied much more with depth or vertical location, and indicated very different hydraulic behavior over a short interval.
Constructive comments from Professor E.C. Kalkani at National Technical University of Athens, Greece and an anonymous reviewer are greatly appreciated. Many relevant staffs of KIGAM (Korea Institute of Geoscience and Mineral Resources) are also appreciated. References Amantini, E., in press. LNG storage in lined rock caverns. GASTECH 2006 (22nd International Conference and Exhibition for the LNG, LPG and Natural Gas Industries) (2006, 4–7 December, Abu Dhabi). Amantini, E., Chanfreau, E., 2004. Development and construction of pilot lined cavern for LNG underground storage. LNG 14 (14th International Conference and Exhibition on Liquefied Natural Gas), PO-33 (2004, 21–24 March, Doha). Amantini, E., Chanfreau, E., Kim, H.Y., 2005. The LNG storage in lined rock cavern: pilot cavern project in Daejeon, South Korea. GASTECH 2005 (21st International Conference and Exhibition for the LNG, LPG and Natural Gas Industries), (2005, 14–17 March, Bilbao), pp. 1–16. Anderson, U.H., 1989. Steel lined rock caverns. In: Nilsen, Olsen (Eds.), Storage of Gases in Rock Caverns. Balkema, Rotterdam, pp. 1–10. Bérest, P., Bergues, J., Brouard, B., Durup, J.G., Guerber, B., 2001. A salt cavern abandonment test. Int. J. Rock Mech. Min. Sci. 38, 357–368. Cha, S.S., Lee, K.K., Lee, D.H., Bodin, J.L., in press. Efficiency test for evaluation of drainage system around lined pilot cavern for underground cryogenic LNG storage. Tunn. Undergr. Space Technol. Chanfreau, E., Amantini, E.M., 2004. Development and construction of lined cavern LNG storage pilot. 2004 Spring Meeting of AIChE (American Institute of Chemical Engineers), 76b, (2004, 25–29 April, New Orleans). Choi, S.O., Park, H.D., Park, Y.J., Kim, H.Y., Jang, H.D., 2000. Test running of an underground food storage cavern in Korea. Tunn. Undergr. Space Technol. 15, 91–95. Dahlström, L.O., 1992. Rock Mechanical Consequences of Refrigeration—A Study Based on a Pilot Scale Rock Cavern. PhD thesis, Chalmers University of Technology, Gothenburg, Sweden. Dahlström, L.O., Evans, J., 2002. Underground storage of petroleum and natural gases. 17th WPC, pp. 128–129 (2002, 1–5 September, Rio). ECE (Economic Commission for Europe), 1999. Underground Storage in Europe and Central Asia. Survey 1996–1999. UN/ECE, Geneva.
S.-S. Cha et al. / Engineering Geology 84 (2006) 229–243 EIA (Energy Information Administration), 2004. The Basics of Underground Natural Gas Storage. Energy Information Administration, Washington, DC. Glamheden, R., 2001. Thermo-mechanical Behavior of Refrigerated Caverns in Hard Rock. PhD thesis, Chalmers University of Technology, Göteborg, Sweden. Glamheden, R., Curtis, P., 2006. Excavation of a cavern for highpressure storage of natural gas. Tunn. Undergr. Space Technol. 21, 56–67. Glamheden, R., Lindblom, U., 2002. Thermal and mechanical behavior of refrigerated caverns in hard rock. Tunn. Undergr. Space Technol. 17, 341–353. Houlsby, A.C., 1976. Routine interpretation of the Lugeon water-test. Q. J. Eng. Geol. Hydrogeol. 9, 303–313. Kim, T., Lee, K.K., Ko, K.S., Chang, H.W., 2000. Groundwater flow system inferred from hydraulic stresses and heads at an underground LPG storage cavern site. J. Hydrol. 236, 165–184. Lee, D.H., Kim, H.Y., Gatelier, N., Amantini, E., 2003. Numerical study on the estimation of the temperature profile and thermomechanical behavior of the rock around Daejon LNG pilot cavern. International Symposium on the Fusion Technology of Geosystem Engineering, Rock Engineering, and Geophysical Exploration, pp. 233–237 (2003, 18–19 November, Seoul). Lee, J.Y., Lee, K.K., 2000. Use of hydrologic time series data for identification of recharge mechanism in a fractured bedrock aquifer system. J. Hydrol. 229, 190–201.
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Lee, J.Y., Yi, M.J., Hwang, D., 2005. Dependency of hydrologic responses and recharge estimates on water-level monitoring locations within a small catchment. Geosci. J. 9, 333–342. Neretnieks, I., 1993. Solute transport in fractured rock-applications to radionuclide waste repositories. In: Lee, C.H., Farmer, I. (Eds.), Flow and Contaminant Transport in Fractured Rock. Academic Press, San Diego, pp. 39–127. Park, H.D., Synn, J.H., Park, Y.J., Kim, H.Y., 1999. A pilot study on the design of an underground food storage cavern in Korea. Tunn. Undergr. Space Technol. 14, 67–73. PB-KBB, Inc., 1998. Advanced Underground Gas Storage Concepts: Refrigerated-Mined Cavern Storage. DOE DE-AC26-97FT3434901. PB-KBB, Inc., Houston, TX. Poten, Partners, 2004. Korea evaluates rock cavern storage for LNG. LNG in World Markets. October 2004. Synn, J.H., Park, C., Park, Y., Kim, H.Y., 1999. Analysis of heat transfer and heat load on underground cold storage cavern. 9th International Congress on Rock Mechanics, International Society for Rock Mechanics, (1999, 25–28 August, Paris, France), pp. 57–60. Thomson, M., Davison, M., Rasmussen, H., 2003. Natural Gas Storage Valuation and Optimization: A Real Options Application. University of Western Ontario, Ontario, Canada.