Tightness and suitability evaluation of abandoned salt caverns served as hydrocarbon energies storage under adverse geological conditions (AGC)
Introduction
The consumption of energy, such as oil, natural gas and electric power, is characterized by noticeable periodically and seasonality [1], but the supply of energies is incapable to keep the consistency with the use of energy all the time. Thus, large-scale energy storage facilities must be constructed along the energy transmission lines or close to the consumption regions so as to support the stability and reliability of the energy market [2], [3]. Usually, energy will be stored when supply is surplus or during ‘low loads’. Reliable energy storage system ensures that energy is available at any time of peak demand, and ensures a supply of energy in the event of a failure of the power transmission routes or encountering emergencies [4]. Pumped hydro storage [5], [6], compressed air energy storage [7], [8], [9], thermal storage [10] and oil and gas storage [11], [12], [13] are the main methods to store energy.
Among all the types of energy storage, the China government pays most concerns and attentions on the storage system of crude oil and natural gas. All around the world, China has the 14th ranked of the oil and gas geologic reserves, but has the 2th ranked of the consumption amount of oil and gas. For instance, as of 2015, China consumed 5.43 × 108 tons of crude oil, in which the amount of import occupies approximate 60%, and consumed 1910 × 108 m3 of natural gas with an imported part of 624 × 108 m3 [14]. At present, China not only has an extremely high dependence of the imported hydrocarbon, but also has a poor system of the hydrocarbon storage, which furtherly increases the China government’s concerns on its energy safety.
As is known, the members of the United Nations are recommended to own at least 90 days of the annual consumption as their strategic oil storage in civil. As that of China, the total oil storage amount is only 31 days at present; simultaneously, the natural gas storage should be of the international level of 15–18% of the annual use [2], but that of China is as low as 3%, far from enough to guarantee a reliable natural gas market. One main reason for these deficiencies is the lack of sufficient and available storage space. Moreover, the currently existing storage facilities of crude oil are mainly aboveground tanks located in coastal regions. Taking the first phase of the China’s strategic petroleum reserve for example, the facilities are all concrete/steel tanks and located at four offshore cities, those are Qingdao, Dalian, Zhenhai and Zhoushan, respectively [15]. These tanks are efficient to provide easy handling of the stored product, but their impact on the environment and on the safety of the neighboring inhabitants has given rise to serious protests and public distrust [16]. The government had to switch from the construction of surface tanks to much cheaper and safer underground storage facilities. As the main regions of energy consumption, central and eastern China lack depleted oil/gas reservoirs and aquifers, but have abundant bedded rock salt formations to allow the construction of large-scale underground hydrocarbon storage caverns, e.g., the Jintan Salt Mine, Huai’an salt mine and Yingyun salt mine [17]. These salt mines all are located close to the pipeline of the West-East natural gas transmission and Sichuan-East natural gas transmission of China [3], [18]. To achieve the target of energy storage, up to 2020, the China government plans to build 5 large oil/natural gas storage sites are planned for construction in salt formations in the Central-Eastern Regions of China, with a total storable volume of up to 4 × 107 m3 [12], [13].
Salt caverns alone can support large scale energy storage applications, and all around the world, they are regarded as the safest facilities to store natural gas, crude oil [19], [20], [21] and hydrogen [22], [23], to store compressed air and even to permanently dispose of radioactive nuclear wastes [24], [25] due to the impermeability and rheology of rock salt [26], [27], [28]. For instance, in Germany, 198 × 108 m3 of natural gas was stored in salt caverns by the end of 2005 [13]. In the USA, salt cavern gas storage represented 23% of the total natural gas underground storage in 2011 [29].
However, there are several inevitable disadvantages to build new salt caverns, such as: (i) long construction time: a single cavern with a volume approximately of 2.0 × 105 m3 usually takes 4–5 years to leach; (ii) high cost: a single cavern costs as much as $ 10–12 million to build; and (iii) strict requirements for technologies: in China, there is little experience in cavern construction, particularly in thinly bedded salts, so the construction tends to be more time-consuming and uneconomic. Nevertheless, the successful conversion of abandoned salt caverns to underground natural gas/oil storage would be a favorable solution for the prevention of geological disasters, for environmental protection, and for hydrocarbons storage.
Utilizing existing underground space, such as coal and salt mines where mining has been completed, to store gas/oil has a long history throughout the world. In Europe, hydrocarbon storage in abandoned salt caverns has been practiced since the early 1950s [30]. Examples include the Manosque facility in France and the Etzel salt dome near Wilhemshaven, Germany, which has been used since 1971 for crude oil storage. Starting in 1961, the Leyden mine was used to store natural gas for Colorado peak-day demands for almost 20 years [31]. Yang et al. [12], [13] demonstrated the feasibility of storing natural gas in six abandoned salt caverns in Jintan, Jiangsu Province of China and carried out field surveys on the caverns’ volume under operational conditions. The results show a satisfactory running state of the caverns. Evans and Chadwick [32] introduced the current situation and perspective of underground gas storage in the UK. They stated that, although hydrocarbons have been stored in abandoned mines, most of the facilities have been closed due to leakage. Therefore, prior to selecting caverns for storing hydrocarbons, it is necessary to evaluate the suitability of the caverns in which hydrocarbons are to be stored. The most important criterion is that gas/oil cannot leak from the cavern into aquifers and groundwater and thereafter to the surface [33], that is, the tightness of the cavern. However, because the critical conditions of the integrity of the host rocks are very strict, the conditions for converting abandoned salt caverns to underground hydrocarbon storage (UHS) are rare, even for the caverns in homogeneous salt domes.
As of the end of 2014, the total volume of all of the abandoned salt caverns in China has reached approximately 2 × 108 m3, and in the latest years, it has a rate of approximate 2 × 107 m3 per year to increase. After abandonment, the caverns are completely filled with saturated brine. The high-pressure brine exerts a certain restraint on the wall to sustain the cavern’s stability, and the almost impermeable rock salt simultaneously prevents the brine from seeping into the surrounding strata. In this case, the cavern shrinks very slowly, and the brine pressure increases slightly until it balances with the geostatic stress [20]. Such a process typically takes place when the caverns are situated in salt domes or in thick salt deposits with a sufficient thickness and high grade of halite. However, the salt beds in central and eastern China are all complex bedded structure, usually consisting of numerous thin salt layers and non-halite interlayers [34], [35], [36], as shown in Fig. 1(a). The evolution of a cavern after abandonment is much different from that in a salt dome or in a thick homogeneous salt bed. So the conversion of them to UHS seems more challenging. Especially in some salt formations of China, e.g., Jintan Salt Mine, Huai’an Salt Mine [37], the interlayers are silt mudstone, silt sandstone and argillaceous sandstone, their permeability is as high as 10−15 m2 or even higher. Brine easily penetrates into these permeable interlayers, leading to a series of negative consequences, such as rock matrix softens, pore pressure increases in wall rocks and internal pressure decreases in caverns. If the caverns are closely distributed with narrow pillars (Fig. 1(b)), the shrinkage of the cavern and the instability of the pillars and roof will be aggravated [38], [39]. These adverse consequences may develop further and cause excessive subsurface subsidence and even collapse, posing serious threats to the safety of the aboveground infrastructures, such as residential buildings, factories and transportation facilities [38], [40], [41]. Moreover, brine is likely to flow upward along fracture zones or overlying faults to overlying aquifers, contaminating groundwater, or causing bearing-capacity loss of foundations, land salinization and even regional geological instability [39]. Thus, it has become an urgent and primary duty to investigate these caverns to assure the sustainable development of salt mining, and the key is how to eliminate the effect of brine and simultaneously maintain the stability of caverns. These adverse conditions are defined as adverse geological conditions (AGC), and the one main focus of this paper is how to evaluate and utilize such caverns under AGC to reduce the risk of potential disasters.
The petrophysical and mechanical properties of different bedded salt mines differ from each other, so the suitability of caverns in these types of mines is also different. Some caverns are suitable for storing natural gas, and some are suitable for storing crude oil; however, others may be unsuitable for storing any type of hydrocarbon. However, at present, there are no widely recognized references or conditions to distinguish the feasibility of using different caverns for different uses. Considering the urgent demand for using abandoned caverns under AGC and the need for large-scale underground storage space for gas and oil, as well as the high standards for storage facilities, we conducted a feasibility analysis on the utilization of abandoned caverns under AGC to store natural gas and crude oil. As tightness is a decisive factor for storing hydrocarbons in underground caverns, we focused on the tightness properties of the caverns to check their feasibility for storing different media. First, laboratory tests, including mechanical tests, porosity and permeability tests, and XRD, are carried out to determine the mechanical properties and basic parameters. Then, conditions for tightness are proposed and numerical simulations of gas seepage around caverns are implemented for different cases. Third, tightness and suitability evaluations of caverns for the storage of natural gas or oil are conducted. As a result, the relation between UHS in abandoned caverns and the improvement of the underground geological environments are discussed. The present study provides a basic reference for utilizing abandoned caverns in bedded rock salt, as well as for similar engineering practices.
Section snippets
Physical and mechanical tests and their results
Physical and mechanical properties provide the basis for analyzing the stability and tightness of energy storage caverns. Bedded rock salt has the mechanical performance of stratified materials. Thus, systematic mechanical experiments should be conducted to determine its properties [12], [34]. Although attention has primarily focused on the tightness of the cavern, favorable mechanical properties and high integrity of host rocks are preconditions for tightness. Damage and seepage channels will
Permeability
According to the measured data, the permeability of the wall rock seems somewhat higher than the value reported [37], [48], [49]. This may be related to the damage that occurs when specimens are prepared and the lower confining pressure loaded for permeability testing. In addition, the confining pressure in the tests (2.5 MPa) is much lower than the in-situ stress around the caverns, and the laboratory measurement time is too short for the rock salt damage to self-heal. We estimated the stress
Gas seepage around the caverns
According to the numerical simulation results, the gas seepage in the vicinity of the caverns over a lifespan of thirty years, including the gas seepage range and seepage pore pressure, as well as FOSs, were obtained to evaluate the tightness and suitability of the caverns under AGC. We list the results for each phase in detail, viz. after 5 years, 10 years, 20 years and 30 years. This is because we intend to determine the service life of the cavern before halting storage or to convert the cavern
Seepage characteristics of different fluids
- (1)
Seepage characteristics of different gases
Zhang et al. [64] stated that different permeabilities were measured for the same rock mass if different pressures or different gases were adopted. For different gases migrating through the same rock, the lighter the gas, the higher the permeability. Fig. 17 shows the permeability measured for the same rock using three different gases.
According to Fig. 17, the tightness of the salt cavern will change if another type of gas other than natural gas is
On treatment of geological disasters
Before considering an abandoned salt cavern for use as a gas or liquid hydrocarbon storage facility, several steps should be taken, of which evaluation of the tightness and suitability is only one, including: (1) a geological survey to get the depth, thickness and distribution of the salt beds in geo-scale; (2) field tests: (a) sonar measurements to obtain the cavern shape, sizes and geological depth and (b) field tests of gas seals to indicate the tightness at each key location, including the
Conclusions
With respect to the tightness and suitability evaluation of abandoned salt caverns under adverse geological conditions (AGC) and to decide whether they are suitable to serve as hydrocarbon storage facilities, petro-mechanical measurements, evaluation conditions, and investigations of gas seepage around caverns have been considered. The main conclusions and suggestions are as follows:
- (1)
The mechanical properties of the target bedded rock salt are satisfactory for the stability of the hydrocarbon
Acknowledgements
The authors would gratefully like to acknowledge the financial support from the National Natural Science Foundation of China (Nos. 41472285, 51574048, 51304256, 51404241), the China Postdoctoral Science Foundation (Nos. 2015M582520, 2015T80857), the Doctoral Program of Higher Specialized Research Fund (20130191130003), the visiting scholar funded Project of the State Key Laboratory of Coal Mine Disaster Dynamics and Control (Chongqing University) (No. 2011DA105287-FW201401), the International
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