A New Research Approach Regarding the Storage of Different Gaseous Products Under Pressure in The Eligible Romanian Salt Caverns

. The paper analyzes the technical and safety factors for the storage of different gaseous products (air, carbon dioxide, hydrogen, methan gas, etc.), in the eligible salt caverns from Romania, especially in the compact salt caverns created by dillution process. This new approach to research and project results, will be an important contribution to the evaluation of large-scale storage capacities in Romania and is necessary for decision-makers to consider these technologies as some of the most appropriate in terms of risks. Especially was studied the geological and lithological diagrams and colected data from the conserved salt mines in order to analyze the storage conditions. The paper analized the Slanic river area, a Trotus tributarry, where disollution method was well implemented, near Targu Ocna town, Bacau County, Romania. The mining area is located near a well-known industrial center for chemical processing, Borzesti Chemical Works, with a great interes in the gas storage facilities. The storage of different gases implies different technologies for gas compression and gas transportation, this beeing in the near future a new challenge for research for reducing greenhouse gas emissions, according to the UN 2030 Agenda for Sustainable Development.


Introduction
The storage of different gaseous products was determined by originally projects related to the long-term geological storage of carbon dioxide in deep geological formations (depleted hydrocarbon deposits, salty aquifers, unexploitable coal seams, etc.), and has recently a growing interest for using the salt caverns created by dillution process, as storage capacities for gaseous products like carbon dioxide, hydrogen, methane, air, etc.The paper analyzes the technical and safety factors for the storage of gaseous products (air, carbon dioxide, hydrogen, methane, etc.), in salt caverns created by dillution process in Romania, an activity without a history until present.Also, the hydrogen storage as a most importan issue is specifically studied, as a strategical one for our country.These wells are of similar construction to those used in the oil and gas industry, where our country has over 160 years of experience in crude oil production and refining.At the same time, the exploitation of salt for centuries, and more recently the exploitation by dissolution, has created hundreds of underground voids and a new economic interest regarding the use of these caverns as storages of gaseous products.
The concept of carbon capture and storage was introduced in Romania, as early as 2001, with GeoEcoMar's accession to ENeRG (European Network for Research in Geo-Energy) [1] and participation in the CASTOR research program [2].In 2010, with the preparation of the launch of the NER300 financing program [3], the Romanian Government started the procedures for the implementation of a CCS demo project in Romania under the name Getica CCS [4], a project that included the entire CCS chain: capture, transport and storage of carbon dioxide (CO2) in salt mines, but due to various socio-political and economic reasons, the project was postponed for an indefinite period.However, based on the Romanian experience in the oil and gas industry, and also, the exploitation of salt in solution from the National Salt Company of Romania SALROM [5], a governamental company, and on the results of the research of specialized institutes in the country, we will analyze the technical factors specific to the storage activity in the caverns of salt, by identifying the correlations with the thousands of cases in the specialized literature known around the world.
The salt in Romania is done by the National Salt Company SALROM in the salt mines from Ocna Dej, Ocna Mures, Praid, Cacica, Târgu Ocna, Ocnele Mari -Valcea and Slanic Prahova, see the figure 1 [6].There are also studied areas, and one of a high interest is located offshore at Vadu, Constanta County, with two strates between 705 ÷ 905 m and 1,542 ÷ 1,728 m, both with thicknesses of about 200 m [7,8].

Fig. 1. Exploitation of salt in Romania
At Ocna Dej, Praid and Slanic Prahova, the salt is mined dry, using the method with chambers and pillars (square or rectangular).At Târgu Ocna and Ocnele Mari, the salt is mined dry and by dissolution.In Ocna Mureș and Cacica, the salt is exploited only by dissolution.A special analyse will be done to the salt mines from Targu Ocna town, Bacau County, Romania.

Geological and hydrogeological conditions of the Târgu Ocna salt deposit
The Târgu Ocna deposit is located on the middle course of the Trotuş River, at its confluence with the Slănicul, being territorially located in the perimeter of the town of Târgu Ocna, Bacău County.
Miocene salt is brought over the Helvetian deposits of the Pericarpathian unit from the Vâlcele Window.In the mining area, the salt sheet extends in the N -S direction for about 1 km and in the E -W direction for about 600 m, with a maximum thickness of about 350 m in the central part see the figure 2 [9].

Fig. 2. Geological section through the western well field
The Paleogean, Neogene and Quaternary age salt deposit is divided by a fault plane into two compartments: -the northern compartment, located north of the transversal fault, where two salt sheets can be distinguished, one with a higher position and the other lower, with thicknesses of a few meters in the western part until thinning, and in the northeastern part they unite, reaching a thickness of approx.200 m, plunging into the depth and enveloping a core of tailings with a thickness in the east and northeast of more than 100 m, reducing to the west to a few meters.
-the southern compartment (much better known), in the eastern and western parts, is limited by a fault.In the central -northern area of this compartment, the thickness of the salt reaches maximum values of approx.350 m.Towards the south, the deposit is gradually laminated.
The roof of the salt is made up of Aquitanian deposits formed by clayey sands and sandy clays, with intercalations of sandstones, and the thickness varies from 42-54 m in the south, to 135 -161 m in the eastern part, reaching thicknesses from 179 -371 m towards north and northeast.Over these are placed the Eocene and Oligocene deposits belonging to the medio-marginal sheet, predominantly gritty and gritty-calcareous, subordinately clayey or breccia.
The knowledge of the salt deposit was obtained through the information obtained from the digging of 69 wells, of which 35 wells were or are being used as exploitation wells.The https://doi.org/10.1051/matecconf/202438900073SESAM 2023 roof of the salt deposit is made up of Aquitanian deposits made up of clayey sands and sandy clays, with intercalations of sandstones.The salt massif presents numerous sterile intercalations (clay and clayey breccias, shale clays, siliceous sandstones, marls), with variable thicknesses, located at different levels, towards the cover, bedding and in the extremities of the deposit.Their thickness varies from a few centimeters to 10-15 cm.
The current salt mining activity at Tg. Ocna takes place in two mining fields: -Trotuş Mine -rock salt by dry route -Gura Slănic -by kinetic dissolution, with the help of probes Dry rock salt extraction takes place in the Trotuş mine, located south of the old bellshaped mines.Exploitation is done descending in floors, by the method of small rooms with square pillars, with a safety floor between floors.In the Gura Slănic deposit, exploitation is done with probes by kinetic dissolution, the dissolution agent being water or unsaturated brine.
Currently, the exploitation of salt by dissolution is carried out in two areas: the western field, which includes 29 exploitation wells, located along the valley of the Slănic stream, on its western bank and the eastern field, entered into production relatively recently with 6 wells, located on the hill on the right bank of the Slănic stream.The wells in the western field are numbered 251, 253-260, 263-278 and 280-285, see the figure 2 from above and those in the western field are numbered with 1E, 4E, 5E, 6E, 11E and 12E.
3 Reservoir engineering, including volumetric calculations of the cavern where the injection will be done.

Preliminary calculation of storage pressures
We consider the case of a cylindrical cavern, excavated in a layer of salt 200 m thick, at a depth of 270 m, see the figure 3  The roof of the salt is made up of Aquitanian deposits consisting of clayey sands and sandy clays, with intercalations of sandstones.The cave will be analyzed using hexahedral volume finite elements, has the following constructive and location dimensions: https://doi.org/10.1051/matecconf/202438900073SESAM 2023 -depth of 265 m, in a salt layer of approx.200 m thickness -cylindrical cavern, diameter 34 m and height 120 m; Volume =108,950.43m 3 ; will be considered a medium of 100,000 m 3  -the roof of the cave consists of 120 m of salt and a layer of 145 m of sterile The 3D model is created using the physical-mechanical and rheological characteristics of the cavern.It is represented a quarter, due to symmetry conditions.
The average elastic and rheological properties in the table 1, constituted input data for the analysis, considering the creep behavior of salt as a power law with a single component.For the creep problem the constitutive power law with a single component was chosen, see the equation below [11]: where the parameters  = 3.9 • 10  and  = 4.9 were obtained experimentally.
For this example, a pressure gradient of 0.013 Mpa/m is considered, which at a depth of 265 m creates an overburden of 3.445 Mpa.The boundary conditions for the analyzed 3D model are those shown in the figure 5 and correspond to the initial stress state created by the compression stresses: σzz = σxx = σyy =−3.5 Mpa.The distribution of stresses in the massif of salt in the vicinity of the cave is shown also in the figure 4 [11], where the maximum value of the minimum principal stress is 2.928 Mpa.Cavern operation is determined by the demand for the stored products, which impose a pressure regime between a maximum pressure that does not exceed the local geostatic pressure and a minimum one (15-20% of the maximum), but not "zero", to avoid the collapse of the cavern.Thus, the working regime for this cavern is given by the maximum pressure of 29 bars, respectively the minimum pressure of 5.8 bars.

Fig. 4. Distribution of stresses at maximum pressure
The maximum operating pressure must be selected to avoid fracturing of the cavern wall, which in principle occurs when the pressure in the cavern is greater than the minimum principal stress in the rock mass.It is recommended to carry out numerical calculations and verify that the selected operating mode does not lead to the unfavorable redistribution of secondary stresses in the rock mass, which can produce cracks in the rock mass leading to leaks of the stored product.

Some calculations regarding the gas injection in the cavern
We consider that the preliminary calculations can be used with extrapolation to other extraction wells.In the table 2 from below, we have some of the closed wells in the perimeter of Gura Slanic, Tg.Ocna, with their possibilities for storage.If we see at the well no.S282, we see that is very close in dimensions with the preliminary calculated one, see the above paragraph, but we have not yet data for the availability of it for exploitation in the storage purpose.Also, the variation of the flange elevation, show a low stability of the cavern, and a careful monitoring of the sinking evolution is required [9].Only one well for storage purposing at this point is the well no.254, that is theoretically available.The volume of the caverne of the well no.254 is quite huge, about 580,000 m 3 , but this will be not an inconvenient as we can vary the volume as much as we want, by salt brine volume variation.
We will consider just 100,000 m 3 , that will be a fixed one for all type of gases and the calculations will be done for a two-stage compression assembly with three compression units, two of low pressure from 1 to 5 bar (abs) with 2,500 Nm 3 /h and the third one of high pressure from 5 to 45 bar (abs) belongs to National Research and Development Institute for Gas Turbines COMOTI, with a total volume flow of 5,000 Nm 3 /h or 120,000 Nm 3 /day [12].The COMOTI's oil-injected compressors can be used for all types of neutral gases and for combustible gases like natural gases or mixtures of natural gas with hydrogen.In a short time COMOTI's compressors will be available for 100% hydrogen compression.It is considered that the temperature in the cavern will be stabilised at 20°C after injection, by internal heat transfer between the brine and the gas and, also, a minimum storage pression at 5.8 bar (abs) and a maximum storage pressure at 29 bar (abs) [11].
Taking into consideration that we have a volumetric compressor, the fluid flow rate in kg/s will be determined accordingly to the volume expressed in Nm 3 at the initial conditions.The calculations are done for the following fluids: air (0% humidity), nitrogen (N2), carbon dioxide (CO2), methane (CH4) and hydrogen (H2), see the table 3 below.Thereby, we have the following values of mass differences stored in a cavern of 100,000 m 3 , using the values from the table 3 making the differences between the density at 29.0 bar (abs) and the density at 5.8 bar (abs), and will be calculated the filling time using the flow rate from the table 3, considering that the temperature balances at an average temperature of 20°C, see the table 4.An interesting behavior is observed that when gases of different densities are compressed, the time to fill a cavity is approximately the same (a medium of 18.9 days), from the point of view of storage pressures, which facilitates the calculations regarding the energy costs required to store a certain amount of gas.
The reversible (ideal) work for compression of each type of gases in the case of oilinjected compression units, delivered by the National Research and Development Institute for Gas Turbines COMOTI is given the equation [13]: where, k * is a transformed k heat ratio with the following formula: The Pratio is the ratio between Poutlet and Pinlet, mg is the mass flow of the gas, ml is the mass flow of the cooling oil, cp is the specific heat constants for constant pressure, cv is the specific heat constants for constant volume, cl is the oil heat capacity, Z is the compressibility factor, R is the specific gas constant, Ti is the inlet temperature.The reversible (ideal) work reguired to pump the oil for the oil-injection compression process, is given by the equation: where,  is the density of the oil.
The total reversible (ideal) power of the compressor is given by the equation: The calculated power consumed in the process, taking into consideration the efficiency of the compression unit (the sum of efficiencies of gear box and coupling and volumetric efficiency and also the isentropic efficiency),   = 0.723, is: and the calculated values for the parameters, power and energy are given in the table 5, taking a medium filling period of 19 days ~ 456 hours.The calculation of the power consumption was done separatelly for the 1-st and 2-nd stages and totalled.The compressibility factor also has various values for different gases and for different pressure and temperature [14,15].The inlet temperature for each stage is 20°C.The inlet pressures have been establishet to 1 bar (abs) for the first stage and 5 bar (abs) for the second stage.The outlet pressure of the second stage is 45 bar (abs).It is considering that after the second stage it is not made a cooling of the gases, because the balance of the temperature in the cavern.It is observed that are not great variations in the energy consumed for different gases, but for hydrogen compression and carbon dioxide compression the deviation from the medium energy consumed is over 20%, and this is an important value for the gas storage price.Without taking into consideration the compressibility factor for the gases the errors for CO2 for example will be about 15%, that is an important value for the quantity of gases stored in a large cavity.From the hydrogen it is observed a very high outlet temperature at the first and second stages, that shows that compressing in just two stages it is not correct, for a same compression unit type, used for other heavier gases, and should be divided into 3 or 4 stages for the compressed volume.Also, as the molecule of the hydrogen is very small, it is required a higher quantity of oil for cooling the gas into the compression unit, and a higher power need for intercooling [16].

Conclusions
In the present paper, were not addressed the problems related to the monitoring of caves and the problems related to the variation of the pressure in time, due to escapes through the pores of the cave or through the defects caused by rock inclusions, etc., but were addressed problems related to the calculation of the minimum and maximum gas storage pressures, calculated by finite element methods with the help of FLAC 3D software, as well as the filling times given by an existing compressor belongs to National Research and Development Institute for Gas Turbines COMOTI, with a capacity of 5,000 Nm 3 /h.The existence of a cavity in conservation, namely at the well S254, located outside the habitable zone and outside the Natura 2000 protected areas, was also taken into consideration.The existing well has a cavern capacity of 580,000 m 3 , but the intention is to gradually use the capacity by gradually removing the existing brine solution from, left inside for stabilization of the cavern.The injection and, also, the withdrawal of the gases, was considered at a constant rate, but in reality the process is done at a variable rate due to the value of the pressure inside of the cavern.For a lower pressure inside, the injection rate is high and for a higher pressure inside, the injection rate tends to 0, until the pressure is equal with the maximum pressure of the compressor, and conversely for the withdrawal of the cavern [17].
In the future work, will be addressed the problems of round-trip efficiency for a given configuration, with more involved parameters, like termical variation of the gases inside of the cavern, caushion gas capacity, etc.A more accurate calculation will be involved for the cavern stability with the data collected by a sonar device named "cavernometer", that measures the cavern profile and displays volumetric results in two-dimensional (2D) or isometric and three-dimensional (3D) plans [18].The measurement principle is based on scanning the cave walls point by point by ultrasound signals that are digitally recorded and continuously monitorized.All signals are transmitted as a general section diagram along with positioning data and are used for interpretation.Gas storage softwares are also available, see also the "CavBase GasStorage", used by Socon [19], for about 23 years in natural gas storage and it is used for simulations for more than 130 caverns in Germany.
Another interesting salt deposit for further studies is the Vadu area, Constanta County, with a strategical position, situated in the Black See offshore geological bed, with two strates at 700 m and 1,500 m of about 200 m thickness [7], that can be used for storage of hydrogen for suplying the ships, comecial or militarry, that can be an important factor in decarbonising of the atmosphere and for reducing the CO2 footprints, according to the European Climate Law, overseeing the greenhouse gas emissions with net reduction target of 55% below 1990 levels in 2030 [20].
Although the number of storage caverns in Romania is increasing, and there are fewer and fewer accidents.One reason for this is that the recent caverns are equipped with a double column anchored in the salt layer or, columns with an annular space filled with water or brine.If the cavern passes the integrity and tightness tests, the injection of the stored product into the cavern may begins.The problems of the loss of the tightness of the cavern come from problems in the well, and less caused by fractures or microfractures in the perimeter of the . The simulations are done by a Socon software [10].

Fig. 3 .
Fig. 3. Vertical sections through salt caverns in the perimeter of Gura Slamic Tg.Ocna, FLAC 3D simulation (a.The mesh of the cavern; b.The dimensions of the cavern)

Table 1 .
Mechanical and rheological properties of the model.

Table 2 .
Some closed wells in Târgu-Ocna and availability for CO2 storage.

Table 3 .
Mass flow calculation for the fluids used in calculation.

Table 5 .
k*, Toutlet, Z and the theoretical energy consumed for compression/filling period.