MATEC Web of Conferences 342, 06006 (2021)
UNIVERSITARIA SIMPRO 2021
https://doi.org/10.1051/matecconf/202134206006
Study of the possibilities of CO2 storage in the
underground caverns of dissolution salt mines
Sorin Popescu1,*, Mirela-Ancuța Radu2, Stela Dinescu1, and Iulian Vladuca3
1University
of Petrosani, Department of Mechanical, Industrial and Transport Engineering, 20
University Street, Petrosani, Romania
2National Institute for Research and Development in Mine Safety and Protection to Explosion INSEMEX Petroșani, 32-34 G-ral V. Milea Street, Petroșani, Romania
3National Research and Development Institute for Gas Turbines COMOTI Bucharest, 220D Iuliu
Maniu Avenue, Bucharest, Romania
Abstract. Evaluating the possibility to store CO2 in salt mines is made
complex because of the lack of necessary data and different mechanisms
that act on different time scales. This analysis can be made using
traditional methods of geographical survey, lab experiments and digital
simulations. Storing of CO2 needs to be done in a stable geological area.
The cavity must be thoroughly analysed for dimensions, depth,
permeability and porosity. Based on studies made by the University of
Petroșani some areas subject to collapse have been monitored using
topography and sonic measurements with the aim to understand their
possible evolution. Monitoring caverns and underground cavities is done
using sonar. The sonar is a good instrument to manage a cavern, it
increases its safety while it is being exploited and offers important data for
the geomechanics model. CavInfo software package has been specially
created to analyse and show individual caverns or systems of caverns.
CavView II software allows for results of cavern sonar surveys to be
displayed in a variety of ways, with the added possibility to compare
surveys, analysis of caverns and export of data.
1 Introduction
Salt caverns are constructed in naturally occurring thick salt domes, deep underground. Salt
formation can be found in almost every part of the world with some exception around the
Pacific Rim. Salt caverns are a proven medium for hydrocarbon storage as salt acts as a
natural sealant, trapping the natural gas inside the cavern. Salt caverns for gas storage use
are formed with a leaching process by pumping hot water to dissolve the salt and removing
the resulting brine via a single well, which then serves for gas injection and withdrawal.
The storage capacity for a given cavity volume (several hundreds of thousands to several
million cubic meters) is proportional to the maximum operating pressure, which depends on
the depth [1]. Sequestration of CO2 in salt caverns allows significantly higher sequestration
efficiency (by at least one order of magnitude) than geological sequestration of CO2 by
*
Corresponding author: sorin.popescu@toposystem.ro
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons
Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).
MATEC Web of Conferences 342, 06006 (2021)
UNIVERSITARIA SIMPRO 2021
https://doi.org/10.1051/matecconf/202134206006
other means. Although CO2 is not as hazardous as methane and other natural gases
commonly stored in salt caverns, CO2 leakage should nevertheless be avoided. Geological
sequestration of CO2 is a mitigation option for significantly reducing CO2 emissions into
the atmosphere. This technology is immediately available and technologically feasible. A
cavern filled with supercritical CO2 will close in, thus reducing its volume, until the
pressure inside the cavern equalizes the external stress in the salt bed. A single cavern of
100 m in diameter may hold 0.5 Mt of CO2. A single cavern may not satisfy the needs of
large CO2 emitters, but arrays of such caverns can be built as a regional repository in the
extensive and thick salt beds, without impairing general security of the repository [2, 3].
2 Objectives and challenges
The transposition and implementation of CCS Directive into Romanian legislation and the
application of CO2 capture and storage in the domains of Romanian economy determine a
series of benefits, besides the ecological ones, and the contribution to providing the balance
of the global climate system, such as:
the increase of technical knowledge that regard the capture, transportation and
storage of CO2 and of technological innovation.
the decrease of energy production costs in the case of the units that implement CCS
technology, through employing the mechanism of trading the shares of greenhouse gas
emissions.
continuing to operate thermo-electrical plants that use lignite, including coal mines,
which provide raw materials and related transportation networks.
generating new jobs along the whole implementation chain, from planning,
execution and operating to monitoring.
generating new professional specializations and new educational programs for
technical colleges and universities.
determining the foundation of a new specialized field of industry.
extending the implementation of CCS technologies to all the operators that generate
greenhouse gas emissions.
integrating Romania within the European CO2 transportation infrastructure.
increasing the capacity of exploiting the crude oil and natural gas reserves through
injecting CO2 in the deposits considered exhausted, owing to the increase of the retrieval
coefficient.
3 Project desired implementation area
Figure 1 and Table 1 shows the characteristics of some closed wells in the Tg Ocna area.
Based on a number of previous studies and monitoring the stability of these wells over the
time, we have established the opportunity to use them as CO2 deposits for as long as
possible. Thus, closed wells S254, S251 and S268 are proposed for CO2 storage. In case of
a request from an investor, an offer could be submitted to NAMR (National Agency for
Mineral Resources) to obtain the exploitation authorization for the geological storage of
carbon dioxide, in accordance with the provisions of Government Emergency Decree no.
64/2011 on the geological storage of carbon dioxide, approved with amendments and
completions by Law no. 114/2013 and its subsequent amendments and completions [4].
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MATEC Web of Conferences 342, 06006 (2021)
UNIVERSITARIA SIMPRO 2021
https://doi.org/10.1051/matecconf/202134206006
Fig. 1. Area of the well S268 under preservation in the mining perimeter of Targu Ocna.
Table 1. Some wells under preservation in the mining perimeter of Targu Ocna.
Well no.
S254
S255
S282
S251
S280
S275
S268
S280
Elevation
variation
[mm]
0
-13.60
-14.10
+1.10
-13.70 ÷ -14.5
-19.10
+2.7 ÷ 4.9
-12.9 ÷ -14.5
Period
[years]
150
36.74
35
454
34.4
26
∞
34
Volume of the
cavern
[m3]
580714.88
351193.25
105416.82
Diameter
[m]
200078.68
67.6
50
200078.68
50
178
91.6
44.6
Wells for CO2
storage
(Yes/No)
Yes
Yes
No
No
Yes
No
Relying on the research carried out between 2004 and 2015, the areas in danger of
sinking were monitored through topographic and sonic measurements; meanwhile, the
prognosis of the evolution of those areas was analyzed through mathematical methods
based on geo-mechanic studies. As a result, relying on the analytical models, a prognosis of
the phenomena for a medium and long term might be carried out. Maximal registered
sinking is much lower than the maximum of 2.5 m, namely 1.7 m that represents the
allowed sinking calculated in relation with the rheological parameters that might determine
the beginning of the uncontrollable dislocation process. These results match the calculation
regarding the safety coefficient of a cavern. In the analyzed case, R>1.6, meaning that
resistance to rock is higher than failure tension, a fact that allows us to conclude that the
analyzed cavern does not show any risks in terms of its roof, walls or surface stability.
FLAC 3D software for geotechnical analyses of the soil, rocks and underground waters
will analyze the movements of the rocks in the salt mines. FLAC 3D employs an explicit
formulation of the finite volume that captures the complex behaviors of the models
displaying a nonlinear material behavior or are instable. The analysis may be applied to the
engineering planning of civil, mining and geotechnical digging (for instance, slopes,
tunnels, caverns, etc.) and constructions (dams, foundations, walls, etc.) on soil, intact rock
and mass rocks (for instance, strongly fractured rock) [5].
The analysis of the phenomenon of sinking and change of the surface should be carried
out in accordance to certain data regarding the physical and mechanical features of the
rocks in the area as well as in accordance to the size of the caverns. Owing to the fact that
the features of the rocks may be clearly observed even from the exploration stage, the main
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MATEC Web of Conferences 342, 06006 (2021)
UNIVERSITARIA SIMPRO 2021
https://doi.org/10.1051/matecconf/202134206006
issue of the process of modeling is the identifying of the manner the size and shape of the
cavern having resulted from salt dissolution progress.
4 Monitoring
Monitoring is a compulsory requirement demanded by governmental regulations and is also
required for safety purposes. From a technical point of view, monitoring may provide
important data for increasing productivity; it might avoid exploitation accidents, calibrate
geo-mechanical models and increase exploitation safety.
A unitary monitoring system, called ecological or integrated, has been created that
works globally. The questions that the monitoring system must answer are [6]:
the purpose of monitoring
monitoring control
monitoring methods
time scale and frequency of data collection
the chosen variables and processes
data analysis methods
data interpretation
the amount of data needed for a maximum of information.
So, the monitoring and surveying of land sink phenomena, both through exploiting salt
in the underground owing to the dry method and owing to dissolution salt mining, is a
subject of interest included within our preoccupations.
The monitoring was also imposed by the package of procedures created by National
Agency for Mineral Resources from Romania, in order to geologically store carbon dioxide
in our country [4].
The software package CavInfo Software Suite created by SOCON Sonar Control
Kavernenvermessung GmbH, is specially designed for the analysis and display of
individual caverns and entire cavern fields. It contains the individual programs CavViewII,
CavMap and CavWalk Pro, which are perfectly coordinated with each other for smooth
data transfer [7].
The echometric measurements for a single cavern may be analyzed by way of
CavViewII. CavMap allows the simultaneous evaluation of several caverns belonging to a
mining camp. CavLog allows the processing of the recording made during the
measurements in the cavern. CavWalk Pro allows the users to create easily animated,
complex 3D models, tridimensional measurements and a lot of other functions.
SOCON Sonar Control Kavernenvermessung GmbH is a services company specialized
in monitoring caverns and underground cavities and, owing to its products, and become a
part of the exploiting, exploring and research activities in the salt industry in our country.
Sonar (figure 2) measures the profile of the cavern and displays volumetric results in
two-dimensional plans (2D) or in isometric and tridimensional ones (3D). Drilling may be
carried out periodically in order to estimate the change of the volume of the caverns and of
the profile of the caverns.
Sonar is a good device for managing caverns, increasing exploitation safety and offering
important data for the geo-mechanic model. It is efficient for the topography of salt caverns
with a radius up to 300 m.
Distance determining is made by way of measuring response time. The measuring
principle relies on scanning the walls of the caverns point by point. Owing to the fact that
sound speed depends on complex physical relations, it is determined in situ by way of a
special drill module.
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MATEC Web of Conferences 342, 06006 (2021)
UNIVERSITARIA SIMPRO 2021
https://doi.org/10.1051/matecconf/202134206006
Fig. 2. Profile of the cavern measured by a sonar.
4.1 Interpreting echographic signals
Echographic signals are digitally registered, continually monitored and employed with a
view to optimize measurement processes. All signals are transmitted as an overall diagram
of the section together with the positioning data and are used for interpretation. Figure 3
shows a typical recording of the signal (echogram) for a horizontal section (360 degrees).
Fig. 3. Typical recording of the signal (echogram) for a horizontal section (360 degrees).
4.2 Cave integrity testing
SoMIT ® (Sunnar Mechanical Integrity Testing) drill, in Figure 4, is a drill conceived by
SOCON with a view to be used in caverns for testing pressure leakage (losses), temperature
or the change of the brine level.
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MATEC Web of Conferences 342, 06006 (2021)
UNIVERSITARIA SIMPRO 2021
https://doi.org/10.1051/matecconf/202134206006
Fig. 4. SoMIT ® (Sunnar Mechanical Integrity Testing) drill.
4.3 Driling visualizinfg and analysis
CavView II is able to display the results of sonar drillings within caverns in a variety of
manners (Figure 5). The integrated display options vary from horizontal and vertical
sections to volume graphics and even tridimensional representations. Meanwhile, it
includes functions for comparing drillings, for analyzing cavern’s data as well as for data
export.
Fig. 5. results of sonar drillings displayed by CavView II.
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MATEC Web of Conferences 342, 06006 (2021)
UNIVERSITARIA SIMPRO 2021
https://doi.org/10.1051/matecconf/202134206006
4.4 Visualing and editing several caverns
CavMap allows the simultaneous display of several caverns (Figure 6). Such a cavern
mining field may be visualized in plan. As a result, the user may separately determine, for
each case, the most favorable type of display for caverns or pillars. Distances between
adjacent caverns may also be determined as well as the visualization of various sections.
Geological data may be correlated with the geometrical data of the caverns. Owing to the
functions specially adapted for cavern fields and to its open architecture, CavMap is also
suitable as a system of information about operations.
Fig. 6. Simultaneous display of several caverns by CavMap.
Fig. 7. CavWalkPro-3D display for measuring the caverns.
A series of software and CavWalkPro applications allow the user to literally move
among the caverns and cavern fields, including the option of integrating the 3D models
defined by the user as DXF format (for instance, geological formations in the roof or floor)
from an outer source, see the Figure 7.
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MATEC Web of Conferences 342, 06006 (2021)
UNIVERSITARIA SIMPRO 2021
https://doi.org/10.1051/matecconf/202134206006
5 Conclusions
Studies were also made on the thickness of the pillars between the caves, the increasing of
the cave numbers in a salt dome, the depth of the caves and the gas pressure introduced into
the caves being variables that must be taken into account when calculating the stability of
caves over time, for long storage periods > 1000 years. The allowable width of pillars
between two adjacent bedded salt caverns, should be 2.0 – 2.5 times the cavern diameters
when the vertical stress, deformation, plastic zone, safety factors, and seepage pressure are
considered [8]. Once the caverns are closed and storage ready these will pass through the
Mechanical Integrity Test (MIT) using pressures between 90 % and 120 % of the initial
stress at the top of the cavern. The tests will be done with dryed compressed air in steps of
5 bars from 0 to 80 bars. After the tests, the brine will be reintroduced in the cavern thereby
preventing the collapse of it. If the cavern passes the integrity and tightness tests, it starts
the injection of CO2 gas into the cavern. The problems of loss of cavern tightness storing
CO2 comes from problems in the well and never as a result of fractures or micro fractures in
the cavern perimeter, as long as the correct geomechanical design of the cavern is made,
especially in a cavern that will be abandoned indefinitely with pressurized CO2 gas. Today
there are only studies about the CO2 interaction with salt rock, determination of CO2
thermodynamic state variables, CO2 phase diagrams, compressibility Isothermal curves,
and geomechanical analysis of CO2 storage in salt caverns [9].
University of Petroşani, in colaboration with private companies and also with the aid of
the other research centers like National Research and Development Institute for Cryogenic
and Isotopic Technologies from Ramnicu Valcea, National Agency for Mineral Resources,
the National Salt Company S.A., National Research and Development Institute for Gas
Turbines COMOTI fom Bucharest and so on, with National and European founds, is
determined to participate in solving the major emerging global warming for CCS (Carbon
Capture Storage) technologies with a key technology: to use more energy from renewable
sources and minimalise the impact of using the fossil fuel in the energy production.
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