Natural gas under griund storage in the UK and why we need it. Illustrate your essay with specific examples.

Executive Summary

The need to secure energy sources is of vital importance to the development of a country’s economic activity. Selecting the right strategy and finding the most suitable technologies to implement those measures is essential.
In the current market and legal frameworks, the UK is heavily dependent of the supply of imported fossil fuels, in particular natural gas. As a part of the energy security strategy, the UK has focused on developing ways to guarantee that the best possible use of those supplies is made.
The aim of the strategy includes developing adequate infrastructure to guarantee that reliable, cost effective and environmentally sound solutions are put into place. Amongst these measures, the storage of natural gas is of particular importance.
The purpose of the study was to define the main aspects which need to be addressed to assist policy makers in assessing the geological safety and risks of gas leakage from underground storage facilities when dealing with UK applications to develop this kind of sites.
In this context, underground gas storage in salt caverns has been identified as a safe, economic and efficient method that would assist the UK in improving the security of supply of fossil fuels.
There is sufficient evidence in of past cases to understand the risks that could result from the construction of new specially created caverns or the adaptation of existing ones from mining activities and their operation.
A systematic analysis has to be made to minimise safety and economic risk and environmental impacts, by including their consideration in all aspects of the gas storage facility lifecycle.

1. Introduction

Fossil fuels currently represent ninety percent of the UK’s energy needs and will continue to be the main energy source in the foreseeable future. In the short run, most of these supplies will be imported, especially oil and gas.
When considering natural gas as the main source of energy supply, the concerns of the UK government lie on the risks of depending from foreign exports. These include the limited access to reserves, which in many cases, are controlled by the exporting countries’ national oil companies; the constraints on the significant investments required to develop new fossil fuel reserves and transport infrastructure; the political instability, turmoil and terrorism in supplier countries; and accidents and natural phenomena.
To guarantee supply, the UK government has planned to take measures to ensure that the energy system is resilient and flexible, both in the international and local contexts (DTI, 2006). The International Energy Agency (IEA) does not expect the Oil and Gas production to peak until beyond 2020 (IEA, 2006). In the UK, Oil and Gas will remain highly competitive fuels. The UK projects that dependence on Oil and Gas will increase from 74% of the nation’s primary energy use, to 85% in 2020 (DTI, 2006).
In the international context, the UK’s energy security strategy focuses on promoting open international market frameworks, to facilitate investment in the different energy sectors; to arrange contingencies to prevent having lack of stocks due to shocks in the world oil markets; and to encourage political and economic stability in regions which are suppliers of fossil fuels.
In the local context the government will encourage oil and gas producers to maximize production, by investing in full in productive fields. Still it will heavily depend on foreign imports. Hence, to facilitate the access to those imports, the government considers that an essential issue is to develop new infrastructure, as gas storage capacity, to guarantee the supply of foreign gas (DTI, 2003).
In this market and legal context, this study looks into the different types of storage that the UK may use for natural gas and analyses its risks and impacts, by looking at state of the art risk analysis methods and real case studies.

• Chapter 2 will provide a background to understand the issues related to gas storage. It will describe the different types of available stores, focusing on the geological and technical aspects.
• Chapter 3 looks into the case of storage for the UK and describes the policy and standards related to storage.
• Chapter 4 focuses on the impacts and risk assessment methods used for the selected type of underground geological storage of gases.
• Chapter 5 analyses real case studies in the UK and Chapter 6 will provide the findings.

2. Background

This chapter introduces the concept of gas storage looking into the market reasons for storing gas and it describes the existing types of gas storage.

The need for gas storage

Natural gas may be stored to supply both the expected demand of fuels consumption, or base load, and also to cope with unexpected increases in the demand, particularly during colder seasons, known as peak load.
The capacity and requirements for these two types of demand are based on the fact that base load storage will be used for covering seasonal demands, whilst peak load is like an emergency supply for unexpected increases (HSE, 2008).
Gas storage development may serve the purpose of satisfying fossil fuel demand and is also a scientific and technological learning step for the development of other gas capture technologies. Fro example, carbon dioxide could also be stored in the same conditions that natural gas, these techniques could also help the UK to achieve its greenhouse gases emission reduction targets. These mandatory targets are set under the United Nations Framework Convention for Climate Change and its instruments, such as the Kyoto Protocol.

Types of gas storage

The concept of geological storage has been derived from the oil and gas industry, in which geological formations have been capable of retaining fluids (both liquids and gases) for thousands of years. In the same way that oil and associated gases are extracted from underground wells, these fluids may be pumped back into formations for storage. The injection of the associated gases back into the oil wells from where they were produced is a commonly used practice in the oil and gas industry. Its purpose is to maintain the pressure in oil wells, to facilitate the fluid recovery, and is known as enhanced oil recovery or secondary production. The first successful underground stores of gas were in fact depleted gas reservoirs in the United States, in 1915 (naturalgas.org, 2008).
The main aspect that must be considered when studying formations for gas storage is their geological and geographical conditions. The aim of underground storage is to create a suitable natural vessel that will serve as a safe container for storing the fluid.
In the first case, the two characteristics that are most relevant for successful storage of gas are the underground formation rock´s porosity and the permeability. The porosity will determine how much fluid may be stored, since it will fill those pores or gaps. The permeability is a physical property which is related to the rate or speed at which the fluids flows across the rock. This would serve as a limit for injection and removal of the gas from those rocks. A formation would be a better underground storage if it has higher porosity and permeability. That is the cases for depleted oil and gas reservoirs.
In the second case, geographical conditions relate to the fact that supply of gases and their storage must be close either to the places in which gas is received in the UK, or close to the markets for that gas.
Not only depleted oil and gas fields are available for storage but other suitable geological formations, such as saline aquifers and salt caverns may be used for the purpose of storing gases. Aquifers are geological formations which act as natural water reservoirs. The ones used for gas storage are those not suitable for human consumption and would be very costly to desalinate. The latter can be specifically created for storage purposes (naturalgas.org, 2008).
It is worth mentioning that smaller volumes of natural gas may also be stored in a compressed form, as liquefied natural gas (LNG) in facilities specially dedicated for that use. These are tanks which are normally located near the areas of consumption. LNG tanks require high pressure conditions and are used for rapid response to increases in demand, known as consumption “peak shaving”.
Depleted oil and gas reservoirs
In order to use an underground geological formation as a suitable vessel for storage, a set of actions must be taken to condition it, to ensure it will guarantee the permanence and recovery of the fluids stored.
For example, in the case of hydrocarbon wells which are newly drilled, the higher the pressure at the bottom of the well, the easier it will push the fluids out. In other words, the extraction of the fluids is driven by the pressure difference between the bottom and head of the well. The natural flow of gases occurs until the pressure of those fluids decreases, to a level below that of the wellhead. Hence, although there are fluids remaining in the reservoir they might not be recovered unless artificial mechanisms are used, such as mechanical pumping or reinjection of gases.
In the context of gas storage, this phenomenon is translated into the fact that some of the gas stored might not be recovered. This is commonly denominated “physically unrecoverable gas”, and it stays embedded in the reservoir permanently. To ensure that the gas is recovered, a portion of it must be kept in the storage to maintain the pressure for recovery. This is known as the “base or cushion gas” (naturalgas.org, 2008).
Finally the gas that can actually be stored and recovered is known as “working gas” and is the term commonly used to describe the storage capacity of an underground storage facility (UST).
The operation of the storage facility will vary with time based on the pressure differential between the store and the well head or surface valve. When operation begins, and pressure is at its highest levels, the rate of gas recovered is at its maximum level and it will decrease as the volume and the associated pressure of the gas in the storage decreases.
Depleted reservoirs have the advantage of being well studied geological structures and they have also have the existing infrastructure required to inject and recover the produced gas, including the wells, gas pipelines, valves, control systems.
Aquifers
Aquifers may be conditioned to be used as natural gas storage formations. They are considered less suitable than depleted hydrocarbon reservoirs because there is less knowledge about them. To use an aquifer as a gas store it would require to first investigate the geology very thoroughly, including seismic testing, which is very costly. The aim of those studies is to understand the properties of those water reservoirs in terms of the porosity, permeability and pressure. This will be used to determine the capacity (HSE, 2008).
Aquifer formations do not have good “retention capabilities” as depleted reservoirs, which means that if some of the natural gas that is injected escapes. It must then be collected and extracted by special wells, designed to gather the gas escaping from the aquifer. Moreover, when gas is stored in a liquid medium, it has to be dehydrated prior to transportation. Saline waters are very corrosive and would damage pipes. Therefore, to condition an aquifer to be used as a gas store involves the development of the entire infrastructure, such as the installation of: wells, extraction equipment, pipelines, dehydration facilities, and possibly compression equipment.
Therefore, aquifers are more expensive and less desirable as stores for natural gas.
Salt caverns
Salts, chemically known as halites or sodium chloride, naturally deposit in sedimentary layers to form salt beds or domes. The former are thin formations which are normally not more than a thousand feet in height. The latter are thicker layers which can be as much as thirty thousand feet in height (naturalgas.org, 2008).
Salt caverns may occur in nature as empty spaces inside those formations (beds or domes) or can be created to store gas. They have sound properties for gas storage, since the walls of salt caverns are not permeable to the flow of fluids and they have high structural strength. Creating artificial caverns from salt domes is preferred to natural occurring ones because they can de designed and built to ensure that best available safety and operational standards are applied.
Salt caverns are created by dissolving the salts in the formation using water to create open spaces, as depicted in Figure 1. This is made by drilling a well into the dome, injecting water and recovering the water through the same well. The process is known as “salt cavern leaching”. The initial salt caverns where created in “solution mining” of the chemical industry for the production of brine or chlorine (naturalgas.org, 2008).
Salt caverns have several advantages as gas storage vessels. Since they are non porous, they retain less gas. This allows a higher gas recovery than the one of the other types of formations and also the requirement of smaller volumes of cushion gas to maintain them pressurized. Their development needs significant less surface area than that of a depleted gas reservoir.
Halite salts have very high ductility and when they are subject to stress, they undergo a crystal plastic deformation processes known as creep. This means that it has a low susceptibility to fracturing, but when it occurs, the halite flows, allowing fractures and cracks to seal. This last concept is known as viscoplastic nature.
These properties make salt caverns the preferred underground formation choice to be used as a natural vessel for gas storage (HSE, 2008).

3. UK Gas supplies and storage

As the gas production from the UK Continental Shelf declines, the UK gas demand is set to continue growing over the next fifteen years, driven mainly by increased demand from the power sector. By 2010, imports could be meeting up to a third or more of the UK’s total gas demand, potentially rising to around 80% by 2020 (UK DTI, 2007). As a result of this increase in imports, the UK will benefit from a greater diversity in supply, but will face bigger risks and impact of any overseas disruptions to energy sources as supply routes become longer and across more countries.
A way to ensure that reliability of supply is met is by having the capacity to store gas in the UK in significant volumes. Hence, the UK government is set to increase substantially the storage capacity available.
The Energy White Paper of 2007 predicts that if all of the planned storage projects go ahead, the proportion of peak day demand that could be met by storage operating at its maximum level would increase from 24% in 2006/07 to between 40% and 60% by 2015/16 (UK DTI, 2007).

Storage Capacity

The UK Health and Safety Executive has stated that the storage of gas in onshore and offshore formations is only possible in certain areas, which have the required geology and geological structures. These are present in a limited number of locations in Great Britain.
In the case of salt formations, suitable salt strata are thought to occur in the Wessex, Cheshire and West Lancashire basins and along the Yorkshire North Sea Coast, as shown in Figure 2.
The existing UK Underground storage facilities and gas infrastructure are shown in Figure 3.

Standard and Legal frameworks

Governments must provide the adequate structure, in terms of legal framework and economic resources, to allow the development of energy supply. The right incentives, in the form of policies and taxation must be developed to achieve a more balanced economic structure.
Amongst these policies is the promotion of technological innovation. Technology must advance at a proper pace to achieve the objectives of securing energy supply. If left to the market, the rate of progress is unlikely to address social and environmental equity. The way in which governments can address this matter is either by setting more stringent laws that promote the emergence of new technologies and by investing and supporting research either with public funds or policies.
Stringent environmental and safety laws are seen as a significant driver for technological innovation. Taxation must be set into place in a level that encourages dynamically efficient innovation, that tackle environment, competitiveness and employment simultaneously. Those profits earned from taxation can serve two purposes. They can either be used to remediate or take action against existing pollution, or could be used to compensate the fiscal balance and reducing other taxes.
The major legal documents that regulate the gas storage activities in the UK are:

1. The Control of Major Accident Hazards Regulations of 1999 (COMAH), which legalises safety issues during construction and operation of the site;
2. the Borehole Sites and Operations Regulations of 1995 (BSO), also dealing with operations;
3. the Pipelines Safety Regulations of 1996, focuses on oil and gas field storage pipelines running to and from gas storage sites; and
4. the Dangerous Substances and Explosive Atmospheres Regulations of 2002, looking into the environmental aspects.

When an operator plans to build a new storage facility, the COMAH regulations require that they submit information regarding safety, pre-construction and pre-operational, which is reviewed by the competent authorities. The aim of this practice is to ensure that the operator takes all of the necessary measures to prevent any accidents and minimise environmental impacts (UK HSE, 1999).
Also, underground gas storage formations are covered by three British and European Standards, BS EN 1918: 1998, which explain the concept and requirements to develop these facilities.
The standard for salt caverns is BS EN 1918-3: 1998 and states that salt caverns are generally seen as suitable and preferential sites for the storage of oil and gas, due to the almost zero permeability to gas and the structural strength of their walls and their viscoplastic nature, which leads to the healing of any cracks and faults.
The BS document also lists a series of properties and measurements that are required for the design of caverns, which are the following:
“1. The “mechanical disturbances” for cavities that require determination and include

1. The change in volume loss by creep in the salt formation (convergence)
2. The distribution of the cavity wall and floor deformation
3. The distribution of the stress induced by the cavity in the surrounding rock

2. The “principal stability parameters” that need to be defined within the cavity design and include:

1. The cavity geometry (shape, height, diameter, roof guard)
2. The positioning (e.g. well pattern, depths, pillars, distances to caprock, bedrock)
3. The distance to subsurface neighbouring activities
4. The maximum operating pressure, which shall always be less than the overburden (lithostatic) pressure
5. The minimum operating pressure to prevent closure of the cavern by salt creep” (BS,1998)

The above calculations and assessments are determined by taking samples, which are subject to rigorous laboratory or in situ stress and strain tests. The aim of the tests is to demonstrate that the cavity will be mechanically stable and capable of containing gas under the proposed or permitted operating conditions (BS, 1998).
The standard also requires that consideration is given to the cavern´s operation and performance under emergency conditions and procedures that will enable safe shut down of the facility.

4. Impacts and risk assessment

The hazards and risks associated with gas storage in geological formations are a key issue to investigate when plans to build an UGS are discussed.
The hazards and risks associated with storage of natural gas relates to many disciplines, such as economic risks; health, safety and environmental effects; public perception and trust.
The major hazards associated to an underground natural gas storage facility are considered to be the likelihood of a leakage taking place. Risk is defined as “the likelihood of actual damage or loss resulting from leakage into adjacent and overlying formations and thence to the surface, which carries two very contrasting risks” (Lippmann & Benson, 2003). The main effects are:
– that the stored product may escape, reaching ground surface where it could generate damage to human health and the environment,
– the loss of an economic commodity (stored gas), and
– mismanagement or unexpected events lead to the collapse of the caverns, allowing the release of brine, that may pollute ground waters, or generate subsidence of the ground.
Although economic costs and its associated risks are the common variable to analyse for most projects, this study focuses on the safety, health and environmental (SHE) risks. Still, it is worth noting that various processes and consequences will be common to both the economic and SHE aspects.
The mechanisms for which the salt caverns may collapse are related to its geology, and there are the following are the principal factors:

1. salt creep,
2. uncontrolled leaching and
3. the presence of anomalous zones in the salt formations
4. the presence of other minerals such as gypsum or anhydrites
5. fractures of non-halite interbeds
6. wet rock heads and subsidence

The first part of this chapter describes those risks. Also there are external factors which can lead to risk, such as seismicity, which are described.
The gas storage by injection, which is the process by means of which a volume of gas is pumped into and underground cavity, defined as a process that typically includes phases of: separation of the gaseous stream from liquids (capture), preparation, compression, transportation and injection. For underground gas storage the risks can initially be studied by breaking down their life cycle into three phases:

during construction of the facility,during operation and following closure and abandonment of the facility.
The second part of this chapter looks at the mitigation measures for each phase.

Failure mechanisms of salt caverns

Salt creep
The mechanical properties of salts are a key characteristic to consider when analysing their suitability for storage. Salts behave as a fluid since they flow or creep under small stresses, temperature and the rate of deformation.
In terms of temperatures, when subject to conditions below their melting point, which is close to 800 °C, salts deform in a plastic manner. This means that they become stronger but more brittle.
Under stresses, salts might suffer of micro fractures which can lead to faults. The UK HSE report recognizes the following conditions as those in which rock salts may fail due to brittleness (UK HSE, 2008):

• when exposed to high rates of strain and the shallow depths, having lower temperatures and confining pressures;
• when losing the confining pressure very rapidly. This case might happen during cavern creation;
• in thin salt beds or those which have layers of mudstones; and
• when the salt crystals contain other materials, known as inclusions, which harden the rocks.

Uncontrolled leaching
When constructing salt caverns, the removal process is known as leaching, as described in Chapter 2.
To control the rate of dissolution of salts there are two main mechanisms:
– injecting and withdrawing the dissolved brine, known as compensated mode and
– injecting diesel to be uses as a “blanket” fluid to protect the created cavity.
When the leaching process in not controlled then the solution rate of the salts is faster than the salt creep process. This may lead to creating a cavity in the rock bed which is unstable or shaped in inefficient ways. In the worse case conditions, uncontrolled leaching could even lead to the collapse of the cavern roof and walls.
Presence of anomalous zones
Salt rock beds are uneven formations which contain not only halites but also other salts, and are known as anomalous zones.
These may be:

1. highly soluble salts, such as potash salts;
2. fractured areas between salt bed; and
3. areas which have suffered of natural leaching and might contain fluids, such as pressurized brine or gases (methane or nitrogen).

The presence of these zones will affect the shape of the constructed caverns and in some cases the operations of the storage.
The presence of gypsum or anhydrites
Some salts deposits normally contain two minerals which can have adverse effects on the development of UST in salt beds. These are gypsum, hydrated calcium sulphate (CaSO4.2H2O) and anhydrite (CaSO4), calcium sulphate. These two mineral species are found in chemical equilibrium, according to the humidity of the formation.
Gypsum can be a serious hazard since it can increase dissolution that may lead to collapse of the overlying areas of soil subsidence. During cavern closure, gypsum may convert into anhydrite and change in volume, which can result in damage of the structure.
It is important to identify the presence of these minerals prior to the selection of the site to develop the storage.
Fractures of non-halite interbeds
Despite the fact that salt beds rarely have fractures, and since they have a viscoplastic nature, they tend to seal over geological times, the presence of fractures could have impact in the gas tightness and safety of the storage. Fractures can develop in the walls of caverns generating a reduction in the pressure of confined fluids.
Fractures are more common in non salt interbeds or enclosing mudstone layers. They may develop in a number of ways, mainly being sedimentary or a consequence of tectonic movements.
The presence of fractures both in salt beds and non salt interlayers must also be checked when selecting the site for storage development.
Wet rock heads and subsidence
Salt beds normally occur in the subsurface as a consequence of the dissolution generated by flowing ground waters. Halites are the most soluble minerals, in three orders of magnitude when compared to anhydrite or limestones, under normal conditions.
As unsaturated ground waters flow they go around mudstones areas that are in layers or beds with the halites, forming breccias. The formation of breccias occurs in spots known as “wet rock head”.
The importance of this issue for storage is that is relevant to the stability of the soil, the movement of brines and the potential that further dissolution could result in soil subsidence.
Seismicity
The natural geological conditions of areas in which salt formations occur are main elements to consider when evaluating the potential for underground storage.
Areas in which seismic activity is significant are more prone to producing fractures in caverns. The rock formations may be fractured, known as having faults, which can lead to displacements. These faults can lead to the leakage or failure of gas tightness of the storage caverns.
Hence, modeling of the behavior of the faults both under the construction and operation phase must be undertaken, to understand the likelihood of gas releases or cavern damage.

Mitigation Actions

Based on the exposed risks and impacts, the following section describes the main actions taken to mitigate them.
Site selection and design phase
The location of caverns depends mainly of geological conditions, the availability of suitable salt bed formations, their relative location to gas consumption areas, and their proximity to existing gas transmission infrastructure.
Caverns are designed to facilitate operation and enhance safety. They are normally ellipsoidal, with a longer vertical axis, to increase stability. Caverns are constructed as small, confined networks, to maximise the efficiency of gas flow but also to protect them from future anthropogenic uses, once the storage activities are terminated.
Salt domes used for storage are located normally between 450 to 2000 m below surface. The beds are normally not more than 300 to 400 m in thickness, as presented in Figure 4. Since storage pressure increases with depth, it is desirable to create caverns as deep as possible, to maximise the volumes of gas to be stored.
Cavern design and spacing is conducted both by performing computer based simulations and also materials testing in the lab. Cavern design encompasses the definition of the mechanical and principal stability parameters specified in the standard BS EN 1918-3:1998 as listed in chapter 3.
Construction phase
Construction of cavers must be done using techniques that avoid uncontrolled leaching. As described in previous section of this chapter, there are several methods to control the shaping of caverns, to avoid future failures of the storage, namely:

1. Brine circulation, which involves injecting and withdrawing liquids. This is done through a set of concentric borehole pipes. The outer, wider diameter pipe is known as casing and the inner tube through which the fluids circulate is the tubing. The process is shown in Figure 5.

o Direct circulation: fresh water is injected through the tubing and the brine is collected from the annular space between the tubing and the casing. Injected water has a lower density than the brine; thus, by being driven by a pressure differential, it causes convection which leads to salt dissolution. This process creates cavities which have a more cylindrical shape.
o Reverse circulation: in this case fresh water is injected through the annular space and brine is collected from the tubing. In these cases, the cavities tend to have wider roofs than bases.

1. Use of blanket fluids, such as diesel, to control the area of dissolution.

Post-Construction phase
The main aspect to consider when caverns are completed is to ensure their air tightness. This is done by conducting pressure and integrity tests. A Mechanical Integrity Test (MIT) is used to test cavern and wells tightness. It involves air tightness testing of the wells and the cements or casings to verify that there is no leakage behind the casing. A sonar survey is also generally performed.
Operational phase
The way in which gases are retrieved from the storage may compromise its integrity, since the pressure distributions in the cavern might affect its structure and lead to well failure.
A common way of operation underground storages is the “brine-compensated” mode, consisting of the injection of an equal volume of brine as of the fluid being removed. Natural gas caverns may be operated in this way, but they generally rely upon the pressure of the stored gas to generate pressure or “lift” to recover the gases.
During the storage of natural gas, pressure may build up as the gas is injected and then falls as the gas is withdrawn. If the well’s casing or wellhead fail and are not checked, the entire working gas volume of the entire cavern could be expelled. This release may happen very suddenly and fast, depending on the initial gas pressure and the leakage rate.
The failure of the wellhead can also result in a rapid and catastrophic loss of pressure of the cavern and release of gas. A consequence would be the severe stressing of the cavern walls, which may lead to collapse. These risks are prevented in the design phase by including fail-safe valves below the wellhead, which is a common practice in the oil and gas industry.
Abandonment phase
Despite considering that no stress would affect a cavern after its use and operation has ceased, special plans should be made for its decommissioning. When caverns are meant to be abandoned, they require to be pressurized, to a level just below of their lithostatic pressure (the overburden pressure) and then sealed.
Still, leakages and changes in the pressure may occur, which must be modeled and monitored to ensure the integrity and safety of the abandoned sites.
The main causes that may lead to pressure changes are: salt creep, thermal expansion of the cavern fluid, transport of the fluid out of the cavern into surrounding porous layers through porous non saline beds, leakage along the well path and additional dissolution and precipitation of the salt in the cavern, driven by temperature differences and convective currents.

5. Case studies in the UK

This chapter looks into a set of gas release scenarios from UGS facilities and describes real cases that have occurred in the UK. The purpose of the study is to define the key issues that need to be addressed to assist policy makers in assessing the geological safety and risks of gas leakage from underground storage facilities when dealing with UK applications to develop UGS sites.
The UK Health & Safety Executive has conducted a thorough review of the existing cases. That study has identified 65 reports at facilities from America and Western Europe. Those incidents were of varying severity and nature. They have resulted in 9 fatalities, almost 62 injuries and at least 6700 people having been evacuated (UK HSE, 2008).
The HSE report identified a vast set of release scenarios and their consequences, which are presented in the following section. These scenarios have been used as a framework to analise release cases both in the UK and other countries.

Economic and safety impacts scenarios

Case study analysis

The following table lists the cases identified by the HSE report, under the previously mentioned scenarios in Canada, France, Italy, Poland, Romania, the US and the UK. These cases were found in brine fields and salt mines and most of them resulted in catastrophic ground failure. Those failures lead to economic and environmental losses.
The table also lists the main aspects that should be considered for new designs.

6. Conclusions

Underground gas storage in salt caverns has been identified as a safe, cost efficient method that would serve the UK’s strategy of improving the security of supply of fossil fuels.
There is sufficient evidence in past cases in the last fifty years to understand the risks that could result from their construction and operation. Measures can be devised to minimise those risks and environmental impacts, by including these dimensions in all aspects of the gas storage lifecycle.
The measures to mitigate risks for the different stage of the lifecycle include:

1. Site selection:the adequate selection the site, based on the natural geological conditions, gas markets locations and infrastructure availability;
2. Design: modelling the caverns construction and operation process to ensure maximum safety conditions, and following the requirements of existing legislation, such as the COMAH and British standards;
3. Construction: using the best available engineering and mining techniques;
4. Operation: conducting a thorough monitoring of operations to ensure that release scenarios are avoided and following the conditions established in regulations;
5. Closure:taking precautionary measures to guarantee fluid tightness and cavern stability.

The minimum requirements pointed out in thus analysis include:

1. Characteristics of storage
2. Prediction of results (simulation of cavity behavior)
3. Technology used during the injection through wells
4. Monitoring and verification of storage
5. Analysis of risk, mitigation and remediation
6. Abandoning wells and strategies for leaving intact the halite rocks

All of these aspects need to be considered by policymakers to promote the use of these gas storage techniques.