GAS STORAGE

In Denmark, gas consumption varies throughout the year, topping during winter time.

On a cold winter day, gas consumption may reach about 30 – 33 million Nm³ on a daily basis. The maximum delivery of natural gas from the fields in the North Sea amounts to about 22 – 24 million Nm³ on a daily basis. In order to manage this dif-ference, gas storage facilities are necessary. During the summer, when consumption is low, natural gas is pumped down into the storage facilities, and during the winter the gas stores are used to cover consumer requirements.

In addition, the storage facilities function as emergency supplies in the event of an interruption in supplies from the Danish North Sea gas fields, or if there is a gas pipe-line rupture in the transmission network. The storage facilities are dimensioned so as to handle supplies of natural gas to the uninterruptible gas market, which for instance comprises domestic heating, for a period of about 60 days. This is the estimated time it takes to repair a gas supply rupture in the North Sea.

There are currently two gas storage facilities in Denmark. The locations of these two facilities are shown in figure 3.1.

One facility is located at Stenlille on Zealand, where gas is stored in porous sand-stone layers at a depth of around 1,500 m. This facility is owned by DONG Energy.

Approximately 1.5 billion Nm³ of natural gas is injected into the facility at Stenlille, of which approximately 580 million Nm³ can be utilized (working gas).

The other gas storage facility is situated at Lille Torup in northern Jutland. At this facility, the gas is stored in seven large subsoil caverns that have been created by the flushing out of a salt diapir. Box 3.1 explains what a salt diapir is. The caverns, which are situated at depths of 1,000 – 1,700 m, are 200 – 300 m high and 40 – 60 m in dia-meter. This facility is owned by Energinet.dk. At the Lille Torup facility, approximately 700 million Nm³ can be stored in the seven caverns, of which approximately 440 mil-lion Nm³ of gas is utilized (working gas).

In 2007, the DEA received an application for a licence to establish and operate a new natural gas storage facility at Tønder. The application was filed by the company Dansk Gaslager ApS. The application is currently being processed by the DEA.

STORAGE OF CO2

The potential for reducing atmospheric CO2 emissions is a topical issue in many contexts. One possibility is to capture and then store CO2 from major point sources such as power stations. This technology is often referred to as ‘CCS’, which stands for

‘Carbon Capture and Storage’.

Technology is available today to remove CO2 from the flue gases of power stations, and research is being carried out to improve and develop new technology to separate the CO2 from power stations more energy-efficiently. The technology involves the CO2 being trapped at the power station and then compressed and transported in liq-uid form to a special subsoil storage facility. The compressed CO2 will be transported in pipelines.

The subsoil storage of CO2 must take place at locations with suitable geological conditions. In Denmark, this will typically be porous and permeable sandstone layers at depths of more than approximately 1,000 m. Storage at this depth means that the CO2 will be in liquid form due to the higher pressure. The sandstone layers must form a structure where the injected CO2 can be trapped in the porous layers. Above the sandstone layers, there must be a tight clay formation, which is impermeable to CO2, so that the stored CO2 cannot escape. Such optimal geological conditions for the storage of CO2 exist in many parts of the Danish subsoil, both on land and offshore.

The layers and structures in the subsoil that can be used for CO2 storage may also be used for other purposes, such as storing natural gas or geothermal heat. Consequently, prioritizing the use of the subsoil is necessary.

Another possibility is to inject the CO2 into the oil fields of the North Sea. This has the added benefit of enabling more oil to be produced from the fields, as the injection of CO2 in an oil field can release more oil from the layers, oil that would not other-wise be recoverable. Some of the injected CO2 will however be extracted together with the oil, and it is therefore necessary to separate this CO2 from the oil and rein-ject it in the subsoil. This method is not yet being used in the oil fields of the North Sea, primarily because the method is considered to be very expensive, one reason being that it would require many new installations as well as modifications to existing installations in the North Sea.

In 2008, licences were issued to both Vattenfall and DONG to undertake preliminary investigations of the Danish subsoil with a view to assessing the potential for storage of CO2. In this context, in autumn 2008 Vattenfall performed a 2D seismic survey of the subsoil northwest of Aalborg in order to map the Vedsted structure. This seismic

survey is discussed in chapter 1, and the location of the seismic lines is shown in figure 1.7.

The existing Danish Subsoil Act addresses the use of the subsoil for storage purposes, including CO2 storage. In connection with EU's climate and energy package, a new directive addressing carbon capture and storage in the subsoil has now been adopted.

The directive contains a system for allocation of exploration and storage licences in connection with the deployment of carbon capture and storage. In addition, the direc-tive contains a number of regulations regarding monitoring, etc. of the stored CO2. It is still up to the individual member states to take a position as to whether they want to use this technology and, if so, determine the areas in which CO2 will be stored. The new directive must now be implemented into Danish legislation. This is expected to take place by an amendment of the Danish Subsoil Act, among other statutory provi-sions.

The production of oil and natural gas impacts the environment, both through emis-sions to the atmosphere of gases like CO2 and NOx and through the discharge of chemicals and oil residue into the sea. Initiatives to reduce this impact are launched on an ongoing basis. In 2008, specific attention was focused on reducing energy consumption offshore and on the most recent follow-up of the Minister for the Environment’s Offshore Action Plan, which aims to limit the discharge of foreign substances into the sea to the greatest extent possible.