WP 3.3

Despite of all caution during construction of gas reservoirs in the geological subsurface there is a remaining risk that stored gas can leak out of the reservoir due to an accident and ascend into near surface aquifers. Such a leakage can occur at low rates, which are not discovered for a long time period, or at spontaneous high rates, e.g. if a well casing is damaged. The leaking gases may have harmful effects on shallow aquifers which might be used for drinking water supply.

The injection of gases into pore reservoirs causes an increasing pressure in deep aquifers filled with saline water. Since the pressure increase can propagate over long distances, there is a risk of saline water migrating into shallow aquifers, if suitable pathways are present. In contrast to saltwater intrusion in coastal areas it is not predictable where the saline water will intrude into the shallow aquifer. The geometry of a local saltwater intrusion also differs from the well-known coastal salt water intrusion.

For approval of gas reservoirs such risks will have to be addressed in the required environmental impact assessment and plan approval procedure. Suited geophysical methods for detection and exploration of leakages have to be present before a leakage occurs.

WP 3.3 investigates:

• Which geological parameters control the spreading of gases and saline water in shallow aquifers?
• How large can gas phase bodies and saltwater intrusions become in shallow aquifers and what is their geometry?
• Which geochemical changes can the gases cause and how do these changes spread in groundwater?
• Which geophysical methods are suited for initial detection and detailed exploration of leakages?

The intended investigations are performed using numerical models. The simulation program TOUGH2 is used to simulate the spreading of gases and saline water in simple scenarios.

A variation of geological parameters shows which parameters influence the spreading of gas and saline water and which parameters have to be determined at real sites with care. WP 1.2 contributes geological structures and parameters for the few sites in Schleswig-Holstein where the geology has been investigated in detail. At one of these sites the migration of gas will be simulated to identify typical spreading patterns. Geochemical changes due to dissolving gas will be simulated with the program OpenGeoSys (WP 2.1, WP 2.2, WP 2.3).

The simulation results are transferred to the workgroup Geophysics. Forward simulations demonstrate which detection output would result from different measurement methods. The comparison with the originally simulated distribution of gas or saline water will show which detection method is suited to detect gas or saltwater at least under the idealized conditions of the models.