Martin Sauter, Torsten Lange, Karolin Brosig & Wiebke Jahnke, Universität Göttingen
The presentation addresses in particular the assessment of the degree of leakage of fracking fluids in the subsurface geological environment, the operational and subsurface processes that could possibly lead to leakage of fracking fluids into the overburden above of the gas reservoir and the spatial and time scales to be considered to evaluate potential impacts of leaking fracking fluids on humans as well as on the ecosystem - by reviewing previous experience and State of the Art knowledge about fluid migration in barrier rocks, assessing the role of local (Münsterland, Lower Saxony Basin) barrier and aquifer formations in the spreading of the fracking fluids, characterising the effective hydraulic parameters of the barrier materials and aquifers, assessing the impact of fault zones on vertical groundwater and solute transport, defining model scenarios that describe the flow of fracking fluids from the reservoir layer, including the respective boundary conditions and by formulating recommendations for preventative and monitoring measures to provide assistance for the decision making process for future fracking operations.
The determination of the fluid spreading is based on the cumulative effect of several unfavourable factors, i.e.:
- Unfavourable combination of spreading processes (only advective transport processes are being considered, potentially retarding processes such as adsorption and matrix diffusion, slowing down the solute transport are being neglected, biodegradation, i.e. a process that describes the metabolic transformation of the harmful substances, is not considered either.
- Unfavourable system geometries (the entry position of the fluids right below the permeable groundwater system, the fluids enter directly into a fracture system (fault zone)
- Unfavourable system parameters /generally highest, but still plausible hydraulic conductivities and lowest effective porosities were selected, both for the barrier rocks as well as the fault zone)
- Unfavourable boundary conditions (the upkeep of the pressure pulse for the total duration of the operations, i.e. maximum solute mass enters the system (high vertical hydraulic gradients between the Cenomanian-Turonian carbonate aquifer system (Münsterland) and the shallow Quaternary aquifer, the assumption of large quantities of fracking fluid penetrating into the Cenomanian-Turonian carbonate aquifer system, the definition of a mobilised methane volume and a constant flux boundary condition for the migration of methane from the gas-bearing-layer into to the shallower layers).
In sum, the results to be expected from the simulation process, although physically possible, are highly unlikely to be observed in nature, but they assist, together with a sensitivity analysis, in assessing the bandwidth of the extent of fluid migration and therefore help to realistically assess an upper margin of the risk, emanating from the subsurface fluid transport processes. We believe that such an approach is reasonable and recommendable in order to demonstrate to the general public the relative risk from the different types of activities, i.e. surface operations, well integrity issues, etc.. In order to still provide hydrogeological credibility, implausible combinations, such as high hydraulic conductivities paired with very low effective porosities were excluded.
The scenarios selected for the illustration of the migration of the fracking fluids analyse two types of conditions:
- A short-term, local scale upward flow phase of the fracking fluids due to the high pressure gradient imposed during the fracking operation (2 h duration plus ca. 10 h of pressure relief and recovery phase). The fluids enter into an overburden, intersected by a permeable vertical fault zone. The overburden allows for anisotropy effects, i.e. fluid flow can occur horizontally along the maximum hydraulic conductivity in the horizontal direction (differing from the vertical by several orders of magnitude).
- A long-term, regional scale transport in the deep carbonate system of the Münsterland Basin, driven by the regional hydraulic gradient from the Teutoburger Wald southwards. Fracking fluids are assumed to enter directly into the groundwater system, i.e. no barrier formation is accounted for between the top of the fracture and the deep groundwater system, following the general philosophy of assuming unlikely, highly unfavourable, but still physically feasible conditions. Furthermore, the regional carbonate flow system is intersected by a high hydraulically conductive fault zone (conductivity corresponds to shallow conductivity of a permeable fault zone; at depth, the pathways tend to be closed, resulting from high lithostatic stresses).
- A long-term migration of methane into shallower layers driven by gravitational and capillary forces.
Two different hydrogeological systems are being analysed: the Münsterland Basin (explored for coal bed methane resources) and the Lower Saxony Basin (explored for shale gas and tight gas resources). These two different regional areas are also distinct with respect to their geological architecture and in particular their hydrogeological flow system. While the Münsterland Basin is characterised by a pronounced hydrogeological flow system in the Cretaceous Cenomanian-Turonian carbonate rocks, regional flow can be assumed to be negligible in the Lower Saxony Basin because of its undifferentiated topography.
In order to account for the local variability of hydrogeological characteristics and still be able to generalise the outcomes of the results, the characterisation and modelling activities concentrate on different so-called geological settings that describe the spectrum of local geological and hydrogeological features, such as aquifer and barrier rock formations, account for the local geological and hydrogeological variability, reduce the flow system to those elements relevant for the migration process, provide the basis for an efficient modelling process. The variables for the definition of the settings considered include regional attribution, thickness of overburden and presence of sealing salt formations
The respective settings also required detailed geological data, which is why the settings are generally associated with the presence of an existing borehole.
The parametrisation of the individual settings is presented for the individual lithological units. Based on these and the relative proportion of the individual lithologies in the stratigraphic column, the effective vertical and horizontal hydraulic conductivities were evaluated by applying thickness weighted harmonic or arithmetic mean values respectively. Evaporite layers dominate the vertical hydraulic conductivities (log K = -11) at depths below 700 m.
