Objectives

The "Hydromechnical behaviour of geothermal reservoirs in the stress field of geological structures" project is a sub-project of the Geothermal and High Performance Drilling Technology Research Association in Niedersachsen (gebo). It is a sub-project (G5) of the geosystems main research activity which covers the geoscientific aspects in the overall association. The stress distribution as well as the hydromechanical fracture behaviour around complicated geological structures is to be characterised in this sub-project developed from a 3D underground model typical of the north German area. The aim is to gain an additional understanding on the hydromechanical reservoir behaviour during and after drilling and stimulation measures, with the objective of optimising exploitation concepts. 

First results

3D models of the stress field at the Hannover-Groß-Buchholz GeneSys location are shown which were undertaken with the COMSOL multiphysics program (finite element method). Modelling was based on the geological structure plans of the geotectonic atlas NW Germany, as well as well logs of the density and the seismic velocities. The diagram shows the outcropping of the upper layers on the surface, and the salt domes, as well as the associated doming. The colour of the grid lines shows the formation, the block colouring shows the depth.

A comparison of the results of the well analysis of the stress field reveals a good correlation with the simulation. The observed depth-dependent changes in the main stress directions at great depths in particular can be reproduced by the model. Circular stress field patterns can give rise to deviations between measured values and the stress field simulations. The absolute values determined for the horizontal stress are slightly lower than the values derived from the first results of a mini-frac test, and it is therefore assumed that a role is played by additional tectonic stresses. Moreover, the rheological description of the salt domes is shown to have a major influence on the stress field, and primarily, on the effect of external tectonic stresses.

Outlook

The results achieved so far already reveal good correlation with the in situ values even without taking into consideration external tectonic stresses. However, to achieve the previously assumed absolute values, a description of the salt is required which differs from the simple Hook's evaluation based on the dynamic measurement values. More research work is required for this purpose. In addition, clarification is still needed on whether a graben structure in the Upper Jurassic could be responsible for the differences between the measurement and the simulation results. These investigations will then be used as the basis for looking at the hydromechanical reservoir behaviour more closely during hydraulic stimulation.