Constraining lithologically differentiated minimum horizontal stress gradients in hydrostatically pressured and overpressured parts of the North Alpine Foreland Basin in SE Germany

A detailed understanding of the present-day stress state is key to understanding naturally occurring seismicity and safely and successfully conducting subsurface operations. For example, in the case of geothermal energy applications, the role of faults and fractures in both productivity and induced seismicity critically depends on the present-day stress state. In addition, knowledge of the subsurface stress state is also of significant importance to mitigate drilling risks. Here, in particular, the least principal stress controls the maximum allowable wellbore pressure before the drilled formation is unintentionally fractured. In its simplest form, the state of stress can be described by the magnitude of vertical stress and two horizontal stresses and their azimuthal orientations. Ideally, the state of stress includes the counteracting effect of pore fluid pressure (short: pore pressure) and is described as the effective stress tensor (effective stress is the difference between stress and pore pressure).

In this study, we investigate the magnitude of the least principal stress (minimum horizontal stress) in the North Alpine Foreland Basin in SE Germany using stress measurements such as Formation Integrity (FIT) and Leak-Off Tests (LOTs).

Whilst pore pressure magnitudes have been extensively studied and published in numerous publications in the North Alpine Foreland Basin in SE Germany, knowledge of the prevailing least principal stress is still quite limited, particularly in overpressured formations. So far, only subsets of the available FIT/LOT data, mainly concentrated around Munich, have been investigated. Recently, additional FIT/LOT data became available covering greater depths (up to 4,2 km) and overpressured formations. We investigate this new dataset in combination with data from previous studies to establish a minimum horizontal stress gradient model, which considers both pore pressure and rock type. To do so, we consider the ratio between the measured minimum horizontal and vertical effective stress using a previously established pore pressure magnitude model. The resulting effective stress ratio model is tested against the least principal stress measurements of deep geothermal wells in the study area's hydrostatically and overpressured regions, showing that considering both lithological and pore pressure variations is necessary to predict the least principal stress magnitudes. The established model can be used to improve the efficiency and safety of future drilling campaigns in the study area and can also serve as an input for mechanical subsurface modelling, e.g. for a better understanding of deformation or natural and induced seismicity.