Reservoir-Management and Seismicity - Strategies to Reduce Induced Seismicity

In Northern Germany seismicity is reported which occurred close to recent gas production. The goal of the study is to better understand and constrain underlying geomechanical boundary conditions by addressing the following key questions: a) How critical is the initial state of stress for fault reactivation in Northern Germany ?, b) What are the characteristics of the induced earthquakes of Northern Germany? ; c) How and where is the state of stress modified by depletion?; d) Are the observed Coupling Coefficients (Reservoir Stress Path) indicative for the whole field?, e) What effects does the depletion on the earth's surface have and can it be differentiated from other causes?, f) Is there differential subsidence across the Soehlingen field?; g) How do the poro-elastic effects resulting from production modify the state of stress at Wustrow, Dethlingen and Havel reservoir levels? and h) Can reservoir management help to reduce the seismicity ? The geomechanical interpretation of the processes is based on rock mechanical tests of reservoir analogue rocks to obtain the strength of intact and weakened reservoir rocks, on observations of vertical displacements (Satellite data and leveling) on measured pressure and minimum horizontal stress data in wells. Based on a unique database from borehole data of the considered gas field it was possible to determine initial stress conditions and the evolution of stress and pore pressure within the field and two of its reservoirs over time. Laboratory experiments on 53 cores from 35 blocks of rocks show that neither the intact nor the weakened reservoir rocks fail under initial stress conditions (natural stress field prior to production). During production, the stress field is modified through pore pressure stress coupling. Under certain circumstances (e. g. compartment boundaries, compartments with different properties) stress changes can lead to failure of weakened reservoir rocks (pre-existing faults). Modelled and observed deformation (INSAR and levelling studies) agree well, which can lead to the conclusion that INSAR methods can also be used for monitoring gas fields at great depth even in challenging conditions (salt above). The modelling indicates that seismicity is most likely to be induced at internal or reservoir bounding faults due to stress changes in the reservoir and the still high pore pressures outside the reservoir. Since the reservoirs and thus the internal and bounding faults are of limited size, this limits the maximum magnitude of induced events. If the seismicity would be triggered outside the reservoir, higher magnitudes could become possible. A reservoir management including the consideration of pore pressure stress coupling and natural tectonic stress field can help to reduce risk of large scale fault reactivation inside and at reservoir boundaries.