Assessment of geological factors potentially affecting production-induced seismicity in North German gas fields

Abstract

Recovery of hydrocarbons has triggered seismicity in some Rotliegend reservoirs in the Netherlands and in northern Germany. In Germany, these earthquakes have caused negligible structural damage, but raised public concerns in a region of low historic seismic activity. Production-induced seismicity has been successfully addressed by analytic poroelastic models that explain the development of critically stressed faults with pore fluid pressure depletion inside and around reservoirs. The available analytical approaches fail to account for the mechanical stratigraphy and structural complexities occurring in reality, i.e., reservoirs compartmentalized by faults. The present study aims to increase the quantitative understanding of the geological factors that potentially affect production-induced seismicity in North Germany. In the first part of the paper, a series of 2D finite element models is presented to investigate a parameter space for reservoir depth, reservoir thickness, mechanical reservoir and host rock properties, and compartment geometries typically observed in North German Rotliegend gas fields. The second part addresses the impact of mobile salt layers with variable thickness atop the reservoir on fault stability. The results indicate preferred fault reactivation for steeply dipping faults, large Biot–Willis coefficients, inhomogeneous overburden loads, large reservoir thicknesses, a shallow reservoir position and short distances of the salt layer to the reservoir. In the third part we investigate the effects of fault dip and throw along a fault loaded by three depletion scenarios in a compartmentalized intra-graben setting. Maximum fault loading was obtained for the case that fault throw is half of the reservoir thickness and production is exclusively from the footwall block. The main finding is a preferred reactivation of steeply dipping faults (>60°) caused by the dominant contribution of reservoir compaction to fault loading. Compaction-loading constitutes the main difference to seismicity driven by far field tectonics dominated by horizontal strain, for which faults with dip angles of approximately 60° are predicted favourable for reactivation.