Time-dependence of subsidence above a producing gas reservoir with salt top seal: A finite element model study of the role of salt flow

ABSTRACT Numerous producing and actively subsiding gas fields worldwide have salt top seals, but surface subsidence records exist for very few. This paper summarizes both reservoir pore pressure data and subsidence measurements obtained for a salt-sealed gas field in northern Europe. The data indicate a time dependence in subsidence evolution, with subsidence rates remaining relatively constant, while pore pressure depletion rates decrease. The creep of the thick, overlying salt may play a part in causing this time-dependence. The present work assesses salt creep flow’s role in controlling the evolution of subsidence through finite element models. Salt creep effects are investigated in isolation, and independently of other factors, incorporating uncertainty in salt flow mechanisms and rheology. Comparing model results with field subsidence measurements suggests that salt flow can account for part of the time-dependent subsidence observed but cannot fully explain the field data, a finding that is also potentially important for other salt-sealed reservoirs around the world. Another important conclusion of the current study is that additional mechanisms that can lead to time-dependent subsidence must be active in the field studied.