Subsidence delay: Field observations and analysis

The objective of this paper is to describe the subsidence-depletion delay from field observations with the aim to explain its cause and constrain its modelling. The nonlinearity in the surface subsidence response to the reservoir depletion can be seen as a shift between the start of the depletion and the (delayed) start of the subsidence. We have analysed data of eight hydrocarbon fields to quantify the subsidence-depletion delay effect and the corresponding time-delay. The time-delays appeared to be in the range of 1.6 to 13 years. The fields were categorised according to their depth, age and rock type. To explain the field observations, the data was tested against four categories of mechanisms: pore pressure diffusion effects; overburden inertia; reservoir compaction behaviour; deformation of the over- under- and side-burden. The relative importance of these mechanisms was assessed from an analysis of the field data. Some mechanisms could be rejected based on theoretical grounds and others appeared to be irrelevant. It is concluded that pore pressure diffusion always contributes to a delay effect but that its effect is too low to explain the field observations. For the shallow reservoirs, natural over-compaction has the potential to cause the observed subsidence-depletion delay. In general, the other reservoir compaction related mechanisms (such as creep, an intrinsic rate effect and an elastic-plastic transition) could also be the main cause of the subsidence-depletion delay effect.