Ground motions induced by a producing hydrocarbon reservoir that is overlain by a viscoelastic rocksalt layer: a numerical model

Abstract

S U M M A R YHydrocarbon reservoir pressure depletion leads to stress changes inside the reservoir and ground deformation which is registered at the surface as subsidence. As reservoirs are often overlain by layers of rocksalt (or other evaporites), which are materials that flow so as to relax stresses inside them, there is the potential for time-varying surface subsidence. This work focuses on understanding the macroscopic mechanisms that lead to rocksalt flow-induced ground displacements. A Finite Element Model is used for this purpose in which the rocksalt layer is represented by a viscoelastic Maxwell material. Two distinct mechanisms that lead to displacement are observed. These are active during different stages of the deformation and have different timescales associated with them. An important observation is that the timescale for deformation that is measured at the ground surface is not equal to the timescale for deformation of a viscoelastic material element, but can be many times larger than that. The sensitivity of the response to the thickness and location of the rocksalt layer is also presented. Conclusions are drawn which allow for the relative importance of the presence of the rocksalt layer to be assessed and for a framework for understanding time-dependent subsidence above producing hydrocarbon reservoirs to be developed. Finally the changes in stress distribution around a producing reservoir are also briefly discussed.

Reconciling the Long‐Term Relationship Between Reservoir Pore Pressure Depletion and Compaction in the Groningen Region

“…Shallow surface deformation should be manifest as short‐wavelength features superimposed on the regional signal (Bourne & Oates, ). Possible near‐surface deformation (e.g., soil mobility and salt flow effects; Marketos et al, ) are expected to be small and are neglected in this work. In addition, a region to the south of the reservoir that comprises the Veendam dissolution salt mine is removed from the data set to minimize the effect from this unrelated shallow surface deformation.…”

Impact of rock salt creep law choice on subsidence calculations for hydrocarbon reservoirs overlain by evaporite caprocks

“…This work focuses on one such mechanism (creep of the rock salt caprock material) highlighting the effects it can have on observed subsidence patterns above a producing gas reservoir. Our previous work [ Marketos et al , , ] has shown that creep of rock salt caprock, in response to shear stresses induced in it by the production process, can, in its own right, produce significant time‐dependent ground deformations that will alter the subsidence patterns and their evolution compared with fully elastic analyses. We showed that, if salt creeps according to a linear viscous flow law, the subsidence bowl initially deepens and narrows and then, over large timescales, becomes shallower and wider.…”

“…We showed that, if salt creeps according to a linear viscous flow law, the subsidence bowl initially deepens and narrows and then, over large timescales, becomes shallower and wider. In that work, we focused on geomechanical models that were the simplest possible, investigating the system response to a step decrease in pore pressure as applied to the simplest possible reservoir shapes, i.e., shapes approaching point sources (a flat cylinder in 2‐D plane strain conditions and a sphere in 2‐D axisymmetric conditions—[ Marketos et al , , ], respectively). We neglected, among other factors, the effect of depletion rate (i.e., reservoir pressure decrease rate) on subsidence evolution.…”

Monitoring land motion due to natural gas extraction: Validation of the Intermittent SBAS (ISBAS) DInSAR algorithm over gas fields of North Holland, the Netherlands

“…In addition, the model is relatively computationally inexpensive compared with the more accurate numerical solutions produced by Finite Element Methods (FEM), commonly adopted for simulating surface subsidence and reservoir compaction (e.g., Minkoff et al, 2003;Marketos et al, 2015). The effort required to adequately characterize complex constitutive models (e.g., material rheology, reservoir geometry, and inhomogeneity) and develop stable mesh attributes mean the repetitive calculations of complex simulations typical of FEM can make these approaches impractical.…”