Advances in geomechanical subsidence modeling: Effects of elasto-visco-plastic constitutive behavior

O&G companies, during normal activities of field development planning and execution, have to assess their projects in terms of environmental sustainability in addition to technical and economic feasibility. With such a primary aim, Eni's commitment to both monitoring and modeling of subsidence has been very strong since the early nineties. Most offshore development platforms have been equipped with GPS stations, able to continuously monitor horizontal and vertical displacements, and radar images are constantly acquired and interpreted for onshore fields. To complete the internal approach, a workflow based on elasto-plastic finite element modeling has been implemented in order to properly reproduce the measured data and to accurately predict the future evolution of the settlement phenomenon. In some cases, it is beneficial to enrich classical elasto-plastic constitutive models to fully reproduce the time evolution of subsidence as recorded by continuously monitoring instruments such as GPSs. Actually, the extension of those models with the introduction of a viscosity component allows for a more detailed match of the recordings, with an increased reliability of the forecasted evolution scenarios. In this paper, the elasto-visco-plastic law by Vermeer and Neher (VNM) is evaluated as a possible constitutive model for the simulation of subsidence occurring above gas and oil reservoirs. This elasto-visco-plastic model can be regarded as a natural extension of the modified Cam-Clay model (MCCM), usually employed by Eni for subsidence modeling. Furthermore, its parameters have a clear physical meaning and can be easily determined from laboratory tests. Oedometer and creep tests performed on sand samples from gas reservoirs are first introduced together with model predictions. Then, a synthetic, nevertheless realistic, case study is analyzed to assess the capabilities of this elasto-visco-plastic model in the simulation of subsidence. Elasto-plastic (MCCM) and elasto-visco-plastic (VNM) simulations are compared to outline the implications of the elasto-visco-plastic modelling in the simulation of subsidence evolution. The results presented herein show that the VNM can adequately reproduce time dependent effects observed in laboratory testing of reservoir materials and, in some cases, can potentially improve the match of complex subsidence histories provided by elastoplastic models.