Abstract
Abstract. From the analysis of GPS monitoring data collected above gas fields in the Adriatic Sea, in a few cases subsidence responses have been observed not to directly correlate with the production trend. Such behavior, already described in the literature, may be due to several physical phenomena, ranging from simple delayed aquifer depletion to a much more complex time-dependent mechanical response of subsurface geomaterials to fluid withdrawal. In order to accurately reproduce it and therefore to be able to provide reliable forecasts, in the last years Eni has enriched its 3D finite element geomechanical modeling workflow by adopting an advanced constitutive model (Vermeer and Neher, 1999), which also considers the viscous component of the deformation. While the numerical implementation of such methodology has already been validated at laboratory scale and tested on synthetic hydrocarbon fields, the work herein presents its first application to a real gas field in the Adriatic Sea where the phenomenon has been observed. The results show that the model is capable to reproduce very accurately both GPS data and other available measurements. It is worth remarking that initial runs, characterized by the use of model parameter values directly obtained from the interpretation of mechanical laboratory tests, already provided very good results and only minor tuning operations have been required to perfect the model outcomes. Ongoing R&D projects are focused on a regional scale characterization of the Adriatic Sea basin in the framework of the Vermeer and Neher model approach.
Highlights
In the last two decades Eni S.p.A. has been developing a robust modeling methodology for production-induced subsidence (Capasso and Mantica, 2006)
Though taking into account inelastic strains is instrumental for describing compaction in clastic reservoirs (Pijnenburg et al, 2019) and the MCCM keeps providing accurate reproduction of monitoring data gathered from almost all the gas fields in the Adriatic Sea (e.g. Gemelli et al, 2015), a further constitutive modeling effort has been recently required for a few of them showing a certain delay between production trend and GPS data – a phenomenon broadly described in the literature (e.g. Hettema et al, 2002)
The modeling approach has been enhanced by adopting the elasto-viscoplastic model proposed by Vermeer and Neher (1999, VNM), capable to describe the viscous response of reservoir sands and derived from the extended overstress theory (Olszak and Perzyna, 1970; Yin et al, 2010)
Summary
In the last two decades Eni S.p.A. has been developing a robust modeling methodology for production-induced subsidence (Capasso and Mantica, 2006). It is based on 1way hydro-mechanical coupling (Gambolati et al, 2005) and elasto-plastic modified Cam-Clay model (MCCM, Roscoe and Burland, 1968). Having the implementation already been validated at laboratory scale and tested at reservoir scale on synthetic hydrocarbon fields (Volonté et al, 2017; Musso et al, 2020), this paper presents a first application of the enhanced approach to the production-induced subsidence analysis of a real gas field in the Adriatic Sea, the GPS data of which exhibit a delay of about 1.5 year (Fig. 1).
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