ABSTRACT:

Despite the overall consistency of chalk in terms of reservoir properties across Danish North Sea fields, most of the constitutive models were successfully applied to one single hydrocarbon field. This study aims at building a comprehensive geomechanical model capable of predicting reservoir deformation under different geological settings and production strategies. After a first phase of data calibration, this second phase consists of a series of 1-D simulations in three locations in the Dan and Halfdan fields. A strain rate dependent constitutive model simulates compaction along a vertical column including the reservoir and underburden. Despite the underlying assumptions during 1-D simulation (e.g., stress path, overburden deformation), the geomechanical model provides a good fit with the observed sea-floor subsidence data for two of the three locations. For the third location, the presence of a major fault may have caused stress arching that has influenced stress path. Two runs of simulation are carried out per location by considering the initial and final yield stresses of chalk. Taking into account the stress range at the elastic to elasto-plastic transition allows to quantifying the uncertainty on the stress conditions at the onset of deformation and thus, improve the accuracy of compaction predictions. In a next phase, the model will be implemented in 3-D to account for the interactions between overburden, sideburden, and reservoir during compaction.

1. Introduction

Changes in in situ stress state and water saturation while producing hydrocarbon alter the petrophysical and geomechanical properties of reservoir rocks. A large amount of laboratory studies, mainly carried out on outcrop chalk, has analysed and quantified these changes according to fluid content, mineralogy, porosity, and stress conditions (Andersen et al., 2018; Hickman, 2004; Leddra, 1989; Risnes et al., 2003). In the southern part of the Danish North Sea, Upper Maastrichtian pelagic chalk deposits that constitute the main reservoir unit are monotonous, cyclic, and with a low clay and quartz content in average (Hardman, 1982). Despite this relative consistency in terms of lithology and geomechanical properties (Havmøller and Foged, 1996), most of the constitutive models were successfully applied to one single hydrocarbon field (Angus et al., 2015; Keszthelyi et al., 2016; Kristiansen and Plischke, 2010; Vejbæk et al., 2014). A comprehensive geomechanical model capable of predicting the compaction behaviour of several chalk reservoirs in the Danish North Sea is still lacking in literature.

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