ABSTRACT
The exploitation of chalk reservoir entails reservoir compaction and seafloor subsidence that can jeopardise the economic viability of a field. Despite the apparent monotonous aspect of chalk in the Danish North Sea, numerous geomechanical models were successfully applied to different fields to forecast rock deformation. The present study aims at building a geomechanical simulation approach capable of predicting compaction and seafloor subsidence in various reservoirs. Laboratory, well log and production data and in situ compaction measurements from different fields are reviewed to build a robust database used for model calibration. A strain-rate dependent constitutive model is implemented into Comsol Multiphysics and tested from core to reservoir scale. The results indicate differences in the elasto-plastic behaviour of Maastrichtian and Danian chalk. The back-analysis of uniaxial compaction tests shows that the hardening effect varies between rock types and a more pronounced effect is observed in Maastrichtian chalk. Model calibration for the rate-dependent deformation uses in situ compaction measurements collected in a producing reservoir over a seven year-long period. The results indicate that the rate-dependent behaviour of chalk is function of porosity and differs slightly between Danian and Maastrichtian chalk. Model calibration represents a key step to quantify chalk properties, which will be then used as input data to simulate compaction in different chalks fields.
The exploitation of a hydrocarbon field causes in situ changes in stress state, formation pressure, rock-fluid interactions and rock physics, especially within North Sea reservoirs that are composed of highly compressible and chemically reactive chalk. These changes entail reservoir compaction, seafloor subsidence, wellbore instability and fault reactivation that can jeopardise the integrity of oil and gas infrastructures and, ultimately, the economic viability of a field (Schutjens et al., 2018). Besides, the efficiency of new technology such as radial jet drilling and time-lapse seismic interpretation in stimulating and monitoring production are also impeded by vertical displacements of and fluid changes associated to compaction (Barkved et al., 2003; Medetbekova et al., 2020). Thus, accurate models predicting the mechanical behaviour of chalk and the overburden under various in situ conditions are essential to assess and mitigate risks. Despite the apparent homogeneous and monotonous aspect of Upper Maastrichtian pelagic chalk, numerous constitutive and rock physics models have been built over the last two decades and successfully applied to different fields throughout the Danish North Sea (Andersen et al., 1992; Angus et al., 2015; Boade et al., 1989; Cassiani et al., 2017; Hickman and Gutierrez, 2007; Keszthelyi et al., 2016; Vejbæk et al., 2014). The present study aims at building a geomechanical simulation approach capable of simulating the chalk compaction and seafloor subsidence observed within several Danish North Sea fields. In a first phase, the mechanical model is here tested against laboratory experiments and in situ compaction measurements for calibration. The reservoir rock types investigated in laboratory are clay-free pelagic chalk of Danian and Maastrichtian age covering a 30% to 45% porosity range.
