InterPore2022

Quantifying the uncertainty associated with reservoir compaction forecasting: Role of the experimental estimation of the hydrostatic yield stress.

30 May 2022, 17:20

15m

Oral Presentation (MS14) Uncertainty Quantification in Porous Media MS14

Speaker

Mohammad Reza Hajiabadi

Description

Geomechanical simulators aim at predicting the irreversible deformation taking place in hydrocarbon and CO2 reservoirs to optimise profits and reduce risks associated to the exploitation of chalk fields. In the context of compaction studies, accurate forecasting of the plastic strain relies on well-calibrated constitutive equations to capture the mechanical response of rocks according to, amongst others, the porosity, water saturation, age of the rock, and stress and temperature conditions [1,2]. The constitutive equations correlating lithological, petrophysical and geomechanical properties under various in situ conditions are based on experimental database that show a non-negligible data scattering [3]. Although raising questions about the reliability of the predicted strain, the uncertainty on the representativeness of these correlation functions to capture the plastic behaviour of chalk is not yet addressed in the literature.
The present study assesses how the change in the hydrostatic yield stress (σ_hy) estimated from laboratory studies impacts the amount and distribution of plastic strain modelled in four depleted reservoirs from the Danish North Sea (Dan, Halfdan, Gorm, and Kraka fields). The selection of the parameter σ_hy is motivated by the difficulty that scientists face to assign a stress value to a specimen tested in the laboratory. The transition from the elastic to plastic regime is not abrupt i.e., occurring at one specific stress value. On the contrary, the elastic-to-plastic transition is progressive taking place over a stress interval delimitated by the initial (σ_(hy,in.)), and final yield stresses (σ_(hy,fin.)). Besides, the method used to determine the representative yield stress (σ_(hy,rep.)) of chalk varies between studies [4–6].
Two 1-D simulation scenarios are carried out per study areas by considering in the constitutive equations the σ_(hy,in.) and σ_(hy,fin.)value of Danian and Maastrichtian chalk. The first scenario is considered as a conservative approach and the latter is an optimistic approach that results in a smaller deformation. The geomechanical simulator is a strain-rate dependent constitutive model using a modified Mohr-Coulomb yield function to capture the change in mechanical properties as irreversible strain accumulates in the rock [7]. The stress paths are reconstructed from repeated formation tests and the reservoir properties are extracted from well log data. Note that the simulation outcomes are quality-checked by subsidence data.
The results indicate that the creep deformation does not contribute to the contrasts in compaction prediction between the conservative and optimistic model. Secondly, changing σ_hy of chalk from its initial to final value obviously shifts the onset of plastic deformation towards high stress conditions. This shifting in σ_hy reduces by a factor of 26% to 73% the amount of strain simulated. The contrast in the simulation outcomes between the conservative and optimistic model is dependent on a subtle interplay between rock porosity, virgin stress, and stress path. Thus, the uncertainty related to the determination of the σ_hy of subsurface chalk can potentially modify crucial decisions taken during a field development such as, platform height design and the drilling trajectory.

References

[1] Angus DA, Dutko M, Kristiansen TG, Fisher QJ, Kendall JM, Baird AF, et al. Integrated hydro-mechanical and seismic modelling of the Valhall reservoir: A case study of predicting subsidence, AVOA and microseismicity. Geomech Energy Environ 2015;2:32–44.
[2] Vejbæk OV, Mohamed FR, Herwanger JV. 4D seismic, 4D geomechanics and hydraulic stimulation in the low permeability South Arne chalk field. First Break 2014;32:139–48.
[3] Hickman RJ. Formulation and implementation of a constitutive model for soft rock (Doctoral dissertation). Virginia Polytechnic Institute and State University, Blacksburg, VA, 2004.
[4] Havmøller O, Foged N. Review of rock mechanics data for chalk. V North Sea chalk Symp., Reims: 1996, p. 1–26.
[5] Amour F, Christensen HF, Hajiabadi MR, Nick HM. Effects of Porosity and Water Saturation on the Yield Surface of Upper Cretaceous Reservoir Chalks From the Danish North Sea. J Geophys Res Solid Earth 2021;126.
[6] Nermoen A, Korsnes RI, Aursjø O, Madland M V., Kjørslevik TAC, Østensen G. How Stress and Temperature Conditions Affect Rock-Fluid Chemistry and Mechanical Deformation. Front Phys 2016;4:1–19.
[7] Papamichos E, Brignoli M, Santarelli FJ. Experimental and theoretical study of a partially saturated collapsible rock. Mech Cohesive-Frictional Mater 1997;2:251–78.

Presentation materials

Interpore_ID566_vf.pptx