Abstract:

Geomechanical uncertainties inherent to the Pre-salt wellbores may be responsible for severe operational and environmental impacts, since drilling deep wellbores in salt rock formations is challenging and risky. The aim of this research is to assess the potential risk to the wellbore related to salt creep. For this purpose, a finite element model of a Pre-salt like synthetic wellbore is used. The evaluation scenario considers an elliptic borehole in which the casing remains uncovered by the cementation sheath. The emergence of plastic deformation in the casing is considered here as a failure event. The salt rock (halite) is modelled as an elasto-viscoplastic material, and creep behavior is simulated using the sophisticated Multi-mechanism Deformation model. Uncertainties associated to the key parameters for salt creep behavior are taken into account as random variables described by probability distributions. The fast probability integration Mean Value methods are adopted to obtain the cumulative probability distribution for the time when the failure event occurs. The probabilistic analyses have highlighted that the most important variables for salt creep and associated threats are those related to the stress state, temperature and creep-rate parameters. In this research, the probability of the casing undergoing plastic deformation before the time predicted by the deterministic analysis is 40%. The results show that even representative geomechanical models with sophisticated constitutive laws can lead to an unreliable solution when parameter uncertainties are neglected.

Introduction

Geomechanical uncertainties inherent to the Pre-salt wellbores may introduce severe operational and environmental impacts, since drilling deep wellbores in salt rock formations is challenging and risky.

Typically, wellbore integrity analysis relies on several geomechanical parameters taken as deterministic values, and the results are directly related to that specific parameter set. Hence, even the most sophisticated geometric models may not provide an accurate response whenever the parameter variability is not considered.

Having in mind the operational threats related to the Pre-salt exploration, one can observe that the uncertainties omitted in the deterministic analyses may represent severe financial and environmental costs. Hence, considering the complexity of this scenario and the duality between risk and benefit, it is highly advisable to take into account the uncertainties related to the geotechnical parameters.

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