Abstract
In oil industry, wellbore instability is the most costly problem that a well drilling operation may encounter. One reason for wellbore failure can be related to ignoring rock mechanics effects. A solution to overcome this problem is to adopt in situ stresses in conjunction with a failure criterion to end up with a deterministic model that calculates collapse pressure. However, the uncertainty in input parameters can make this model misleading and useless. In this paper, a new probabilistic wellbore stability model is presented to predict the critical drilling fluid pressure before the onset of a wellbore collapse. The model runs Monte Carlo simulation to capture the effects of uncertainty in in situ stresses, drilling trajectories, and rock properties. The developed model was applied to different in situ stress regimes: normal faulting, strike slip, and reverse faulting. Sensitivity analysis was applied to all carried out simulations and found that well trajectories have the biggest impact factor in wellbore instability followed by rock properties. The developed model improves risk management of wellbore stability. It helps petroleum engineers and field planners to make right decisions during drilling and fields’ development.
Highlights
Drilling is one of the most important and costly operations in oil industry due to the fact that if the wellbore stability cannot be accomplished, drilling will lead to higher than necessary costs
The tensile failure represents a hydraulic fracture within the formation which normally leads to a loss in system circulation during drilling, whereas the shear failure can be described as the collapse of the borehole wall which may result in stuck pipe and, in severe cases, a loss of the open hole section [2]
This paper presents a model that accounts for uncertainties of rock properties and in situ stresses as well as wellbore trajectories in order to estimate the critical mud pressure during drilling
Summary
Drilling is one of the most important and costly operations in oil industry due to the fact that if the wellbore stability cannot be accomplished, drilling will lead to higher than necessary costs. Unlike conventional stability models that deal with static values, this probabilistic model uses Monte Carlo simulation method in order to study the impact of uncertainties on wellbore instability [4] It was developed by utilizing a linear elastic and isotropic constitutive model in conjunction with the Mogi-Coulomb rock failure law. − 2V (σV − σH sin2α − σh cos2α) cos 2θ, σθz = (σh − σH) sin 2α cos θ, σrθ = 0, σrz = 0 In this case, where the wellbore axis lies along the maximum horizontal principal stress (i.e., α = 0), the stress at borehole wall is described as follows: σr = Pw, σθ = σV + σh − 2 (σV − σh) cos 2θ − Pw, σz = σH − 2V (σV − σh) cos 2θ,. 2√2 3 sin φ, where c is the shear strength or cohesion of the material and φ is the internal friction angle [13]
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