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For wells having complex geometry or projected high degree of drilling difficulty, extensive pre-well planning is necessary to optimize key drilling and drilling fluid parameters. These simulations in the well construction process are typically handled in a single fashion and include: Hole cleaning optimization, Hydraulics/equivalent circulating density (ECD) optimization, Wellbore stability modeling (WBS), Torque and drag modeling, Circulating temperature modeling

Heretofore, predictions of wellbore stability have used surface drilling fluid density as a key input parameter. However, in the normal drilling process, the wellbore wall rarely experiences the surface drilling fluid density, but rather a higher density reflecting the ECD or sometimes a lower density caused by swab effects. Especially for oil-based muds (OBM) or synthetic-based (SBM) muds, the effects of compressibility and thermal expansion on the density of the drilling fluid components cannot be ignored. Also, the impact of drilling fluid-rock interaction on wellbore (in)stability must be accounted for while estimating the safe operating mud window.

The effects of surge and swab are also investigated in terms of wellbore stability In the case discussed here, the effects of a 0.3 lbm/gal swab, equivalent to POOH at 60 ft/min, was calculated using hydraulic modeling. Wellbore stability simulations using chemoporoelastic methods were then performed. Time-dependency is included in this simulation to demonstrate the progressive collapse failure of the borehole. In Fig. 5a , the predicted wellbore collapse is modeled for a short interval of 2 hr. under swab conditions. Here, wellbore failure principally along one axis is predicted, with breakouts of 2-3 in. maximum. Fig. 5b contains the results after a period of 24 hr. and predicts failure along nearly the entire wellbore perimeter. While it would be quite rare to maintain swab conditions for 24 hr. , the results are useful in that they show wellbore failure increasing over time.

Source: www.aade.org

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