Abstract

The effects of filter cake buildup and filter cake properties variation with time on the wellbore stresses and wellbore stability have been plaguing the industry for long. The increasing use of lost circulation materials (LCM) in recent years for wellbore strengthening in weak and/or depleted formations nessesitates real-time models that can predict these effects. However, the complexities of effective stresses and pore pressure real-time evolution, i.e. while drilling, coupled with the time-dependent properties of mud filtration have delayed such modeling attempts. In this paper, the analytical solutions for the time-dependent coupled effects of mudcake buildup on the wellbore stresses and formation pore pressures controlling wellbore collapse and fracturing are derived. The transient effects of mudcake buildup, mudcake permeabilities, and mudcake buildup coupled with its permeability reduction during filtration on the safe drilling mud window were illustrated through numerical examples. The results showed that the safe mud weight windows are greatly affected by the buildup of filter cake and its permeability variation. In particular, the analysis for the mudcake with permeability of 10-2 mD showed a widening of safe mud weight window by 0.9 g/cc after 2.5 hrs post excavation compared to that of a wellbore without mudcake. On the other hand, analysis with a 10-3 mD mudcake showed a widenining of mud weight window by as much as 1 g/cc. In addition, analyses revealed that even for mudcake permeability as low as 10-3 mD neglecting the permeable nature of the mudcake results in overestimation of the safe drilling mud weight window.

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