Improved Wellbore Stability Modeling in Fractured Formations
One of the most prominent features of the Earth’s upper crust is the presence of joints and fractures. Many petroleum reservoirs are situated or inter-bedded in these fractured porous formations. For instance, fractured carbonates and fractured laminated shales are prevalent in the giant and prolific fields of the Middle East; the Ekofisk formations in the North Sea comprise fractured chalks. Drilling in these fractured formations presents wellbore stability challenges because the fractured rocks have larger shear failure zones and smaller fracture gradients. This results in a narrower safe mud-weight window, and wellbore instability-related costs can become great because of lost time or, in the worst case, a lost well. A double porosity poroelastic model has been proposed and the finite-element method was applied to obtain a wellbore stability solution. This method considered the effects of both fractures and porous media on wellbore stress redistributions, and hence can better model fractured porous media. A case study in Oman was examined, where claystones were weak and carbonate rocks were heterogeneous and contained natural fractures. Wellbore-stability incidents, such as drilling fluid losses and massive wellbore breakouts, frequently occurred while drilling. These problems were mainly caused by the narrow mud-weight window available for maintaining wellbore integrity. Increasing mud weight caused drilling fluid losses, but lowering mud weight induced wellbore breakouts. After modeling, analysis, and calibration in the offset wells, the correct mud-weight window and casing intervals were recommended for proposed wells to avoid wellbore shear and tensile failures.
AAPG Search and Discovery Article #90077©2008 GEO 2008 Middle East Conference and Exhibition, Manama, Bahrain