Definition - What does Wellbore Strength mean?
Wellbore strength can be defined as the ability of a borehole to resist failure due to mud loss. Mud loss is frequently encountered while drilling and happens when the mud weight essential to maintain wellbore stability and well control exceeds the fracture resistance of the formation, especially in depleted reservoirs.
A failed wellbore has to be abandoned, causing heavy financial losses. However; there are methods to strengthen wellbores to prevent failure.
Trenchlesspedia explains Wellbore Strength
Wellbore strengthening is not thoroughly studied yet due to the scope of the subject. But it helps to better understand and treat lost circulation. Depleted reservoirs require investigating deeper for more hydrocarbon.
As the reserves decline, there is a drop in pore pressure, weakening rocks that contain hydrocarbon. The neighboring low permeability rocks or shales may maintain their pore pressure. The mud weight necessary to support the shale is greater than the fracture resistance of the sands and silts, making drilling extremely difficult or impossible. Wellbore strengthening can enable reaching these deep reservoirs.
Strengthening can be achieved by different methods such as temperature changes to alter the stress state around the wellbore, mud heaters (in some cases) to increase near-wellbore stresses, adding granular particles to the mud to seal fractures, etc.