Shale and Wellbore Integrity

Clay‐rich shale poses substantial challenges to establishing well integrity during drilling, operation and abandonment of wells used in oil and gas production, CO2 sequestration, and other waste disposal activities. We review problems created by shale for borehole stability, zonal isolation, and long‐term (100s of years) integrity. Shale mechanical weakness and susceptibility to infiltration of drilling fluids can result in hole enlargement and collapse, stuck drilling tools, and difficulty cementing, among other issues that compromise well integrity. These problems can be managed by adjusting the drilling mud density and manipulating the drilling mud chemistry (oil‐based versus water‐based). Loss of zonal isolation in a well results in fluid escape through the cement‐filled annulus between casing and rock. Experimental studies of bond strength between cement and shale show that strength is a complex function of the extent of drilling damage to shale and the presence of residual drilling fluids and filter cake. Mechanical modeling of stresses induced by drilling and well operations can be used to characterize risk of damaging the integrity of the cement‐shale bond. CO2 sequestration poses new challenges with the requirement of well integrity over periods of 100s of years. Evidence of chemical reactions involving CO2, cement and rock that promote self‐sealing of leakage pathways show promise for maintenance of long‐term integrity. In addition, concerns of degradation of cement and steel over these long periods may be mitigated by the capacity of some shale to creep under long‐term stress potentially maintaining zonal isolation as demonstrated in some wells in the North Sea.