The costs associated with lost circulation—the loss of drilling mud into the formation—resonate throughout the E&P industry. As horizontal wells become longer and more complex, and as drilling moves into more complex areas, lost circulation due to wellbore integrity promises to be an issue of growing importance.
In the Gulf of Mexico alone, wellbore integrity issues such as stuck pipe, wellbore collapse, sloughing shales, and lost circulation account for as much as 44% of nonproductive time (NPT). Fluid loss typically occurs through fractures caused by drilling; once fractures are created, fluid can flow partially or completely through them into areas known as thief zones that effectively steal drilling fluid from the wellbore. Because the pressure required to lengthen a fracture is often lower than that required to initiate it, these fractures tend to spread easily.
The industry is meeting the threat of lost circulation through the application of diverse materials and methods aimed at maintaining wellbore integrity. Collectively, these approaches are called wellbore strengthening and include strategies that alter stresses around the wellbore. While there are differences in how these mechanisms are applied, the overall effect is to elevate the pressure at which losses occur, thus widening the drilling safety margin.
A recent Oilfield Review article, “Stabilizing the Wellbore to Prevent Lost Circulation,” discusses the prevailing theories and practices of wellbore strengthening championed by industry experts and designed to combat fluid loss into thief zones during drilling.
Read the full article by visiting the Oilfield Review Web site.
Cook J, Growcock F, Guo Q, Hodder M and van Oort E: “Stabilizing the Wellbore to Prevent Lost Circulation,” Oilfield Review 23, no. 4 (Winter 2011/2012): 26–35.