Casing Drilling is a process in which a well is drilled and cased simultaneously. This innovative technology has been successfully practiced for the past decade. The original purpose of developing Casing Drilling was to eliminate Non Productive Time (NPT) associated with tripping drill pipe and running casing. However, during early implementation of the technology, other benefits were observed while drilling with large diameter casing. This paper reviews four of these advantages which are lost circulation reduction, wellbore strengthening, improved wellbore stability, and drilling-induced formation damage mitigation.
Casing Drilling reduces mud lost to the formation in two ways; the Plastering Effect seals off the wellbore and prevents fluid transfer between the borehole and the formation; secondly, in the worst cases where the losses can't be cured, drilling will be continued with minimized losses until the casing reaches the total depth. Reduced mud loss to the reservoir section can be directly correlated to reduced skin due to drilling induced formation damage. This leads to improved productivity of the wells drilled with casing in the reservoir section.
As the Plastering creates an impermeable mud cake on the wellbore wall the pressure containment of the borehole is augmented. This process increases the fracture gradient of the formation in near wellbore area which results in a wider operational mud weight window (Wellbore Strengthening). Wells with stability problems are among the best candidates for Casing Drilling and could be drilled trouble free. The wellbore stability benefits of Casing Drilling are due to no tripping, less mud formation exposure, gauged well, superior hydraulics and borehole cleaning, etc.
With all the mentioned benefits Casing Drilling is heralding the way to the future of the drilling industry. There have been a lot of drilling practices developed to gain every one of the discussed benefits; however, Casing Drilling offers the whole package with one unified solution.