Operators are always looking for ways to reduce risk in high cost environments and maximize value. Many challenges exist in the drilling environment today including depleted sands, lost circulation zones, weak formations and rubble zones. Casing and liner drilling has found its way from a niche market into the mainstream drilling environment through recent successes in a variety of these applications. There exist several options to drilling down casing or liners. These include retrievable BHA's with traditional measurement and logging equipment and non-retrievable BHA's that are drilled or reamed into place and cemented.
This paper presents several case studies where non-retrievable casing/liner drilling technology has either improved the economics of a challenging application or made the application technically viable. These applications vary from onshore wells with tight fracture gradient schemes, shallow onshore wells looking to improve economics in surface hole intervals to offshore wells reducing risk by drilling or reaming down casing and liner strings. Applications will be taken from the Gulf of Mexico, South Texas, West Africa, Australia and Norway. The justification of using casing/liner drilling technology on these wells will be explored, as well as the lessons learned to apply to future drilling programs.
Casing and liner drilling covers a very broad range of drilling technologies and applications. Figure 1 shows a generic picture of casing and liner drilling systems, Most are commercially available with all having been utilized in test applications over the last 10 years. The relative success behind the development and application of each of these systems has been driven by the application need, ease of implementation and the potential economic prize available by proving the technology as a viable alternative.
Vincent, et al1 discussed the history of casing and liner drilling dating back as far as 1907 with the Baker casing shoe yet it still remains a niche technology in many geographical areas. Throughout the majority of the 20th century, drilling with casing was seen as a means to combine the casing and drilling in one operation, thus saving on tripping time. While this continues to be a focus for the technology, the application range has extended to combat problems caused by depleted sands and tight pore pressure gradients as well as to minimize unscheduled drilling events.
As an introduction to casing and liner drilling technology, a service provider has developed a system that is easy to implement and practical in application. This system focuses on the non-retrievable casing and liner method with typical systems being:
Drill down casing method: This method involves a specially designed casing drilling bit (Fig 2a) beneath several casing joints (stabilized as required) and a float collar. The casing is latched into the top drive at surface either through a specialized casing drive system (Fig 2b) or with a drilling spear and pack off, providing a secure and convenient method of transferring rotary torque and hydraulics to the casing string for drilling.