Over 300 commercial well intervals have been drilled with casing and retrievable BHA systems in the last five years. This activity has been focused mainly on drilling vertical wells with only about a dozen of these wells being directional. However, there is growing interest in applying the techniques to directional wells.
Steerable positive displacement motors (PDM's) have been used for most directional wells drilled with casing. However, rotary steerable systems (RSS's) have recently been run while drilling with casing. This provides another avenue for improving directional casing while drilling (CwD) performance.
The combination of rotary steerable systems and CwD is a natural solution to directional wellbore stability problems. It capitalizes on the pinpoint control of RSS's while rotating the casing to heal wellbore wall problems. This paper presents a comparison of casing directional drilling with steerable motors and rotary steerable systems and discusses the procedures that were developed to make casing directional drilling routine.
Casing while drilling (CwD) has moved from being a new technology with unproven value to a technology that is now recognized as a practical method of solving particular drilling problems and reducing drilling costs.1,2,3,4,5,6,7 It is being used in a variety of commercial applications that range from drilling entire onshore wells from spud to TD to offshore applications where only a single hole section or two are drilled with casing.
Most commercial CwD activity to date has been focused on drilling vertical intervals where the techniques are proven and the benefits are evident. The technology is still in its infancy in terms of market growth, but some operators have embraced it and are actively pursuing new applications.
The simplest CwD applications are ones where a special bit is attached to the casing to drill vertical wells. The bit can be drilled out to run subsequent casing strings. Other applications have used a conventional bit run on the final casing string that is simply left in the well. However, many other vertical applications have been with a fully retrievable and re-runable drilling assembly that allows the bit and BHA to be changed without tripping the casing.
CwD systems have demonstrated advantages in vertical applications because:
The time required to drill a section and run the casing is reduced.
Well control and lost circulation events are reduced and often eliminated.
Difficulty in tripping out and running the casing after the hole is drilled is eliminated.
The number of casing strings in some wells is reduced.
Drilling fluid lost to the production zones may be reduced, which decreases formation damage and increases production rates.