Expandable Drill Bit Provides New Method Of Drilling Increased Diameter Hole

Abstract

Abstract It has long been recognised within the drilling industry that the ability to drill an increased diameter borehole, having passed through a restricted casing and/or tubing diameter, can provide significant advantages. Among these is the ability to incorporate novel well construction techniques, using both solid and/or expandable tubulars, as well as Drilling with Casing possibilities, in which the casing is run immediately behind the bit. The opportunity to successfully incorporate these new techniques can thus enhance well design to yield significant economic and well control gains. Current techniques for drilling such increased diameter holes, such as bi-centered bits and under-reamers, have limitations on their ability to expand, whilst at the same time compromising the cutting structure presented to the formation. Additional limitations include limited reaming and back reaming capabilities, as well as non-uniform directional response. This can lead to well construction problems and reduced drilling performance. One embodiment of the technique of Drilling with Casing requires a bit which can be retrieved through the existing casing string. To achieve this and to maximise drilling performance requires a device with a formation cutting structure, indistinguishable from a standard PDC drill bit but capable of being withdrawn through a restriction significantly smaller than the borehole size just drilled. This concept leads to the possibility that a viable solution to the task of drilling increased diameter holes could be a bit that offers substantial expansion capabilities while still presenting full cutting structure to the formation. In this paper the authors will discuss the earlier problems and solutions and then present a new approach with an "Expandable Drill Bit". They will discuss the design concept, development and prototype construction of such a bit and will also present details of the successful field trials and the minor design changes that resulted from them. They will conclude that use of an "Expandable Drill Bit" is a viable tool for a variety of drilling applications. Background to the Invention Traditional well construction techniques, with individual concentric lengths of pipe to progressively line the borehole have been employed for many years. This leads to considerable structural redundancy of the casing used as a bore-hole liner as well as extra cost involved in both materials and logistics. The need to isolate sections of the well during the drilling operation depend upon a variety of geological factors, such as abnormal pore pressure, wellbore instability and hydrocarbon or fresh water bearing zones. The challenge is to develop well construction methods that address the traditional goal of lining the bore-hole from top to bottom, but in ways which are more predictable, reliable and cost-effective. Current methods will never permit a single diameter bore hole to be drilled along its entire length, but new techniques have been recently developed which require a radical new approach to the drilling process itself. Two techniques directly applicable to, and positively impacted by utilization of an "Expandable Drill Bit" are the use of expandable casing and drilling with casing. The ductility of certain grades of steel permit significant amounts of plastic deformation to be achieved without detrimental work-hardening such that it is feasible to increase the diameter of a pipe evenly and without unequal thinning of the cross section. The development of pipe connections able to accommodate this increase in diameter has made the concept of mono-bore wells a reality. In situations where borehole stability is a concern, lining the hole at the same time as drilling removes some of the uncertainty from the operation and achieves reduced annular space when used with concentric casing strings. It may also be used with a mono-bore construction. Drilling with casing, however, also creates a whole new set of challenges and requires a re-think of traditional drilling practices.