A detailed test program was performed with an eccentric tool at the Baker Hughes Experimental Test Area (BETA) field research facility to evaluate the feasibility of its use in an Expandable Tubular Technology application in the North Sea. The testing used a 9–7/8" Drill Out Steerable Ream While Drilling (DOSRWD) tool in conjunction with 6–1/2" pilot bits (both PDC and roller cone). Motor bent housing settings included 1.0°, 1.5°, 1.75° and 2.0° bends to evaluate directional and stability response. Surface speeds were varied from 0, 35, 50 and 75 rpm at each motor housing setting. Caliper logs including four and six-arm and ultrasonic borehole imaging (UBI) tools were used to characterize the borehole under all conditions. The analysis included directional tendencies, down hole vibration monitoring and borehole diameter, quality and degradation over time. The test results show the 9–7/8" DOSRWD system is capable of providing the high quality wellbore required for expandable tubular technology, ensuring the casing can be run, expanded and isolated across the formation.
Introduction
Expandable tubular technology has the potential to significantly reduce well construction costs. Conventional well construction results in telescoping of the well size from the wellhead down to the reservoir. Apart from resulting in large expensive surface casing, wellheads, trees and operating equipment, the method can result in an unworkable small hole size at the required depth. This could then lead to compromises in well operability or in worst case failure to reach the final objective.
Expandable tubulars can help solve difficult drilling challenges posed by high-pressure zones, deepwater environments and troublesome sub-salt plays.1,2,3,4,5 Its innovative characteristics allow operators to explore in remote geologic regions and exploit reserves once considered unprofitable if drilled with conventional technology. Instead of using progressively smaller diameter pipe as drilling progresses deeper, Expandable Tubular Technology allows tubular diameters to be expanded with specially designed "pigs," or mandrels. This reduces well tapering while preserving borehole size. Expandable technology can also extend the profitable life of mature fields by internally cladding existing wellbores to isolate troublesome zones.
This developing technology has created a need for improved understanding of the directional tendencies of eccentric drilling tools run on steerable assemblies and the wellbore geometry and quality that can be achieved with these tools. Consistent wellbore diameter is of particular concern for expandable tubulars. If the wellbore diameter is too small, expansion of the pipe with a fixed diameter cone might not proceed properly across sections of firm formation. Worse yet, the expansion cone could become stuck requiring remediation or sidetrack of the well. A wellbore that is too large could affect the sealing effectiveness depending on the sealing system used. For example, a closer diameter tolerance would be required if the seal mechanism is an integral part of the casing (elastomer bonded to the outside of the casing).
