Drilling with Casing Reduces Cost and Risk

Abstract

More than 1.2 million ft of hole has been drilled in over 260 wells with retrievable casing-while-drilling tools. These wells have been drilled with seven casing sizes ranging from 4–1/2″ to 13–3/8″. The drilling system is composed of downhole and surface components that provide the ability to use normal oil field casing as the drillstring so that the well is simultaneously drilled and cased. The casing is rotated from the surface for most operations. The experience gained while drilling these wells has led to a better understanding of the advantages of the process. It's most effective applications have been to reduce the cost and risks associated with wells which routinely experience lost circulation when drilled conventionally; wells in which well control is difficult for tripping drillpipe; wells where excessive fluid loss into low pressure reservoirs impairs the production and ultimate recovery; and wells where it is difficult to get the casing shoe as deep as needed. Introduction Simultaneously drilling and casing a well by using conventional oilfield casing as the drillstring has been a dream of a few drillers since the rotary drilling process was introduced in the late 1800's. One patent1 dating back to 1890 describes a rotary drilling process for simultaneously drilling with the casing (including a sand control screen) and then retrieving the hydraulically expandable bit. Another patent issued in 1926 list the advantages (Table 1) of drilling with casing with a retrievable bit.2 Inventors kept up a steady stream of ideas aimed at fielding a practical system for drilling with casing and a retrievable bit.3–10 Considerable work was done in Russia beginning in the late 1930's to develop a system for replacing the drill bit without having to trip the drillstring. This work was stimulated by the desire to reduce the time spent tripping to replace bits that had relatively short runs.11, 12 Work done by Brown Oil Tools13 in the late 1960's was the first broad effort to develop all the surface and downhole components needed to drill with casing and retrievable bits. This "casing-drilling system" included components forrotating the casing with an electric top drive,gripping the casing without utilizing its threads,locking a wireline retrievable drilling assembly to the bottom of the casing, andusing an underreamer to enlarge the borehole. Ultimately, this work failed because of the inertia in the drilling industry, but it did stimulate the development and commercialization of the top drive. Tesco started development of a casing-while-drilling (CwD) system in 1997 that included the complete system needed to drill with casing. While others had failed to accomplish this earlier, improvements in materials (e.g. PDC cutters for underreamers), increased market applications (particularly related to problems encountered in drilling marginal fields), and the brute determination and financial commitment of Tesco management successfully produced a CwD system that has gained commercial acceptance. This system has proven to be a practical method of solving drilling problems and of reducing drilling costs.14–18 It is currently being used in both onshore applications where entire wells are drilled with casing and offshore applications where specific hole sections are drilled with casing.