Development And Testing Of A Downhole Replaceable Continuous Chain Drill
Authors: Varnado S.G. , St. Clair J.A. , Togami H.K. 
Published: December 2007

This work was supported by the U.S. Department of Energy.

Abstract
The continuous chain drill is a diamond drill bit in which the cutting surface can be replaced while the bit is still in the hole. Successful field testing of this bit has recently been accomplished. This paper describes the development and testing of this prototype.

Introduction
The continuous chain drill is a unique system in which the cutting surface can be replaced without removing the drill from the hole. This system is being developed by Sandia Laboratories which manages the Geothermal Drilling and Completion Technology Program for the Department of Energy, Division Program for the Department of Energy, Division of Geothermal Energy (DOE/DGE).
Economic studies of the chain drill system have been made which show the system to be commercially viable when modest performance parameters are achieved, i.e. in order to parameters are achieved, i.e. in order to attain breakeven, each cutting surface (5 chain links) must have a life of 1/3 of a conventional bit if an equivalent penetration rate is achieved. Since the economic advantage is closely associated with time saved in tripping the bit, the cost savings are most significant where rig cost is high and/or drilling is deep as shown in Figure 1. However, appreciable savings are available in shallow drilling.
The first prototype chain drill was tested in September, 1977 at the Drilling Research Laboratory in Salt Lake city. The most noteworthy achievement in this initial phase of development was the demonstration of phase of development was the demonstration of a viable cutting surface. This was essential for further development of the chain drill and was accomplished by a team effort between Sandia Laboratories and Christensen, Inc., in Salt Lake City. It represents a refinement in diamond bit technology in terms of diamond setting and bit hydraulics. During these tests, it was shown that the chain cycling mechanism was not reliable. However the concept of changing the cutting surface downhole was demonstrated.
The results of testing the Prototype I chain drill were encouraging, but a new system for reliably cycling the chain drill was needed. It was decided that a downhole cycling system must be compatible with current drilling practices and must employ proven downhole mechanisms wherever possible. proven downhole mechanisms wherever possible. Based on these guidelines, a new system concept was developed. This concept was reviewed with several drilling, downhole tool and oilwell service companies, and their comments solicited. No major changes were recommended although other approaches were discussed. The general consensus was that the development program would take four to five years because of the formidable downhole environment. An accelerated program for design, fabrication, and testing was established to reduce the expected long development time. This program has allowed for a field test of the redesigned Prototype II drill in a period of less than two years.

DESIGN CONCEPT PROTOTYPE II
The Prototype II chain drill is 4-3/4" in diameter and 96 inches long as shown in Figure 2. The body is a two-piece construction joined by splines. The pin end of the body has a fishing neck which will allow the bit to be fished, washed over, and jarred. Energy for drawing the fifty link chain (10 cutting surfaces) is stored in a large helical spring. The two-piece body is locked together with fluid actuated detents. The mud flows through the bit body and exits through five nozzles in the bit head. A single small nozzle in the bit body is used to wash out the chain cavity.