Cement in Place Drilling with Casing System Provides Safe, Reliable Method for Improving Drilling Efficiency
Authors: Galloway Greg 
Published: October 2008

Using Casing as the drillstring has a number of applications ranging from very soft surface hole formations to deep production casing strings. Drilling with casing has been performed in excess of 400 wells worldwide. Utilizing a drillable drill bit attached to the bottom of the casing has proven to be a very simple technique with a growing range of applications. The result has been significant reductions in flat time and more efficient wellbore construction.

As drilling efficiency increases in a number of areas, one area little affected has been the reduction of 'flat time'. The method of drilling with casing significantly reduces operator flat time and streamlines the drilling process. This paper describes the system used to deliver flat time reductions while maintaining and even improving on bottom drilling efficiency. (figure 1)

Introduction

Standard oilfield drill bits, whether roller cone or drag type, are constructed of steel, tungsten carbide, diamond and other hard materials. These materials are highly effective in rock destruction but are not 'drillable'. The development of a 'drillable' drill bit in early 2000 enabled a system for drilling with casing as the drill string that could be cemented in place once casing point has been achieved. The basic system includes a drillable drill bit/casing shoe (DrillShoe? tool), float collar, centralization/stabilization, and a drive system to allow torque transmission from the top drive to the casing string. The system is revolutionary in that for the first time, nothing is required to be tripped out of the hole prior to initiating the next hole section.

Eliminating tripping requirements results in more efficient drilling operations without the problems and safety risks associated with tripping pipe and BHA components. Eliminating trips and associated time significantly reduces flat time in the drilling curve.

The Drillable Drill Bit Cutting Structures

Based on success of a tool designed to ream casing through tight hole, dog legs, ledges and past cuttings bed, the concept was born for a drill bit that could be used on casing to drill soft formations then be drilled through with the subsequent bit. The initial designs were very simple tools that were immediately successful in drilling very soft formations1. These tools consisted of an aluminum alloy inner core and a steel outer housing. The inner core and the outer housing were locked together and the structure was machined to form a bladed profile. (figure 2) Aluminum, being relatively soft material that is susceptible to abrasion, requires hardfacing that would be abrasion resistant but maintain the drillability of the tool. The HVOF (Hyper Velocity Oxy Fuel) process was selected as the method to apply the hard coating without damaging the soft aluminum base materials. The HVOF process uses a fuel/oxygen mixture in a combustion chamber. Tungsten carbide containing powder is continually fed into the gun. The combustion process melts a TC powder and propels it at super sonic speed toward the surface of the blade2. The partials then bombard the blade, resulting in impact fusion. The result is a very dense, low porosity layer of tungsten carbide coating the aluminum alloy blade2.