Benefits of Dual Rotary Drilling in Unstable Overburden Formations
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Benefits of Dual Rotary Drilling in Unstable Overburden Formations

INTRODUCTION
THE DUAL ROTARY DRILLING CONCEPT WAS DEVELOPED by Barber Industries and commercialized with the introduction of a DR (Dual Rotary) rig in 1979. In 1993, this technology was acquired by Foremost Industries, LP of Calgary, Canada. The Dual Rotary method has been proven repeatedly where unconsolidated formations (sand, gravel, cobbles, and boulders) make it difficult to drill a cased hole using conventional drilling techniques. The versatility of this unique drilling rig makes it one of the most efficient and cost effective methods for drilling holes in difficult formations. The primary distinguishing feature of a Dual Rotary drill rig is a lower rotary drive that is used to advance steel casing through unconsolidated overburden. Rotational forces are transmitted to the casing via power-operated jaws. A carbide-studded shoe, welded to the end of the first piece of casing, enables the casing to cut its way through the overburden. A top drive rotary head simultaneously handles a drill string equipped with either a down-the-hole hammer, drag bit or rolling cone bit to drill the center. Initially, the Dual Rotary technique was perfected for use in waterwell and construction applications.
Foremost continues to refine the Dual Rotary concept and in recent years has introduced new DR models and design features that have improved performance and broadened its range of applications. Today, Dual Rotary rigs operate very successfully in Africa, Australia, New Zealand, Canada, Europe, Mexico, Korea, Taiwan, South America, Trinidad, El Salvador and the USA. This paper briefly addresses major overburden drilling methods and explains the important features and benefits of the Dual Rotary drilling method.

BACKGROUND
THE CABLE TOOL (PERCUSSION) METHOD OF DRILLING HOLES has been used successfully for overburden and hard rock drilling for centuries. In 1948, Wendell Reich (USA) is credited with revolutionizing the well drilling industry with the development of a top drive rotary rig with hydraulic pull-down capabilities. The next major advancement in drilling productivity came with the introduction of the down-hole hammer. These two events contributed significantly to the state of hard rock drilling as we know it today. However, technical
progress in overburden drilling has not been as spectacular. Drilling holes in overburden and unconsolidated formations, due to unpredictable variations, continues to present drillers worldwide with challenges such as loss of circulation, boulders, unstable walls, crooked
holes, aquifer contamination and heaving sands. While a variety of overburden drilling methods exist, historically none has been consistently able to overcome all of these challenges. In 1979, Barber Industries, in conjunction with Wendell Reich, conceptualized and built a rig that could simultaneously drill a hole and set casing under the most difficult of drilling conditions and subsequently introduced the first Dual Rotary drill rig.

MAJOR DRILLING SYSTEMS
THERE ARE A VARIETY OF COMMONLY USED TECHNIQUES for drilling in formations that require hole stabilization. The best method in any one situation is a function of the equipment available, ground conditions, environmental nconsiderations, and operator expertise. The major overburden drilling systems that have been successfully used are:
• CABLE TOOL (PERCUSSION/DRILLING AND DRIVING)
• ROTARY DRILLING UTILIZING BENTONITES OR POLYMERS (OPEN HOLE METHOD WITH ‘MUD’)
• ROTARY DRILLING WITH CASING HAMMER
• ROTARY DRILLING WITH UNDER-REAMING (ECCENTRIC/CONCENTRIC)

• THE DUAL ROTARY DRILLING METHOD