Page 72 - Air and gas Drilling Field Guide 3rd Edition

4.1 Rotary Drill String 63

AIR DISCHARGE
WITH GEOLOGICAL
MATERIAL

ROTARY TOP DRIVE

AIR IN

DUAL-WALL DRILL PIPE

BIT SUB

WEAR SLEEVE

TRICONE ROCK BIT

FIGURE 4-3. Dual wall pipe drill string for reverse circulation operations (courtesy of Foremost
Industries Incorporated).

tricone drill bit. The top drive system rotates the entire drill string. The tricone
drill bits used in reverse circulation operations have the same cutting structures
as tricone bits used in direct circulation operations. However, the reverse circu-
lation bits are fabricated to allow the compressed air to flow from the annulus
between the two walls of the dual wall pipe to the bit rock cutting face. At
the bottom of the well the air flow entrains the rock cuttings and flows to the
surfacethrough alarge centerorifice in thedrill bitthatleads totheinsideof
the inner pipe of the dual wall pipe. The drill bit used in a dual wall pipe reverse
circulation operation is selected to have a diameter that is slightly larger than
the outside diameter of the dual wall pipe. Thus, the outside annulus between
theinsideofthe borehole andthe outsideofthe dual wall pipe is kept quite
small. This outer microannulus, together with a skirt structure fabricated on
the drill bit like that shown in Figure 2-9, restricts the return flow of air to
the surface and entrained rock cuttings to the inside of the inner pipe [4].