Page 215 - Air and Gas Drilling Manual

Chapter 5: Shallow Well Drilling Applications 5-57
5.3 Reverse Circulation
Reverse circulation is extensively used in shallow air drilling operations. This
circulation drilling technique is usually used to drill specialized geotechnical and
environmental monitoring boreholes and wells. Reverse circulation drilling,
particularly with dual wall pipe, have important operational advantages for the
drilling and development of environmental monitoring wells [4, 5, 6]. This
circulation technique is very useful in drilling larger diameter shallow wells. Tri-
cone drill bits fabricated for shallow drilling operations have a single large water
course passage through the drill bit body which allows these drill bits to be used for
both direct and reverse circulation shallow operations (see Figure 1-9). As will be
seen in Chapter 11, air hammers are also a very useful downhole tool for both direct
and reverse circulation operations. These tools and their drill bits are designed for
direct or reverse circulation drilling operations. In this chapter only tri-cone drill bit
operations will be discussed.
5.3.1 Minimum Volumetric Flow Rates
In order to initiate the well planning procedure given in Section 5.1, the
geometry of the reverse circulation operation must be defined and the anticipated
drilling penetration rate estimated. Figures can be prepared to give the approximate
minimum volumetric flow rates for a variety of shallow well and drill string
geometry. The calculations to prepare the plots for these figures are carried out using
API standard atmospheric conditions (i.e., 14.696 psia and 60˚F, see Chapter 4).
Thus, the figures developed will give the minimum volumetric flow rate values for
air drilling using atmospheric air at API standard conditions. Once these figures are
developed, the minimum volumetric flow rates can be calculated for any other
atmospheric conditions (surface locations) from the minimum volumetric flow rates
given for API standard conditions. The approximate minimum volumetric flow rate
values are calculated assuming a minimum bottomhole (inside the drill string)
3
kinetic energy per unit volume of no less than 3.0 ft-lb/ft (see Chapter 8 for details).
The basic equations used to determine the minimum volumetric flow rate are derived
in Chapter 7.
The minimum volumetric flow rates are calculated assuming that the boreholes
(surface to bottom) are openholes (not cased). The calculations for determining a
minimum volumetric flow rate is a trial and error process. The equations needed to
determine the minimum volumetric flow rate are outlined below. The critical
equation for determining the minimum volumetric flow rate is derived in Chapter 7.
This is the equation for the pressure above the drill bit inside the drill string, P ai.
This equation is

.
 2 aH  05
i
−
2
T av
P ai =   P ( at 2 + bT av) ebT av 2   (5-12)
i
i
   
2
where P at is the atmospheric pressure at the exit from the annulus (lb/ft , abs),
H is the depth of the borehole (ft),
T av is the average temperature of the borehole over the depth interval (˚ R).