Page 323 - Air and Gas Drilling Manual

8-6 Air and Gas Drilling Manual
fixed solids flow rate, the solids volumetric concentration in the pipe increases and
the wall frictional loss decreases. Thus, the line CD shows an overall pressure
gradient decrease with the decrease in gas velocity. This line indicates that the
decrease in frictional loss component is significantly higher than the increase in the
hydrostatic component. As the gas velocity is further decreased beyond point D, the
hydrostatic component becomes more significant and the resulting curve defines a
minimum at point E. A further decrease in gas flow rate (and velocity) leads to an
increase in the pressure gradient. This is shown as the curve EF and indicates the
dominance of the hydrostatic component. The solids concentration along curve EF
is high and is dense phase solids flow.
In general, for air and gas drilling systems the choking point can be defined as
the inflection point along the curve EF. Fortunately, the choking condition is rarely
observed in actual vertical air or gas drilling operations. This is due to two
important air and gas drilling operational facts. 1) As the drill bit advances, the
rotating drill string breaks up larger rock particles into smaller more easily
transported particles when the larger particles collide with the drill string surface.
This breaking action occurs all along the drill string length, but is likely very
pronounced around and just above the drill collars. This mechanism has been
observed in vertical drilling operations where downhole pneumatic motors have been
tested (drilling with no drill string rotation and with rotation) [11]. 2) As will be
seen in the illustrative examples to follow, the actual volumetric flow rate used in an
air drilling operation is determined by the primary compressor output(s) to be used
at the drilling location. If the approximate minimum volumetric flow rate to the
borehole is determined to be 1,000 scfm, and the compressors to be used to supply
the compressed air are rated at 700 scfm each, then two compressors will be used to
supply the air. Thus, the actual volumetric flow rate to the well is 1,400 scfm.
This gives a factor of safety (above the minimum required) of 1.4. It is standard
engineering practice to use a volumetric flow rate factor of safety of at least 1.2.
This approximate minimum factor of safety (i.e., 1.2) is also used in determining
the actual volumetric flow rate to be taken from a natural gas pipeline for a gas
drilling operation. The use of primary compressor flow rates that are above the
minimum volumetric flow rate would result in an operational point for the air and
gas system somewhere along the curve CD.
8.2.3 Engineering Planning Graphs
In order to initiate the well planning process given above, the geometry of the
well must be defined and the anticipated drilling penetration rate estimated. Figures
with graphs can be prepared for the approximate minimum volumetric flow rates for
a variety of deep well and drill string geometric configurations. These figures are
presented in Appendix E. The calculations for such figures are carried out using API
Mechanical Equipment Standards 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 air at these API standard
atmospheric conditions (see Chapter 4). Once such figures are developed, the
minimum volumetric flow rates can be determined for any other atmospheric
conditions (surface locations) from the minimum volumetric flow rates given for
API standard conditions. The minimum volumetric flow rate values are calculated