Page 162 - Air and Gas Drilling Manual

5-4 Air and Gas Drilling Manual
Unless field test data are available that give the actual rock specific gravity, the
approximate average specific gravity for sedimentary rock is assumed to be 2.7, the
average specific gravity for igneous rock is assumed to be 2.8, and the average
specific gravity for metamorphic rock is assumed to be 3.0 [1].
Illustrative Examples 5.1, 5.2a, and 5.3a describe the implementation of the
basic planning steps Nos. 1 through 6 given in Section 5.1 above.
Illustrative Example 5.1 Using the basic equations above, determine the direct
circulation minimum volumetric flow rate for a well with a 4 1/2 inch openhole
borehole (4 1/2 inch drill bit diameter) and a uniform drill string composed of a drill
pipe outside diameter of 2 3/8 inch. The anticipated drilling rate in a sequence of
competent unfractured limestone rock (sedimentary rock) is assumed to be 30 ft/hr.
The drilling depth is 1,000 ft.
In order to be able to calculate air drilling conditions at other surface elevations
above sea level, all calculations to obtain values like the minimum volumetric flow
rate are carried out for a baseline sea level atmosphere. For these calculations and
most of those in this treatise the baseline sea level atmosphere is the API Mechanical
Equipment Standards standard atmospheric conditions (see Chapter 4). These
conditions are a pressure of 14.696 psia and a temperature of 60˚F. Thus, the
pressure of the gas (in this case air) that flows into the compressor, P at, is
.
p at = 14 696 psia
P at = p 144
at
2
,
P = 2 116 lb/ft abs
at
The atmospheric temperature of the gas (air) that flows into the compressor, T at, that
will supply the drilling operation is
o
t = 60 F
at
T at = t at + 459 67
.
o
.
T at = 519 67 R
Thus, P g and T g become
P = P = 2 116 lb/ft abs
2
,
g at
T = T = 519 67 R
o
.
g at
Using Equation 4-11, the specific weight of the gas entering the compressor is