Page 37 - Air and gas Drilling Field Guide 3rd Edition

28 CHAPTER 2 Air and Gas Versus Mud

Temperature ( F)
40 60 80 100 120 140 160 180 200
0 0
Injection
Exit
−500
−2000

Annulus −1000
−4000
Depth (ft) −1500 Depth (m)

−6000 Inside Drill String
−2000

−8000
−2500
Bit
−10000 −3000
0 20 40 60 80
Temperature ( C)

FIGURE 2-18. Mud drilling temperature versus depth.

air or other gases. Thus, as the drilling mud flows down the drill string and up
through the annulus to the surface, heat is transferred from the rock format-
ions through the surfaces of the borehole, through the drilling mud in the annu-
lus, and through the steel drill string to the drilling mud inside. It is assumed that
the drilling mud is circulated into the top of the inside of the drill string at 60 F

(15.6 C).
As the drilling mud flows down the inside of the drill string, the drilling mud,
because of its high specific heat, heats up as heat flows from the higher tempera-
ture rock formations and drilling mud in the annulus. At the bottom of the well
the drilling mud temperature reaches the bottom hole temperature of 160 F

(71.1 C). The drilling mud flowing up the annulus is usually laminar flow and is

heated by the geothermal heat in the rock formation. The heated drilling mud flow-
ing in the annulus heats the outside of the drilling string, which in turn heats the
drilling mud flowing down the drill string. Because of its good heat storage capabil-
ities, the drilling mud exits the annulus with a temperature greater than the injec-
tion temperature but less than the bottom hole temperature. In this example, the
temperature of the drilling mud exiting the annulus is approximated to be the aver-

age of the injection and bottom hole temperatures, i.e., 130 F(54.4 C).

Figure 2-19 shows plots of the temperature in compressible air drilling fluid as
a function of depth. The compressed air drilling fluid is significantly less dense