Page 42 - Air and Gas Drilling Manual

Chapter 1: Introduction 1-19
Figure 1-18 shows the plots of the temperature in the incompressible drilling
mud as a function of depth. The geothermal gradient for this example is 0.01˚F/ft.
Subsurface earth is nearly an infinite heat source. The drilling mud in a mud
drilling circulation system is significantly more dense than compressed 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 formations through the
surfaces of the borehole, through the drilling mud in the annulus, through the steel
drill string to the drilling mud inside. It is assumed that the drilling mud is
circulated into the top of the drill string at 60˚F.

Figure 1-18: Mud drilling temperature versus depth.

As the drilling mud flows down the inside of the drill string the drilling mud
heats up as heat flows from the higher temperature rock formations and drilling mud
in the annulus. At the bottom of the well the drilling mud temperature reaches the
bottomhole temperature of 160˚F. The drilling mud flowing up the annulus (usually
laminar flow conditions) is heated by the geothermal heat in the rock formation.