BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
Figure 7 - 3
Approximate Mud Weight Required to Control Salt Creep
Coal Formations
Coal is a very brittle formation with low compressive strength. It is usually highly fractured
and in areas of high tectonic stress can instantly collapse into the borehole when the horizontal
stress is relieved by the bit – stuck pipe is often the end result. High drilling fluid densities can
rarely be used to stabilize the coal formation because of their inherent low fracture gradients.
The recognized technique to drill coal is to limit penetration rates so that the stresses are given
a chance to equilibrate and so that the blocky pieces of coal can be removed from the hole.
Good hole cleaning is essential – it may be necessary to modify the cleaning capacity of the
drilling fluid while drilling coal. The properties can be restored to normal when the coal has
been drilled. Alternatively viscous, weighted sweeps can be used to enhance hole cleaning –
this is particularly appropriate if the coal seam is anticipated to be thin.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-10