BAKER HUGHES - Drilling Fluids Reference Manual

Baker Hughes Drilling Fluids
pressure support (over-balance) is lost.
Pressure Invasion: Finally, loss of differential pressure support at the wellbore surface allows
the over-burden pressure (weight of shale pressing down) to destabilize the invaded shale and it
will slough off (third image) into the annulus. In this case, the pressure of the mud column is
equal to the pressure within the shale, leaving no supporting pressure to prevent the overburden
force from sloughing the shale.
Capillary Effects
Neither water-base fluids nor oil-base fluids form a solid filter cake in shale. Under normal fluid
pressures, shales are permeable to water-base fluids but almost impermeable to oil. The stable
behavior of shales while drilling with oil-base fluid or synthetic-base fluids is a result of capillary
action. When oil enters a shale, it has to overcome a threshold pressure caused by the capillary
effect between oil and the pore fluid. The capillary pressure is in the order of thousands of psi and
is generally too large to be overcome by the fluid pressure differential. The threshold pressure,
therefore, acts as an alternative semi-permeable membrane and provides effective fluid support to
the wellbore.
Oil/ Synthetic Based mud
Water Based Mud
Wellbore
pressure
Wellbore
pressure
Water
Pore pressure
Water
Water
Pore pressure
Figure 3-17
Capillary Action in Oil/Synthetic vs. Water Base Fluids
As a consequence, shale instability with oil and synthetic drilling fluids is normally caused by
lack of fluid support, i.e., too low of a fluid density.
Cloud Point Behavior
Definition
Cloud point is a phenomenon exhibited by many glycols. The solubility of these glycols in water
decreases as temperature increases, with materials that are fully soluble at room temperature
forming separate phases at higher temperatures. The temperature at which the glycol and water
separate is known as cloud point, since the previously clear solution becomes “cloudy” upon
separation as shown in Figure 3-17 Capillary Action in Oil/Synthetic vs. Water Base Fluids
Baker Hughes Drilling Fluids
Reference Manual
Revised 2006 3-73