BAKER HUGHES - Drilling Fluids Reference Manual

Water Based Drilling Fluids
Inhibition Studies with Tertiary Clays
Figure 3-16
Inhibition Studies with Tertiary Clays
Pore Pressure Transmission
Baker Hughes Drilling Fluids has explored pore pressure transmission
(PPT) testing for several years. PPT testing measures the formation pore
pressure increase from filtrate invasion in very low permeability formations
such as shales. In highly permeable formations the pressure rise from
filtrate flow is rapidly dissipated in the formation volume and pore pressure
is not affected. However, in very low permeability formations, the pressure
increase from filtrate invasion declines very slowly and the pore pressure
continues to increase with additional filtrate flow. This pore pressure
increase reduces the effective over balance pressure. Over-balance pressure
decline is exaggerated by wall fractures from drilling. These fractures
increase near-wellbore permeability resulting in rapid pressure increase
inside the wall. Reduced overbalance tends to destabilize the wellbore and
promote sloughing. The introduction of cloud point glycols (AQUA-
DRILL), aluminum complexes (ALPLEX) and sealing polymers
(PERFORMAX) by Baker Hughes Drilling Fluids has greatly improved
the osmotic effectiveness of water-based drilling fluids.
Properly Pressured: Ideally, the hydrostatic pressure exerted by the mud
column is higher than the formation (pore) pressure. Here, there is a
pressure differential at the shale surface which serves as a support
mechanism (first image) to offset over-burden pressure.
Over Pressured: Here, drilling fluid pressure invasion (second image)
occurs into the shale matrix. Due to the low permeability of the shale, the
added pressure dissipates very slowly. In emulsion fluids, the capillary
entry pressure effects strongly impede the invasion of pressure from base
fluids (oils, esters or synthetics) because of the wetting characteristics of
water vs. base fluids. However, in water-based muds, the capillary entry
pressure can be very low and therefore water pressure invasion can easily
occur in the matrix. As pressure invasion occurs, the pore pressure at the
shale surface rapidly increases to a level equivalent to that of the mud
column. Then, the differential pressure at the surface is reduced and
Reference Manual
Baker Hughes Drilling Fluids
3-72 Revised 2006