BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
dispersion. The encapsulation action of this polymer enhances the physical integrity of shale
cuttings. It therefore gives more strength against mechanical degradation while the cutting is
in transport out of the wellbore.
The hydrated layer formed when the polymer adsorbs onto the shale particles is permeable, and
other water molecules will eventually react with the shale particles causing shale hydration and
swelling. This process could lead to disintegration, but due to the presence of the PHPA
polymer, the shale particle remains intact. This suggests that the benefits gained from PHPA
are indeed of a mechanical nature and the ability of this polymer to reduce swelling is a kinetic
effect.
Pore Pressure Transmission
The most important factor in maintaining shale stability is preventing pressure increases in the
shale matrix. Pore pressure increases, and differential pressure support decreases, when drilling
fluid filtrate invades the shale. Wall support is lost with the reduction in differential pressure
and the shale can then easily slough into the annulus. Shale (borehole) stability is achieved
when pressure increases in the matrix are reduced and differential pressure support is
maintained. The problem of pressure invasion is exacerbated by shales having low
permeability, which slows the rate at which pressure can dissipate. This typically confines the
added pressure in the shale to the vicinity of the near-wellbore surface.
Pressure transmission in emulsion systems is managed by the generation of a semi-permeable
membrane and the osmotic potential pressure difference between the shale and emulsion fluid.
Sources of the semi-permeable (selective) membrane of OBM/SBM proposed in the literature
include the oil film and emulsifiers/surfactants surrounding the emulsion. Calcium chloride in
the internal phase reduces water-phase activity and creates an osmotic pressure differential at
the borehole wall. Additionally, pressure invasion in emulsion systems is suppressed by the
capillary entry pressure that must be overcome in order to force oil into the water-wet pore
throat.
Conventional water based drilling fluids are poor in drilling weak rocks like shales. These
require an effective radial support stress, provided by drilling fluid overbalance, for stability.
Overbalance, however, also drives the flow of drilling fluid filtrate and diffusion of drilling
fluid pressure into the shale, causing a number of destabilizing effects. The most profound of
these is near-wellbore pore pressure elevation which destroys effective drilling fluid pressure
support. The in situ stresses may now overcome the strength of the shale, causing plastic
deformation and failure. OBM and SBM are restricted from invading shales due to capillary
threshold pressures. Improvement of WBMs involved reducing the filtrate flow J v into shales,
which can be described by:
Where:
J v = k ( ∆P – σ ∆ Π ) / η
k = shale permeability,
η = filtrate viscosity,
∆P = the hydraulic overbalance,
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-13