BAKER HUGHES - Drilling Fluids Reference Manual

BOREHOLE PROBLEMS
σ = the membrane efficiency
∆ Π =- the osmotic pressure defined by:
∆ Π =
RT ln
V w
⎡ a
⎢
⎣a
sh
w
df
w
⎥
⎦
⎤
where:
T = temperature,
R = the gas constant,
V
w
= the partial molar volume of water,
df
a and = the shale and drilling fluid water activities respectively.
sh
w
a w
These equations show that there are four methods for reducing J v:
1. Reducing the flow of drilling fluid filtrate by increasing the filtrate viscosity η ;
2. Reducing the flow of drilling fluid filtrate by reducing the shale permeability k ;
3. Balancing the flow of drilling fluid filtrate into the shale driven by overbalance ∆P by a
backflow of pore water driven by the combined osmotic pressure σ ∆ Π ;
4. Combining the above mechanisms.
Pore Pressure Transmission Inhibition
Emulsion drilling fluids are the ultimate membrane generating systems and provide superior
drilling performance primarily because of their ability to reduce pressure transmission in shale.
Preventing pressure invasion in shale using WBM is a formidable obstacle due to the technical
difficulty of generating an efficient membrane while remaining cost-competitive with
OBM/SBM. A significant technology gap in High Performance Water Based Drilling Fluid
(HPWBM) occurs when the system cannot generate a membrane approaching that of
OBM/SBM. PERFORMAX is designed to closely emulate the shale stabilizing
mechanisms of an emulsion system by generating a selective membrane and creating an
osmotic pressure difference between the drilling fluid and the shale.
The mobility of solutes in shale determines membrane efficiency of drilling fluids. The shale
essentially acts as a semi-permeable (selective) membrane because the clay-rich matrix hinders
the movement of solutes. The shale matrix can be made more selective, thus further reducing
the mobility of solutes, by decreasing shale permeability.
Other water based drilling fluids also generate selective membranes using various products
including cloud point glycols, silicates and ALPLEX®. In PERFORMAX a two-prong
approach is used to generate the membrane. This is done by reducing shale permeability using
both mechanical and chemical means. MAX-SHIELD, a deformable sealing polymer,
having an extremely small particle size, is used to mechanically bridge micro-fractures on the
exterior of the shale matrix.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 7-14