Peh: Drilling Problems and Solutions Publication Information Petroleum Engineering Handbook

Pressure Diffusion. Pressure diffusion is a phenomenon of pressure change near the borehole
walls that occurs over time. This pressure change is caused by the compression of the native
pore fluid by the borehole-fluid pressure, p
wfl
, and the osmotic pressure, p
os
. 
Borehole Fluid Invasion into Shale. In conventional drilling, a positive differential pressure
(the difference between the borehole-fluid pressure and the pore-fluid pressure) is always
maintained. As a result, borehole fluid is forced to flow into the formation (fluid-loss
phenomenon), which may cause chemical interaction that can lead to shale instabilities. To
mitigate this problem, an increase of mud viscosity or, in extreme cases, gilsonite is used to seal
off microfractures.
Several models in the literature address wellbore-stability analysis.
[2]
 These include very-simple
to very-complex models such as linear elastic, nonlinear, elastoplastic, purely mechanical, and
physicochemical. Regardless of the model, the data needed include rock properties (Poisson
ratio, strength, modulus of elasticity); in-situ stresses (overburden, horizontal); pore-fluid
pressure and chemistry; and mud properties and chemistry. 
Other than the mud data, the data are often compounded with problems of availability and/or
uncertainties. However, sensitivity analysis can be conducted by assuming data for the many
variables to establish safety windows for mud selection and design.
Total prevention of borehole instability is unrealistic because restoring the physical and
chemical in-situ conditions of the rock is impossible. However, the drilling engineer can mitigate
the problems of borehole instabilities by adhering to good field practices. These practices include
proper mud-weight selection and maintenance, the use of proper hydraulics to control the ECD,
proper hole-trajectory selection, and the use of borehole fluid compatible with the formation
being drilled. Additional field practices that should be followed are minimizing time spent in
open hole; using offset-well data (use of the learning curve); monitoring trend changes (torque,
circulating pressure, drag, fill-in during tripping); and collaborating and sharing information.