Page 260 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 236

[ ] Well Programming
2.5.8

ical titration of a sample of the water phase (obtained by breaking
down the emulsion of a sample of the mud, diluting with water, and
creating a straight emulsion for test purposes).
A measure of the osmotic gradient that is present can also be
derived from the water activity (Aw) of the mud. This test measures the
percentage relative humidity in air above a sample of mud in a closed
space using a hydrometer.
Originally, it was considered that the osmotic gradient should be
such that it be sufficient only to avoid a net movement of water from
the water phase of the mud to the formation. This was described as a
“balanced activity.” However, the benefits of an osmotic imbalance in
favor of water transport from a shale to the water phase of an oil mud
can be seen in improved hole stability. In fact there are several other
forces acting to create flows of water apart from the osmotic (chemi-
cal)—hydraulic (overbalance) and electrical potentials also play a part.
It is the net of these forces that determines whether and in which direc-
tion flow will take place.
It is possible to define the required water phase salinities or water
activities for any given matrix stress for any shale at any depth. To cre-
ate the osmotic gradient in favor of water transport being directed to
the water phase of the mud, there are two forces to be considered. First,
there is the osmotic pressure generated by the water phase salinity of
the mud. Acting in the opposite direction, there is the osmotic pressure
of the shale. This is related to the matrix stress of the shale.
The osmotic pressure of a shale is generated in two ways. First,
during diagenesis, water is forced out of the shale because of com-
paction (or generation of matrix stress). Diagenetic water leaving the
shale has a lower salinity than water remaining in the pore spaces. The
resultant shale salinity will be higher. Additionally, because of com-
paction, pore spaces are reduced. In drilling a shale, there will be a ten-
dency for the rock to be released from stress at and near the bore hole
wall. This release of stress will tend to cause expansion and resulting
increase in pore volume. If there were to be an increase in pore volume,
there would be a suction potential for water into the shale. In effect,
this would be the opposite of diagenesis. If water can be prevented
from entering the shale, the suction potential will provide stabilization
by minimizing pore volume expansion.

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