Page 262 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 238

[ ] Well Programming
2.5.8

Matrix stress. In a few cases, you may be running a downhole tool
that will tell you the matrix stress. If this is not possible, you have to
use the most accurate measurement of pore pressure and work on the
rough model that matrix stress + pore pressure = overburden pressure
(derived as having a gradient of 1 psi per foot unless it is known more
accurately). For this example, this rough model was applied. The
shale is at 12,000 feet and pore pressure was determined to be equiv-
alent to an 11.1 ppg fluid or 6296 psi. The matrix stress is obtained by
subtracting 6296 psi from 12,000 psi (the assumed overburden pres-
sure at 12,000 feet).
Shale salinity. This may be harder to quantify at the rig unless the
logging unit is measuring it. It can be measured in the laboratory with
a certain degree of accuracy. You may have to make a best guess at shale
salinity. Discussion with geologists may help in respect to shale type. A
marl will have a higher shale salinity than a claystone for example. If
you study the graph, you will see that at low matrix stresses (shallow-
er depths), that variance in shale salinity is more important than at
higher matrix stresses (deeper depths) except for shales with salinities
of 200,000 to 300,000 ppm NaCl equivalent. These shales are not very
common. At shallow depths, the shale salinity is more likely to be in
the range of up to 100,000 ppm NaCl equivalent salinity.
It is worthwhile to study the graph, irrespective of whether you
apply it or not, since it shows how the two parameters—matrix stress
and shale salinity—affect the required water phase salinity.
Properties of the brine phase—emulsion. The water phase,
because it is discontinuous and coated with emulsifier, will behave as a
solid in the oil mud and demonstrate itself in the plastic viscosity
dependent on the strength of the emulsion and the amount of brine
phase present. A tight emulsion will provide for smaller sized droplets
of emulsified brine and a lower plastic viscosity. It should be remem-
bered that the base oil, the insoluble solids in the mud, and the oil wet
condition of the insoluble solids also contribute to the plastic viscosity.
Properties of the brine phase—oil/water ratio. The size of the
water phase in an invert oil emulsion has a significant effect on almost
all the properties of the resultant mud. The size of the water phase, as
well as being measured as the volume percentage of the total mud vol-
ume, can be quantified in what is the most fundamental of parameters
of an invert emulsion oil mud—the oil/water ratio (OWR). This is

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