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13
SECTION 4
CALCULATIONS AND THEORY OF MEASUREMENT
A. Stuck Tendency Coefficient
Using the standard test described in Section 3 "Standard Test Procedure" and the 12.5" (31.75
cm) radius'd plate and 477.5 psi (3292 kPa), the average of 6 "break" readings taken within 30
seconds of one another divided by 1000 is known as the
STUCK TENDENCY COEFFICIENT
(K
st
) of a drilling fluid. This coefficient takes into account both the bulk sticking coefficient and the
cake-building character of the sample (Refer to Fig. 2).
The physical basis for the Stuck Tendency Coefficient is that it takes into account both the static
coefficient of friction of the cake (the bulk sticking coefficient) per unit area of cake and also the
amount of caking that would occur to stick the pipe in the hole. A low caking drilling fluid will not
build up as much around the collars as a high caking drilling fluid. There is a higher bond area for
a higher caking drilling fluid. A very sticky drilling fluid which builds very little cake is safer down
hole than a high cake building drilling fluid with high permeability that sticks a much greater area
at even a factor of one half the bulk coefficient, resulting in a higher Stuck Tendency Coefficient.
B. Selection of Plates--Radius'd or Flat
Generally for direct K
st
readings, use the radius'd plate, since only in the case of high caking
drilling fluids will the flat plate provide comparative readings to the radius'd plate. The flat plate
does not take into account collar-to-well bore or drill stem-to-casing geometry for the caking area
component of the K
st
coefficient. The difference between the flat plate and radius'd plate in the
K
sc
measurements is that the radius'd plate requires measuring stuck area dome diameters less
than 2" (5.08 cm), as well as getting average edge height readings, as is done with both. The
thickness of the cake and the type of plate used determine the parameters for calculation of K
st
and K
sc
.
A. Conditions for type of torque plate and drilling fluid cake types are listed below:
1.
Radius'd Plate 12.5 inch (31.75 cm) Spherical Radius
Sample Contact less than 1 inch (2.54 cm) radius
Sample contact equal to 1 inch (2.54 cm) radius, edge effects ignored
Sample contact equal to 1 inch (2.54 cm) radius, edge effects considered
2.
Flat Plate
Sample contact always equal to 1 inch (2.54 cm), edge effects considered
NOTE: The flat plate will always have a 1 inch (2.54 cm), stuck radius and will
always have at least some edge effects.
In taking into account the edge effects on either plate, the assumption is made that the full pressure of
477.5 psi (3292 kPa) is not achieved on the full height (h) of the cake, but on 2/3 of (h). This is
assumed because the cake is permeable and the pressure drop is a gradient throughout the
thickness of the cake (h) on the vertical edge.