Rheology is a fundamental mud property determining circulating pressures, suspension capabilities and carrying capacity. It must be checked regularly and controlled very carefully to ensure it is maintained within the range specified in the drilling fluids programme.
Plastic Viscosity (PV) indicates solids content and, therefore, general condition of the mud. For a specific mud, at a constant gradient, an increase in PV may indicate that the efficiency of the solids control equipment requires checking. A high PV adds to the circulating pressure and provides no benefits. Consequently, efforts must be made to minimise it.
Yield Point (YP) gives an indication of the carrying capacity of the mud. This is the main rheological control parameter and must be maintained within the programmed specification by addition of viscosifier or thinners as required.
Gel strength determines static suspension capabilities. Development of gel strength is linked to YP, in the sense that the same additives are used to adjust it. When YP is adjusted, it must be ensured that there are no detrimental effects on the Gel strength, either in causing it to increase or to decrease outwith the programmed specification range.
Equipment
Fann Model 35 Viscometer or equivalent 6 speed viscometer with standard R1/B1 rotor-bob-spring combination.
Temperature controlled heating jacket and fitted cup.
Metal thermometer.
Procedure
All rheological measurements must be carried out at 150°F or any temperature specified in the Mud Programme. Do not exceed 200oF when using a hollow bob.
Calculation
|
Parameter |
Equation |
Units |
|
AV |
|
cp |
|
PV |
|
cp |
|
YP |
|
lbs/100 ft2 |
|
Gels |
|
lbs/100 ft2 |
where, q600 is the Fann viscometer dial reading as measured at 600 RPM etc.
Reporting
The following values must be reported:
|
Parameter |
Units |
Accuracy |
|
q600, q300, q200, q100, q6, q3 |
report measured values |
± 1 |
|
AV, PV |
cp |
± 1 |
|
YP, 10” Gel, 10’ Gel |
lbs/100ft2 |
± 1 |
Sources of Error
The mud sample must be raised up to, or slightly above, the mark on the motor, otherwise significant error will result.
The viscometer must be calibrated at regular intervals using the API procedure.
An accurate, recently calibrated, metal thermometer must be used, especially with oil based mud.
Solids settling, stir during heating.