Various properties of the drilling fluids are measured as an indication of the performance of the mud in the hole. Methods for measurement of these properties are stated in API RP 13B “Recommended Practice on Standard Procedure for Testing Drilling Fluids” (Check Also WBM Testing Procedure – OBM Testing Procedure). These procedures are revised and extended periodically as new tests gain acceptance. Tests of WBM or OBM commonly made are:
- Density Or Mud Weight
- Plastic Viscosity
- Yield Point In drilling mud
- Drilling fluid gel strength
- Sand Content
- The Solids, Oil, And Water Content
Density Or Mud Weight
Density Or Mud Weight affords a measure of the hydrostatic pressure of the mud column. When mud weight is reported as pounds per square inch per thousand feet, the pressure at any depth is easily calculated. Density is also reported in pounds per gallon, specific gravity, and per cubic foot. (Check also: Mud Balance Test Procedure)
Viscosity Properties Of Drilling Mud
Viscosity is a measure of the internal resistance of a drilling fluid. This internal resistance, or inertia, results from the attractions of molecules in a liquid and is a measure of the combined effects of these attractions and the natural cohesion of suspended particles. The greater the internal resistance, the greater the viscosity.
Generally, the Marsh Funnel tool determines “Funnel Viscosity”, which refers to the amount of time it takes for a fluid to flow 1 quart of fluid through the calibrated orifice at the end of the funnel. (Check also: Marsh Funnel Test Procedure)
To measure significant rheological properties of drilling mud, one can use a direct-indication viscometer, which operates at shear rates of 600 and 300 RPMs. (Check also: Viscometer Test Procedure)
Plastic Viscosity is defined as the 600 RPM shear stress reading minus the 300 RPM shear stress reading. The Plastic Viscosity depends mainly on the friction between solids and the liquid. Plastic Viscosity is also somewhat representative of high shear rate viscosities encountered at the drill bit.
The yield Point is the 300 RPM shear stress reading minus the Plastic Viscosity. The yield Point measures the attractive forces between active clay particles in the mud under flowing conditions. It is also a measure of the hole-cleaning capabilities of mud.
Gel Strength Properties Of Drilling Mud
Typically, the gel Strength measures the attractive forces of suspended particles in a liquid when that liquid is in a static state. Gel strength is reported in lb/100 sq ft. The gel strength influences the surge and swabbing effects of the drilling fluids when tripping with the drill string, the pressure required to break circulation, the ease of gas release, and the setting of suspended particles in the pits. The direct indicating viscometer measures relative gel strength properties. In addition, mud engineer will report them as 10-second and 10-minute gels.
Filtration Properties Of Drilling Fluid
The filtration test (HPHT Filtration Test– API Filtration Test) is a relative measure of liquid filtered into a permeable formation and of the cake left on the formation. The condition of the mud and the type of solids in the mud influence filtration. There are two standard filtration tests. One is at ambient temperature and 100 psi, and the other at 300°F and 500 psi. The high-temperature-high-pressure test should preferably run under actual bottom hole temperatures and differential pressures in the wellbore.
Sand Content is one of the drilling fluid properties measured because sand is abrasive to the equipment that comes in contact with the mud, and sand may cause trouble by setting in the hole or increasing the mud weight. (Check also: Sand Content Test)
Solids, Oil, And Water Content
Measuring Solids, Oil, and Water Content is important not only for controlling the oil content of emulsion mud but also for monitoring the performance of the mud. The amount of solids in the mud affects its drilling performance and flow properties, so it is crucial to have proper solids control for optimal mud performance.