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
- 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 reported also in pounds per gallon, specific gravity, and pounds 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, is a result of 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.
The Marsh Funnel
The Marsh Funnel is an instrument used to measure what is commonly called “Funnel Viscosity.” The Marsh Funnel viscosity is reported as the number of seconds required for a given fluid to flow 1 quart of fluid through the calibrated orifice at the end of the funnel. (Check also: Marsh Funnel Test Procedure)
Direct-indication viscometer is used to measure more significant rheological properties of drilling mud. Measurements are normally made 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.
Yield Point is defined as the 300 RPM shear stress reading minus the Plastic Viscosity. Yield Point is a measure of 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
Gel Strength is a measure of 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, ease of release of gas, and setting of suspended particles in the pits. Relative measures of gel strength properties are made on a direct indicating viscometer and are commonly reported as 10 sees and 10 min. 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 be run under actual bottom hole temperatures and differential pressures existing in the wellbore.
Sand Content is one of the drilling fluid properties that is 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 by increasing the mud weight. (Check also: Sand Content Test)
Solids, Oil, And Water Content
Solids, Oil, And Water Content are measured not only as a basis for the control of the oil content of emulsion muds but also as an aid in the control of the performance of the mud. Solids content affects the drilling performance and flow properties of the mud. Optimum solids control is essential for overall superior mud performance.