scope:

The objective of this Recommended Practice (RP) is to provide a basic understanding of and guidance about drilling fluid rheology and hydraulics, and their application to drilling operations. The methods for the calculations used herein do not take into account the effects of temperature and compressibility on the density of the drilling fluid.

Rheology is the study of the deformation and flow of matter. Drilling fluid hydraulics pertains to both laminar and turbulent flow regimes.

For this RP, rheology is the study of the flow characteristics of a drilling fluid and how these characteristics affect movement of the fluid. Specific measurements are made on a fluid to determine rheological parameters of a fluid under a variety of conditions. From this information the circulating system can be designed or evaluated regarding how it will accomplish certain desired objectives. Drilling fluid rheology is important in the following determinations:

a. Calculating friction loss in pipe or annulus.

b. Determining the equivalent circulating density of the drilling fluid.

c. Determining the flow regime in the annulus.

d. Estimating hole cleaning efficiency.

e. Evaluating fluid suspension capacity.

f. Determining the settling velocity of drill cuttings in vertical holes.

The discussion of rheology in this RP is limited to single-phase liquid flow. Some commonly used concepts pertinent to rheology and flow are presented. Mathematical models relating shear stress to shear rate and formulas for estimating pressure drops, equivalent circulating densities and settling velocities of drill cuttings are included.

Conversion factors and examples are included for all calculations so that U.S. Customary units can be readily converted to metric (SI) units.

Where units are not specified, as in the development of equations, any consistent system of units may be used.

The concepts of viscosity, shear stress, and shear rate are very important in understanding the flow characteristics of a fluid. The measurement of these properties allows a mathematical description of circulating fluid flow. The rheological properties of a drilling fluid directly affect its flow characteristics and all hydraulic calculations. They must be controlled for the fluid to perform its various functions.