API RP 13D:2006 pdf download
API RP 13D:2006 pdf download.Rheology and Hydraulics of Oil-well Drilling Fluids.
The settling shear rate is used to calculate the viscosity of fluid experienced by the settling particle.
4.6 Fluid characterization
4.6.1 Fluids can be classified by their rheological behavior. Fluids whose viscosity remains constant with changing shear rate are known as Newtonian fluids. Non-Newtonian fluids are those fluids whose viscosity varies with changing shear rate.
4.6.2 Temperature and pressure affect the viscosity of a fluid. Therefore, to properly describe the drilling fluid flow, the test temperature and pressure must be specified.
4.6.3 Some mathematical models used for hydraulic calculations are shown in this subclause.
4.7 Newtonian fluids
4.7.1 Fluids for which shear stress is directly proportional to shear rate are called Newtonian. Water, glycerin, and light oil are examples of Newtonian fluids.
4.7.2 A single viscosity measurement characterizes a Newtonian fluid at a specified temperature and pressure.
4.8 Non-Newtonian fluids
4.8.1 Fluids for which shear stress is not directly proportional to shear rate are called non-Newtonian. Most drilling fluids are non Newtonian.
4.8.1.1 Drilling fluids are shear thinning when they exhibit less viscosity at higher shear rates than at lower shear rates.
4.8.1.2 There are some non-Newtonian fluids which exhibit dilatant behavior. The viscosity of these fluids increases with increasing shear rate. Dilatant behavior rarely occurs in drilling fluids.
4.8.2 The distinction between Newtonian and non-Newtonian fluids can be illustrated by using the API standard concentric-cylinder viscometer. If the 600-rpm dial reading is twice the 300-rpm reading, the fluid exhibits Newtonian flow behavior. If the 600-rpm reading is less than twice the 300-rpm reading, the fluid is non-Newtonian and shear thinning.
4.8.3 One type of shear thinning fluid will begin to flow as soon as any shearing force or pressure, regardless of how slight, is applied. Such fluids are termed pseudoplastic. Increased shear rate causes a progressive decrease in viscosity.
4.8.4 Another type of shear thinning fluid will not flow until a given shear stress is applied. The shear stress required to initiate flow is called the yield stress. These fluids are referred to as viscoplastic.
4.8.5 Fluids can also exhibit time-dependent effects. Under constant shear rate, the viscosity changes with time until equilibrium is established. Thixotropic fluids experience a decrease in viscosity with time, while rheopectic fluids experience an increase in viscosity with time.
4.8.6 Thixotropic fluids can also exhibit a behavior described as gelation or gel strength development. The timedependent forces cause an increase in viscosity as the fluid remains static. Sufficient force must be exerted on the fluid to overcome the gel strength to initiate flow.
4.8.7 The range of rheological characteristics of drilling fluids can vary from an elastic, gelled solid at one extreme, to a purely viscous, Newtonian fluid at the other. Circulating fluids have a very complex flow behavior, yet it is still common practice to express the flow properties in simple rheological terms.
4.8.8 General statements regarding drilling fluids are usually subject to exceptions because of the extraordinary complexity of these fluids.
4.9 Rheological models
4.9.1 Rheological models are intended to provide assistance in characterizing fluid flow. No single, commonly-used model completely describes rheological characteristics of drilling fluids over their entire shear-rate range. Knowledge of rheological models combined with practical experience is necessary to fully understand fluid performance. A plot of shear stress versus shear rate (rheogram) is often used to graphically depict a rheological model.
4.9.2 Bingham Plastic Model: This model describes fluids in which the shear stress/shear rate ratio is linear once a specific shear stress has been exceeded. Two parameters, plastic viscosity and yield point, are used to describe this model. Because these parameters are determined from shear rates of 511 s and 1022 s, this model characterizes fluids in the higher shear-rate range. A rheogram of the Bingham plastic model on rectilinear coordinates is a straight line that intersects the zero shear-rate axis at a shear stress greater than zero (yield poin.
4.9.3 Power Law The Power Law is used to describe the flow of shear thinning or pseudoplastic drilling fluids. This model describes fluids in which the rheogram is a straight line when plotted on a log-log graph. Such a line has no intercept, so a true power law fluid does not exhibit a yield stress. The two reguired power law constants, n and K, from this model are typically determined from data taken at shear rates of 511 s and 1022 s1. However, the generalized power law applies if several shear-rate pairs are defined along the shear-rate range of interest. This approach has been used in the recent versions of API RP 13D.
