API RP 13D:2010 pdf download

API RP 13D:2010 pdf download.Rheology and Hydraulics of Oil-well 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-r/min dial reading is twice the 300-r/min reading, the fluid exhibits Newtonian flow behavior. If the 600-r/min reading is less than twice the 300-r/min 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 time-dependent 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 Bin gham 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_i and 1022 s1, 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 point).
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 required power law constants, n and K, from this model are typically determined from data taken at shear rates of 511 s1 and 1022 s. 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 1 3D.
4.9.4 Herschel-Bulkley Model—Also called the “modified” power law and yield-pseudoplastic model, the Herschel-Bulkley model is used to describe the flow of pseudoplastic drilling fluids which require a yield stress to initiate flow. A rheogram of shear stress minus yield stress versus shear rate is a straight line on log-log coordinates. This model is widely used because it (a) describes the flow behavior of most drilling fluids, (b) includes a yield stress value important for several hydraulics issues, and (c) includes the Bingham plastic model and power law as special cases.