API RP 13D:2003 pdf download
API RP 13D:2003 pdf download.Recommended Practice on the Rheology and Hydraulics of Oil-well Drilling Fluids.
5 Types of Fluids
5.1 DESCRIPTION
5.1.1 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.
5.1.2 Temperature and pressure aftéct the viscosity of a fluid.5 Therefore, to properly describe the drilling fluid flow, the test temperature and pressure must he known.
5.1 .3 Some mathematical models used for hydraulic calcu— lations are shown in this section.
5.2 NEWTONIAN FLUIDS
5.2.1 Those fluids in which shear stress is directly proportional to shear rate are called Newtonian. Water. glycerin, and light oil are examples.
5.2.2 A single viscosity measurement characterizes a Newtonian fluid.
5.3 NON-NEWTONIAN FLUIDS
5.3.1 Most drilling fluids are not Newtonian; the shear stress is not directly proportional to shear rate. Such fluids are called non-Newtonian.6
5.3.1.1 Drilling fluids are shear thinning when they have less viscosity at higher shear rates than at lower shear rates.
5.3.1.2 There are non-Newtonian fluids, which have dilatant behavior. The viscosity of these fluids increases with increasing shear rate. Dilatant behavior of drilling fluids rarely. if ever, occurs.
5.3.2 The distinction between Newtonian and non-Newtonian fluids is illustrated by using the API standard concentric cylinder viscorncter.7 If the 600-rpm dial reading is twice the 300-rpm reading. the fluid exhibits Newtonian flow behavioi If the 600-rpm reading is less than twice the 300-rpm reading, the fluid is non-Newtonian and shear thinning.
5.3.3 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 fluid is termed pseudoplastic.8 Increased shear rate causes a progressive decrease in viscosity.
5.3.4 Another type of shear thinning fluid will not flow until a given shear stress is applied. This shear stress is called the yield stress.
5.3.5 Fluids can also exhibit time dependent effects. Under constant shear rate, the viscosity decreases 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.
5.3.6 Thixotropic fluids can also exhibit a behavior described as gelation or gel strength. 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 begin flow.
5.3.7 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. The circulating fluids have a very complex flow behavior, yet it is still common practice to express the flow properties in simple rheological terms.
5.3.8 General statements regarding drilling fluids are usually subject to exceptions because of the extraordinary complexity of these fluids.9
5.4 RHEOLOGICAL MODELS
5.4.1 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. A knowledge of rheological models combined with practical experience is necessary to fully understand fluid performance.
5.4.2 Bingham Plastic Model: The most common rheological model used for drilling fluids is the Bingharn Plastic Model. This model describes a fluid 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 constants are determined between the specified shear rates of 511 sec.-1 and 1022 sec:1. this model characterizes a fluid in the higher shear rate range.
5.4.3 Power Law: The Power Law is used to describe the flow of shear thinning or pseudoplastic drilling fluids. This model describes a fluid in which shear stress versus shear rate is a straight line when plotted on a log-log graph. Since the constants, ii and K, from this model are determined from data at any two speeds, it more closely represents an actual fluid over a wide range of shear rates.
5.4.4 Herschel-Buckley (Modified Power Law) Model: The modified Power Law is used to describe the flow of a pseudo- plastic drilling fluid, which requires a yield stress to flow. A graph of shear stress minus yield stress versus shear rate is a straight line on log-log coordinates.
