Concentric Cylinder Viscometer Flows of Herschel-Bulkley Fluids Received Sep 09, 2019; Accepted Nov 28, 2019
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Appl. Rheol. 2019; 29 (1):173–181 Research Article Hans Joakim Skadsem* and Arild Saasen Concentric cylinder viscometer flows of Herschel-Bulkley fluids https://doi.org/10.1515/arh-2019-0015 Received Sep 09, 2019; accepted Nov 28, 2019 Abstract: Drilling fluids and well cements are example τ non-Newtonian fluids that are used for geothermal and Cement slurry petroleum well construction. Measurement of the non- Spacer Newtonian fluid viscosities are normally performed using a concentric cylinder Couette geometry, where one of the Shear stress, Drilling fluid cylinders rotates at a controlled speed or under a con- trolled torque. In this paper we address Couette flow of yield stress shear thinning fluids in concentric cylinder geometries. We focus Chemical wash on typical oilfield viscometers and discuss effects of yield Shear rate,γ ˙ stress and shear thinning on fluid yielding at low viscome- ter rotational speeds and errors caused by the Newtonian Figure 1: Example of typical flow curves for fluids involved in well shear rate assumption. We relate these errors to possible construction. implications for typical wellbore flows. Keywords: Drilling fluids, Herschel-Bulkley, Viscometry The operation schedule involves drilling, where the drilling fluid transports out drilled cuttings, displacement PACS: 83.85.Jn, 83.60.La, 83.10.Gr, 47.57.Qk of the drilling fluid from the narrow annular space be- tween the casing string and the formation and replace it by a cement slurry to create a total zonal isolation in the well. 1 Introduction Once in place, the cement slurry is allowed to harden into a cement sheath. A successful operation is dependent on the In the enterprise of constructing geothermal or petroleum complete displacement of drilling fluid from the narrow an- wells, drilling fluids and well cements are used. These nular space outside the casing. This is typically achieved fluids are constructed as a blend of particle suspensions, by injecting a sequence of wash and spacer fluids ahead liquid-liquid dispersions and polymer solutions with a of the cement slurry, and the displacement is optimized large variety of additional chemicals. The fluids show a through careful design of fluid densities and viscosities. As strong non-Newtonian characteristic. illustrated by the example flow curves in Figure 1, the in- As an example, in Figure 1 we plot typical shapes of volved fluids typically exhibit yield stress and thus show flow curves for fluids involved in the operation where acas- shear thinning behavior. Most drilling fluids, and some ce- ing string is cemented to the newly drilled formation. Pri- ments slurries also exhibit a curved shape of the viscosity mary cementing is performed once the current well section flow curve after reaching the yield stress; i.e. yield stress is drilled to target depth and a casing string is run into the shear thinning fluids as proposed by Herschel and Bulk- hole and to the bottom of the section. ley [1]. In addition to exhibiting shear thinning and yield stress behaviour, most drilling and well construction fluids are also thixotropic, i.e. requiring a certain time duration for microstructure equilibration to current shearing condi- tions [2, 3]. In this paper, we neglect thixotropic behaviour *Corresponding Author: Hans Joakim Skadsem: NORCE Nor- and instead focus on the steady state, non-Newtonian flow wegian Research Centre AS, Prof. Olav Hanssensvei 15, N-4021 curve of well construction fluids. Stavanger; Email: [email protected] Arild Saasen: University of Stavanger, N-4036 Stavanger Open Access. © 2019 H. J. Skadsem and A. Saasen, published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License 174 Ë H. J. Skadsem and A. Saasen Stable displacements are normally preferred for pri- For a concentric cylinder viscometer, Krieger and El- mary cementing operations in the laminar regime, imply- rod [7] showed that the angular velocity Ω of the rotating ing that the displacing fluid express a higher frictional cup is related to the shear rate and shear stress in the gap pressure loss than the drilling fluid to optimise displace- via Eq. (2) τ Z 1 ment. In practice this means that the cement should be 1 훾˙ (τ) Ω = dτ, (2) effectively more viscous than the fluid it displaces [4,5]. 2 τ 2 It is also essential to limit the circulating pressures in the κ τ1 well to avoid hydraulic fracturing the formation and un- where τ1 is again the wall shear stress at the inner bob and controlled loss of fluids. This means that to optimize dis- κ = R1/R2 is the ratio of bob to cup radii. As Ω is controlled placement and avoid formation fracturing during pump- and τ1 is measured, Eq. (2) is to be solved for the unknown ing, it is important to reduce the viscosity of the drilling shear rate 훾˙ (τ1). Krieger and Elrod [7] and later Krieger [8] fluids being in the annulus. However, this reduction can- showed that Eq. (2) can be solved for the unknown shear not be so large that the pressure control in the well is lost. rate via an infinite series without making a priori assump- Therefore, it is important to have accurate and reliable vis- tions of the type of flow curve for the fluid. These solutions cosity characterization of the fluids involved in the pro- do not always perform well for fluids with yield stresses [9] cesses. as yield stress introduces a mathematical singularity in the In the field, fluid viscosities are normally measured viscosity curve. Alternative solution strategies based on using a concentric cylinder Couette geometry, where the Tikhonov regularization [10, 11] and wavelet-vaguelette de- outer cup rotates at a fixed angular velocity Ω and the inner composition [12] have been developed, which are capable bob is held fixed by application of a torque M. The oilfield of handling yield stress fluids with no a priori flow curve standard for viscosity measurements are based on equip- assumption. ment description. The dimensions for the conventional oil- The problem of existence of yield stresses is sum- field viscometer were described by Savins and Roper [6]. marised by Watson [13]. However, for our purpose we con- The rotor-and-bob geometry is as follows: Rotor radius is sider sufficiently short time frames where use of an yield 1.8415 cm and the bob radius is 1.7247 cm. The length of stress is applicable. Therefore, as will be described later, the bob is 3.8 cm. These dimensions were partly sought to we will assume an existence of the yield stresses. present the viscosity at the shear rate of 511 s−1 directly in If the relationship between shear rate and shear stress units of centipoise and to allow for measurements of fluids is known a priori for the fluid in question, relations link- containing weighting particles of sizes less than 75 micron. ing angular velocity, applied torque and model parameters According to oilfield standards for well cementing, the vis- can be derived from Eq. (2), see e.g. Li et al.[14]. Following cometer should have at least the following rotation speeds: a different approach, Chatzimina et al.[15] solved the gov- 600, 300, 200, 100, 6 and 3 revolutions per minute (RPM). erning equations for Herschel-Bulkley fluids and found ex- The 600 RPM is used for drilling fluids and spacers, but not pressions for the wall shear rate in a concentric cylinder ge- for well cements as exposure to this shear rate reduces the ometry. They showed that wall shear rates may deviate sig- reproducibility of flow curves. Later, it has become anin- nificantly from corresponding Newtonian values depend- creasing use of viscometers with more rotation speeds in ing on the diameter ratio of the Couette geometry and ma- the oilfields. Typically, additional rotation speeds equal to terial parameters. 60, 30, 20, 10, 2, and 1 RPM can now occasionally be found In this paper, we assume a priori yield stress shear on oilfield equipment. The dimensions remain the same. thinning (Herschel-Bulkley) flow curves and study the ef- In the absence of end effects, the applied torque is re- fects of yield stress and shear thinning on the wall shear lated to the wall shear stress at the bob via rates. We compare the actual Herschel-Bulkley wall shear M rates to those of a Newtonian fluid and quantify the error in τ = , (1) 1 πR2L the common Newtonian assumption for model yield stress 2 1 fluids. The outline of the paper is as follows: Webegin where the submerged bob radius and length are denoted by considering the momentum equation for the Herschel- R1 and L, respectively. This results in a relationship be- Bulkley fluid in a Couette geometry in the next section. We tween angular velocity of the cup and the corresponding derive an equation relating rotational velocity of the cup shear stress at the bob surface. However, a relationship be- to the shear stress in the gap that is equivalent to what tween the shear rate and the corresponding shear stress is can be obtained through Eq. (2). Next we solve the gov- typically what is sought for. erning equation for azimuthal flow velocity in the gap and bob shear rate for a certain, fixed Herschel-Bulkley fluid. Concentric cylinder viscometer flows of Herschel-Bulkley fluids Ë 175 1000 Finally, we consider example Fann 35 model viscometer n =0.5 measurements of well construction fluids, including a typ- 100 n =0.75 n =1.0 ical water-based drilling fluid, a spacer fluid and cement 10 slurry, and investigate the difference between the Newto- actual 1 nian shear rate assumption and the non-Newtonian ) 0.1 wall shear rate.