Creep-Recovery Test: A Critical Tool for Rheological Characterization (2013)

The relaxation time of drilling fluids is small compared to operational time, so under normal steady shear, the viscous response is dominant. However, under infinitesimal deformation or high shearing speed, drilling fluids behave viscoelastically. Viscoelasticity has received much attention from the petroleum industry since many phenomena are related to it. Viscoelastic properties of fluid are the measurement of the gel structure, structure stability and gelling speed. Hence, it governs particle settling and the invasion of fluid into the formation. These viscoelastic properties can be determined through dynamic tests including oscillatory, creep-recovery and relaxation tests. However, current rheological characterization is not sufficient to evaluate these important properties. In addition, a standard test procedure for dynamic test is not available. Therefore, the petroleum industry still lacks a rheological tool to evaluate fluids in term of particle transportation and structure stability. In this search, creep-recovery test is used to obtain the viscoelastic properties of drilling fluids. A standard experimental procedure is designed to obtain creep-recovery data of sixteen drilling fluids. This test procedure provides a guideline for other experimental investigations. Creep compliance, a material function of fluid, is analyzed and compared. A method of using creep-recovery data to evaluate the settling of particles is discussed. The procedure to obtain the zero shear viscosity from creep-recovery test is shown in detail. To mathematically model the viscoelastic response of fluids, a viscoelastic model, consisting of three Kelvin-Voigt models and one Maxwell model, was used to fit the experimental data. The model parameters for each sample obtained from nonlinear regression are presented. The results of this study provide petroleum engineers a practical rheological tool to obtain zero shear viscosity, predict particle settling and evaluate fluid structure stability. Our mathematical model can also be used to model the viscoelastic response of drilling and hydraulic fracturing fluids. Introduction During circulation through the well, the drilling fluid experiences a wide range of shear rates. Shear rates on the order of 10 s are encountered in the drillpipe. In the annulus, drilling fluid experiences a shear field on the order of 10 s. For normal drilling fluids, this operational time is usually long compared to the relaxation time of the drilling fluids. Therefore, the viscoelastic effect may not be significant. In other words, the steady shear of drilling fluid is strongly governed by its viscosity. However, the viscoelastic effect is significant under very high shear rate. Such high shear rate is commonly encountered at the bit nozzles, where shear rates as high as 10 s may develop. In this case, the operation time ( 1   ) can be on the order of the relaxation time of the drilling fluid. Drilling fluid, at this shear rate, will behave viscoelastically. Under infinitesimal deformation, the gel structure of drilling fluid shows viscoelastic response to the deformation. The pressure transient, pressure peak and pressure delay is a clear evidence of viscoelasticity and gel structure formation of drilling fluids. This gel structure is the desired property of drilling fluid to fulfill its complex functions in cuttings transport and solid suspension, and in reducing fluid invasion into the formation. The gel formation, when fluid is at rest, helps to keep the solid particles from settling. The settling of the heavy components such as weighting additives and cuttings may result in severe operational problems. The formation of the gel structure of drilling fluids during shear flow is also important in dynamic conditions to enhance cutting carrying capacity and reduce dynamic barite sag. Also, gel structure formation helps to prevent fluid invasion into the formation and lost circulation problems.