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.