Abstract
AbstractDrilling fluids perform a number of important functions during a drilling operation, including that of lifting drilled cuttings to the surface and balancing formation pressures. Drilling fluids are usually designed to be structured fluids exhibiting shear thinning and yield stress behavior, and most drilling fluids also exhibit thixotropy. Accurate modeling of drilling fluid rheology is necessary for predicting friction pressure losses in the wellbore while circulating, the pump pressure needed to resume circulation after a static period, and how the fluid rheology evolves with time while in static or near-static conditions. Although modeling the flow of thixotropic fluids in realistic geometries is still a formidable future challenge to be solved, considerable insights can still be gained by studying the viscometric flows of such fluids. We report a detailed rheological characterization of a water-based drilling fluid and an invert emulsion oilbased drilling fluid. The micro structure responsible for thixotropy is different in these fluids which results in different thixotropic responses. Measurements are primarily focused at transient responses to step changes in shear rate, but cover also steady state flow curves and stress overshoots during start-up of flow. We analyze the shear rate step change measurements using a structural kinetics thixotropy model.
Highlights
Drilling fluids perform a number of important functions during a drilling operation, including that of lifting drilled cuttings to the surface and balancing formation pressures
Accurate modeling of drilling fluid rheology is necessary for predicting friction pressure losses in the wellbore while circulating, the pump pressure needed to resume circulation after a static period, and how the fluid rheology evolves with time while in static or near-static conditions
In an attempt to model the rheometric flow of the two drilling fluids, we fit measurements of the steady state flow curve and the thixotropy measurements to the structural kinetics model proposed by Dullaert and Mewis [14]
Summary
Abstract: Drilling fluids perform a number of important functions during a drilling operation, including that of lifting drilled cuttings to the surface and balancing formation pressures. Ragouilliaux et al [6] used the same model to interpret creep tests and MRI velocimetry of a model oil-based drilling fluid, and showed how a relatively simple single-structure thixotropic model describes the observed viscosity bifurcation when subjecting the fluid to constant shear stresses below the yield stress. WBMs are normally based on polymers as viscosifiers, while the rheology of OBMs are based on emulsified water or brine droplets in combination with organoclays [13] This results in generally different dynamic rheological behavior even if their steady state viscosities are similar. Motivated by this observation, we study one water-based (WBM) and one invert emulsion oil-based (OBM) drilling fluid, and perform a series of steady state and transient rheological tests to compare the rheological response of the two systems. In an attempt to model the rheometric flow of the two drilling fluids, we fit measurements of the steady state flow curve and the thixotropy measurements to the structural kinetics model proposed by Dullaert and Mewis [14]
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