Session: 11-02-02 Well Drilling Fluids and Hydraulics-2

Submission Number: 180650

Estimation of Relaxation Times and Thixotropic Stress Extremes During Rapid Shear Transients 

Many drilling fluids exhibit a complex behavior that makes accurate flow simulation challenging. For example, the fluids may be, non-Newtonian, thixotropic (time-dependent shear stress) and viscoelastic because of gelling properties. To improve prediction of pressure losses and hole cleaning during drilling operations (including tripping and connections), there is a need for having real-time models that more accurately than today can reproduce the dynamic behavior of modern drilling fluids.

A common way to model thixotropic behavior of drilling fluids is to let rheology parameters be dependent on one or more structure parameter that describe internal connections and bounds in the fluid. In this concept, the structural parameters are modeled using differential equations that describe their rate of change as a function of the current structural state and the applied shear rate. Their values typically range between zero and unity.

Since the flow curve is obtained from steady-state measurements at various shear rates, the internal structure will, for each measurement point, approach an equilibrium state that depends on the shear rate applied at that point. Thus, to reproduce this, the modelled structure parameters must also approach equilibrium values which change between the individual shear rates. This complicates modeling of thixotropic fluids because it is not known from simple field measurements which value of the internal structures a measurement in the flow curve should be related to.

Furthermore, in transient periods, when the structure parameters are different from their equilibrium values, they can in principle have any value between the minimum (0) and the maximum (1). In order to map rheology parameters that describe the flow curve, to the structure parameters, it is necessary that any shear stress on the flow curve can be calculated for any value of the structure parameters.

A part of the current work is to estimate the limiting cases – i.e., to find rheology model parameters (shear stress values) that correspond to both the situation when the structure parameters are zero, as well as situations when they take maximum values. Such situations are here denoted thixotropic stress extremes. Flow curves constructed from shear stress extremes where all structure parameters are zero is presented for different fluid formulas – these are denoted the unstructured flow curve. Similarly, flow curve constructed from shear stress extremes where the structure parameters are at their maximum is also provided – these are denoted the maximum structured flow curve.

Another important step for predicting correct hydraulic behavior in a well is to estimate how fast the internal structures change when the shear rate changes. This part is addressed here by analyzing the observed drop in the shear stresses immediately after a step change. It is shown that these time-responses cannot be accurately modelled by two first-order time-constants, as proposed in some simplified models. More sophisticated correlations, taking into account the internal states or the fluid, the concentrations and the shear rate, are proposed. It is shown how these correlations improve the accuracy in the modeling.
 

Presenting Author: Kristian Gjerstad University of Stavanger

Presenting Author Biography: Works as an associate professor in Department of Energy & Petroleum Engineering at the University of Stavanger. Holds a PhD within petroleum from University of Stavanger (UiS), and a MSc in Engineering Cybernetics from NTNU/NTH.
Research interests include flow modelling and multiphase fluid dynamics for practical real-time applications, and experimental and theoretical rheology of industrial fluids. Other focus areas are automation and optimization of drilling operations, including detection and mitigation of hazardous events, and modelling and optimization of geothermal energy systems.
Works also part time as a numerical modelling engineer at Sekal AS, a company supporting real-time automation and optimization within drilling.

Authors:

Kristian Gjerstad University of Stavanger