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

Paper Number: 105303

105303 - Determination of Drilling Fluid’s Elasto-Viscoplastic Thixotropic Properties From Gel Strength Measurements and Transient Models 

Measurements of the gel strength (stress overshoot) of the drilling fluid are normally performed regularly during drilling operations. These measurements, which are done by using a rotational viscometer, may be among the very few measurements that are available for updating drilling mud properties in (near) real-time. They are further important to predict the downhole drilling conditions, which again is crucial for safe and optimal operations.

The standard measurement procedure is to let the mud be quiescent for 10 s and 10 min after initial stirring and measure the peak values of the shear stress during startup of slow shear motion in a rotational viscometer/rheometer. The rotational speed is normally three revolutions per minute (RPM) and the results are reported as the 10 s gel strength and 10 min gel strength respectively. However, it is in general not straightforward, for the thixotropic elasto-viscoplastic (TEVP) fluids used in many operations today, to determine exactly which properties of the mud these measurements reveal. 

Details for how these tests should be performed at rigsite is described in the API standard for gel strength measurements, and these procedures help in performing the tests consistently within an operation and between different operations/rigs. However, the procedure is based on using quite simple viscometers where test parameters such as the acceleration of the shear rate motion at startup is not controllable and may be unknown, whereas earlier studies have shown that this acceleration rate can have a non-negligible effect on the results. 

In addition, since TEVP fluids in the beginning of the enforced motion, will exhibit a transition from elastic dominated behavior to viscous dominated behavior, the selected target rotational speed may determine which properties of the fluid that dominates the measured peak values. Thus, selecting other input parameters could give other and potentially more useful information about the mud.

Many rheology models that potentially can predict the transient behavior at startup of shear, including the stress overshoot, are based on model parameters that represent physical fluid properties. Thus, in order to predict the fluid behavior as accurately as possible in real-time by modelling, there is a need to know more about the relationship between the gel strength measurements and these key properties of the mud (such as yield stress, elastic modulus and thixotropic time constants).

In this paper we use a scientific rheometer where the mentioned input parameters can be controlled, and we analyze how the stress overshoot is affected by them. We further investigate if published dynamic/ transient models of TEVP fluids can reproduce this transient behavior for different combinations of input parameters, and which model parameters (mud properties) that are associated with the peak values.

Our findings are that none of the candidate models that we selected and tested in this work did reproduce the behavior of the mud in this transient phase. The candidate models where chosen based on which one we expected to best describe the TEVP muds. We therefore modified some of these existing models and thereby introduced a new model that better matched the measurements. 

Based on the new model, we finally show the challenges we have for determining fluid properties when only the API standard measurements are available, and we propose some additional tests that can improve calibration of rheology model parameters, and thereby improve the accuracy of complete flow models in drilling.

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. Research interests include flow modeling and analyzing the rheology of the drilling fluid as well as development of models that describe the transient behavior of various muds. Another focus area is automated drilling and optimization of the operations, including detection and mitigation of hazardous events such as kick/loss, stuck pipe.

Gjerstad also works part time as a numerical modelling engineer at Sekal AS, a company supporting real-time automation and optimization within drilling. At Sekal, development of flow models is a main task. He holds a PhD within petroleum from University of Stavanger (UiS), and a MSc in Engineering Cybernetics from NTNU/NTH.

Authors:

Kristian Gjerstad University of Stavanger