Sensor development and calibration method for inline detection of viscosity and solids content of non-Newtonian fluids

Résumé. Le projet de recherche conjoint EURAMET « Développement et méthode d’étalonnage de capteurs en ligne pour la mesure de la viscosité et du contenu solide» (ENG59-NNL) concerne les fluides de forage, en particulier dans le secteur de l’énergie du pétrole et du gaz. Ce projet regroupe sept laboratoires européens de métrologie, trois compagnies pétrolières, quatre constructeurs d’instruments de rhéologie et des laboratoires de recherches universitaires et industriels. Les fluides de forages contiennent une grande quantité de matières solides. Leur viscosité varie en fonction de la vitesse de cisaillement. Ce sont des liquides non-newtoniens rhéofluidifiant à seuil avec un comportement qui dépend du temps, ils sont thixotropes. Un des buts de ce projet est d’a ider au développement de capteurs de mesures en ligne de la viscosité, de la masse volumique, de la concentration et de la distribution en taille de particules solides de ces liquides. Les autres buts de ce projet, sont: de pallier à une absence de norme sur les dispositifs de mesures de viscosité dans le domaine des fluides non-newtoniens, de permettre d’évaluer ces appareils de laboratoires ou ceux qui dans l’avenir pourraient être utilisés en ligne; de développer des matériaux de référence certifiés similaires aux fluides de forage pour les appareils de laboratoire et pour les appareils sur site; de faire enregistrer des CMCs (Calibration and Measurement Capabilities) par les Instituts Nationaux de Métrologie, pour assurer la traçabilité des instruments de mesures. Le projet est présenté sur le site web (www.eng59-rheology.eu). Les premiers résultats de cette étude sont présentés dans cet article.


Introduction and background
When drilling deep for gas and oil or for the purpose of building bridge piles or taking core samples for a future tunnel, non-Newtonian drilling fluids are used. Only special non-Newtonian drilling fluids have this functionality. While drilling, fluid is pumped through the drill string to the drill bit. Typically a drill string has an inner diameter of 15 cm and a length of 1 to 10 km. The fluid flows through high-pressure nozzles mounted in the bit body against the bottom and flows back to the surface along the inside of the borehole wall taking the drilled cuttings with it. The carrying capacity of the fluid depends on a minimum circulation velocity while the pump pressure required to circulate the fluid should be realistic. Moreover, at the moment that a drill pipe has to be added to the drill string, the pumps have to be switched off. In that situation, despite the fact that additional pressure required for circulation is absent, the fluid should keep the drilled solids in suspension, should still stabilise the borehole and balance the fluid pressure in the pores of the borehole formation. There are laboratories in academic and industrial research : CCMM (Portugal), IFPen (France), SIK (Sweden).

Need for the project
To optimise, control and automate the drilling process, accurate and frequent measurement of the viscosity, particle size distribution, solids content and density of the muddy fluid is required. This is currently done using manually operated, batch type sensors at the surface. However, the accuracy of these devices is limited and not sufficient to perform the required optimisation and automation of the drilling process. Moreover, no in situ calibration procedure that assures a maximum measurement uncertainty currently exists, or directly suitable traceable non-Newtonian reference materials.
Measurement standards exist for producing and measuring Newtonian calibration materials. By using these standards, NMIs are capable of delivering Newtonian viscosity with an accuracy of 0.1 to 0.5%, based on flow through a capillary at low speed. Whether this measurement standard is applicable to non-Newtonian fluids has not been thoroughly investigated, but complications are expected as the flow of shear thinning liquids through a capillary does not have a predictable Poiseuille velocity profile [1].
Therefore, calibration liquids are needed that exhibit non-Newtonian properties. A few such standard reference materials (SRMs) are available. NIST issues two of them, a polymer solution [2] and corn syrup-fine limestone slurry [3]. Furthermore, PTB has completed research to define a polymeric SRM, however this research was discontinued. Since then, non-Newtonian reference materials based on aqueous solutions of synthetic clay particles have come to market. These show little degradation by exposure to shear and over time, which might make them suitable, but they do not possess traceable and consistent properties. There is also no SRM available which can be used to investigate the solid content influence and give a sufficiently good estimation of shear stress at zero strain (gel strength).
In the drilling industry, especially in the oil and gas energy sector, it is very important to have reliable and nearly continuous information on the mentioned physical properties of the fluid. This information is used in models to calculate the down hole pressures and the cuttings carrying capacity. Four problem areas that currently limit accurate and inline measurement of drilling fluid properties are: x Fundamental properties of drilling fluids, i.e.
high apparent gel strength, high shear thinning properties, large amount of suspended solids, strong time dependent behaviour of viscosity.
x Absence of a (European) guide for calibrating equipment when measuring non-Newtonian fluids, including certified reference materials similar to drilling muds and a method to assess the associated measurement uncertainty.
x No CMCs (Calibration and Measurement Capabilities) registered for non-Newtonian reference materials by any NMI at the BIPM website, in order to show the degree of equivalence between measurements and to provide traceability for the instruments used in the field.
x A lack of sensors for inline viscosity, solids content, PSD (particle size distribution) and density measurement that can cope with strongly non-Newtonian fluids with a high gel strength and large amount of suspended solids.

Objectives and activities
The JRP aims to address these problem areas by developing a traceable calibration method for inline measurements on non-Newtonian liquids, with a strong focus on the drilling fluids used in the oil and gas energy sector. The JRP is divided in five Workpackages (WP's).
WP1 will develop a set of calibration standards in the form of standard reference materials (SRM) for complex fluids in the range of viscosities, densities and temperatures; that are of interest for the stakeholders, WP2 will study the physical behaviour of complex fluids by means of existing viscometer techniques and rheometers. Current models of the relation between shear rate and viscosity will be compared to experimental results,

Progress
The technical part of the project is divided in three « Workpackages », themselves divided in different tasks. They are defined below with their progress.

D1.2.3 Preliminary method for the production of the set of non-Newtonian SRMs
Preliminary method for the production of the set of non-Newtonian SRMs has been established. This method will be used for SRMs production.

Conclusion
The energy sector is not the only industry in which it is important to monitor the non-Newtonian properties of fluids. For many manufacturers of food, paint, slurries, body care and pharmaceutical products the viscosity, density and solids content is very important for quality and process control. Similar to the drilling industry, many chemical plants require measurements to be taken in hostile working environments and to be able to do this automatically with suitable inline equipment would be a major step forward.