Rheological Behavior of Drilling Muds, Characterization Using Mri Visualization

P Coussot,B Herzhaft,F Bertrand

doi:10.2516/ogst:2004003

P Coussot, B Herzhaft + Show 1 more

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https://doi.org/10.2516/ogst:2004003

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Abstract

Drilling muds are very complex fluids used to drill oil wells; their functions are various: to carry the rock cuttings to the surface, to maintain a sufficient pressure against the rock formation, to lubricate and cool the bit. There are mainly two families of drilling muds: oil based muds (invert emulsion of brine into an oil phase with various additives) and water based muds (aqueous solutions of clays and polymers). Originally prepared from produced oil, oil based muds formulations have evolved to very complex compositions of various additives. The base oil may be of various nature, and additives are very complex: water droplets, surfactants, organophilic clays, viscosifyers, various solids and others. These additives give specific properties to the mud, particularly regarding rheological properties. Drilling muds are often described as thixotropic shear thinning fluids with a yield stress. Due to their complex composition, drilling muds exhibit an internal structure which is liable to modify according to the flowing and shear conditions, which may lead to non homogenous phenomena. It is therefore important to develop investigation techniques allowing to visualize the internal structure of the fluid in parallel to rheological measurements. In this study, we present rheological experiments coupled to magnetic resonance imaging (MRI). Using this technique, it is possible to determine the velocity profile in a viscometric flow. Conventional rheological experiments performed on two different drilling fluids formulations give similar flow curves: beyond a critical apparent shear rate there is a simple yielding behavior with an apparent plateau at low shear rates; below this critical shear rate there is a simple viscous behavior without yield stress. MRI experiments show that in fact, below this critical shear rate, no stable flow can occur and the deformation is localized in a region of the sample the dimensions of which may depend on the size of the constitutive elements. The (macroscopic) rheological behavior observed from conventional rheometric experiments is then the signature of this sheared zone and does not represent the behavior of the bulk material.

Highlights

Oil well drilling operation is part of the global process implemented to localize and extract from the reservoir the hydrocarbons lying underground
For both materials the apparent shear rate follows different evolutions depending on the applied stress
For a stress below a critical value it progressively decreases in time: for the oil based mud the shear rate eventually reaches an apparent plateau after several tenths of seconds; for the water based mud it goes on decreasing significantly even after several hundreds of seconds

Summary

Introduction

Oil well drilling operation is part of the global process implemented to localize and extract from the reservoir the hydrocarbons lying underground. The classical drilling technique is the rotary drilling technique, invented at the beginning of the 20th century by Lucas for a drilling operation in Texas [1] This technique consists in applying a weight on a bit which is put on rotation through drill pipes string. With the combined effect of the weight and the rotation, the drill bit cut the rock producing a certain amount of rock cuttings. These cuttings are lifted off to the surface thanks to a fluid which is circulated downward through the drill pipe and upward through the annular space between the rock and the pipe.

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