Rheological Behavior of drilling muds, characterization using MRI visualization

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.