A Drilling Well as Viscometer: Studying the Effects of Well Pressure and Temperature on the Rheology of the Drilling Fluids
Authors: Maglione Roberto , Gallino Germana , Robotti Giovanni , Romagnoli Raffaele , Rommetveit Rolv 
Published: December 2007

Abstract
The current need of drilling ultradeep wells involves facing new technical problems, especially concerning the effects of high pressure and temperature values. When acting along all the vertical profile of the well, the pressure and temperature effects heavily influence the rheology of the drilling fluids.
Some of the main drilling parameters which are involved are cutting lifting and hole cleaning efficiencies (resulting both from the variation of the velocity profile of the fluid flow, and from the variation of the rheological parameters), and - of course - the pressure spatial distribution along the well profile.
In order to characterise the real influence of pressure and temperature upon the rheology of the drilling fluids, circulation tests were performed and repeated at different depths inside cased hole (9 5/8" csg at 2973 m) with the 8 1/2" bit off bottom, while making trip (e. g. for the drilling out task of the casing shoe). Complete sets of data are available of circulation tests performed at 2400 m, 1400 m, 800 m and 198 m of the bit depth (while making the POOH trip), obtained by recording the mud flow rate and the corresponding stand pipe pressure (SPP) values.
By developing an original numerical procedure able to determine the equivalent rheological tern (n, k and o of any drilling fluid flowing in the well) which fits the observed SPP data in the best way, it has been possible to characterise the rheological curves corresponding to the different testing depths.
The processed rheological outputs are finally analysed and matched, pointing out the effects of pressure P and temperature T.
The outcoming effects can be remarked with respect to the equivalent viscosity of the considered drilling fluids and to some of the main practical drilling parameters, such as the velocity profile and the pressure spatial distribution along the well profile.
Qualitative trends of the field rheological data versus depth have been compared with the results of laboratory studies performed with an Huxley & Bertram HPHT rheometer with OBM systems. Some interesting analogies have been found with the two different approaches and the changes of the rheological parameters vs depth have been discussed also with reference to the mud structure sensitivity to P and T.

Introduction
The rheological properties of drilling muds under downhole conditions may be very different from those measured at ambient pressures and temperatures at the surface.
High temperatures and pressures can influence the rheological properties of the drilling fluids in several way:–Physically: decreases in temperature and increases in pressure both affect the mobility of the systems and lead to an increase of apparent viscosities and viscoelastic relaxation times. The effect of pressure is expected to be greater with oil based systems owing to the oil phase compressibility.–Electrochemically: an increase in temperature augments the ionic activity of any electrolyte, and the solubility of any partially soluble salt that may be present in the mud. This could alter the balance between the interparticle attractive and repulsive forces and so the degree of dispersion and flocculation of the mud systems. Sometimes, this can also deeply affect the emulsion stability of oil based muds. All these phenomena have a profound impact on rheological properties, especially as far as viscoelasticity and thixotropy are concerned.–Chemically: all hydroxides react with clay minerals at temperatures above 90 C. With many kinds of muds, this can result in a change of the structure and therefore also in a change of the mud rheological properties,
Because of the large number of variables involved, the behaviour of drilling muds at high temperatures and pressures may be very complicated so that it can be very difficult to get general guidelines for each group of muds (water base muds, oil base muds, etc.) or even for the same type of mud (little differences in the composition can result in considerable differences in the rheological behaviour).
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