In-Situ Rheological Characterization of Drilling Mud
- Roberto Maglione (ENI SpA, Agip Div.) | Giovanni Robotti (Natl. Research Council) | Raffaele Romagnoli (Polytechnic of Turin)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 2000
- Document Type
- Journal Paper
- 377 - 386
- 2000. Society of Petroleum Engineers
- 1.14.3 Cement Formulation (Chemistry, Properties), 1.6 Drilling Operations, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.11 Drilling Fluids and Materials, 2 Well Completion, 1.11.5 Drilling Hydraulics, 5.3.2 Multiphase Flow, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.10 Drilling Equipment, 1.6.1 Drilling Operation Management, 5.9.2 Geothermal Resources
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The rheological model of Herschel and Bulkley reported in 1926 can be applied to determine the characteristic parameters of a drilling fluid. In this paper, an in-situ characterization approach is proposed.
During flow tests at fixed drilling depths inside the well the pump rates and the relative stand pipe pressures (SPP's) are recorded. This allows one to determine in-situ the Herschel and Bulkley rheological parameters and the behavior of the drilling mud circulating in the well.
The results are compared to those obtained in the laboratory using a Fann VG 35 viscometer for the same drilling mud. It is found that the rheological triad from the viscometer data does not always coincide with the rheological triad from the in-situ drilling test. Thus, the calculated SPP using viscometer readings could lead to misleading errors for an actual process. This method could be useful not only to calculate and predict the SPP, but also to evaluate with accuracy the annular pressure drop in order to obtain the maximum allowable pump rates without fracturing the formations.
We discuss the sensitivity of the results in relation to the equivalent viscosity of the drilling fluids considered to some of the main practical drilling parameters, such as the flow velocity and pressure spatial distribution along the wellbore profile and, with reference to the mud structure, sensitivity to pressure and temperature.
Considering the drilling well essentially as a viscometer (WAV) enables one to investigate the performance of the drilling hydraulic circuit and also the effects of the true effective viscosity (here called equivalent viscosity) and of the rheological behavior of the muds in all types of wells, and overall in deep wells with great accuracy.
During well drilling operations it is very important to know the exact pressure drop for many reasons:
for optimizing the pressure drop at the drilling bit in order to get the maximum impact force on the formation and, as a consequence, to increase the rate of penetration;
for optimizing the flow rate in the annular gap between the drill pipe and the borehole wall for better transport of the drilling cuttings to the surface and optimizing the hole cleaning efficiency;
to avoid fracture of the formations crossed due to the underestimation of the annular pressure drop;
to detect any unexpected changes of the stand pipe pressure, due to a change in the hydraulic drilling circuit (i.e., washout, plugged nozzles and fluid kick) and make opportune decisions to restore the original conditions;
to better design the mud pumps available on the drilling rig.
In addition, in drilling ultradeep wells, high temperatures and pressures can influence the rheological properties of the drilling fluids in several ways.
Physically, decreases in temperature and increases in pressure both affect the mobility of the system and lead to an increase of apparent viscosities and viscoelastic relaxation times.1 The effect of pressure is expected to be greater with oil-based systems due to the oil phase compressibility.2
Electrochemically, an increase in temperature augments the ionic activity of electrolytes 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 in doing so the degree of dispersion and flocculation of the mud systems. Sometimes this can also deeply affect the emulsion stability of oil-based muds.3 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. For 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 behavior of the drilling muds at high temperatures and pressures may be very complex to explain, so that it can be very difficult to set general guidelines for each group of muds (water-based muds, oil-based muds, etc.) or even for the same kind of mud (small differences in the composition can result in considerable differences in the rheological behavior).
Several different types of rheometers can be used to investigate the mud rheology at high pressure and temperature (HPHT) conditions. The most remarkable studies over the years were conducted by Annis4 and by Hiller,5 who studied effects of high temperatures (up to 150°C) and high pressures (up to 500 bar) on the rheology of water-based muds, considering them to have plastic behavior.
During the same period, Combs and Whitmire6 performed experiments at high temperatures and high pressures on all invert emulsion muds, and measured effective viscosity variations by a capillary viscometer. Sinha7 investigated water-based clay suspensions as well as oil-based muds using a falling bob consistometer, and concluded that the equivalent viscosity of water-based muds, compared to inverted emulsion muds and oil-based muds, is not affected to the same extent by the variations of temperature and pressure. He also concluded that the temperature is the dominant variable in the case of water-based muds. McMordie8 carried out experiments (with temperatures up to 180°C and pressures up to 965 bar) on colloidal suspensions of asphalt in oil-based muds. He proved9 that the viscosity of oil-based muds at constant temperature and pressure may be described well by modification of the power law expression.
Later, Bailey et al.10 studied, using the Huxley and Bertram HPHT rheometer, the behavior of the viscosity of low toxicity inverted oil emulsions at high temperature (up to 200°C) and high pressure (up to 1,000 bar). They concluded that the Bingham model is not that accurate for predicting the rheological behavior of low toxicity oil muds at high temperatures.
At the same time, other experiments on the same kind of muds were performed at high temperatures (from 32 to 150°C) and pressures (from 69 to 1,034 bar) in a coaxial cylinder viscometer by Politte.11 He found that oil-based muds behave as plastic fluids, and concluded that the plastic viscosity is strictly related to the viscosity of the oil at high temperatures and pressures, while the yield point is only weakly affected by pressure, and mostly depends on temperature in a very complex way.
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