In-situ rheological characterization of drilling mud

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 seat accuracy.