A New Model for Predicting Pressure Fluctuations for a Robertson-Stiff Fluid in a Vertical Well

Determination of the reasonable and safe speed for tripping or running casing to avoid downhole problems requires the development of a methodology for modeling the rheology of drilling fluid and pressure fluctuations. Based on the model of flow in a narrow channel, a new calculation model of pressure fluctuations for a Robertson-Stiff (RS)fluid in a vertical well is derived in this paper. Unlike the simplified pressure fluctuations prediction method based on flow in a narrow channel, this model considers the actual difference in stresses between the wall of the moving drill string and the borehole wall, yielding more reliable prediction values. Results indicate that pressure fluctuation values calculated by this model are in good agreement with the values measured under field cjnditions: the average deviation corresponds to 2%, the maximum deviation does not exceed 7%. The pressure gradient directly depends on the hipping speed, rheological parameters of the fluid, and the ratio of the inner and outer diameters of the annular space, i.e. the gradient increases with an increase in any of these parameters. The findings presented in this paper may be of interest for determining safe operation modes for wells with enhanced safety requirements, such as drilling with casing or in deep-water drilling, including selecting the tripping speed, the rheological parameters of drilling mud, and the optimal sequence of casing installation.