Finite Element Modeling of Annular Flows With Application to Slim Hole Drilling Hydraulics

During the exploration and exploitation phases of an oil field, it is necessary to drill wells at strategic points of the field in order to obtain information about the geology, lithology and oil bearing potential of the reservoir. Due to the financial and technical advantages of applying slim hole drilling and coring to exploration, increased effort in the analysis of the performance, safety and environmental impact of the operation has been observed. One of the major issues in slim hole drilling and coring operations is the accurate prediction of pressure losses in the annulus, in order to guarantee optimum operating conditions. As has been pointed out in the literature, the small annular clearance and the high rotational speeds of the drillstring are factors which preclude the vii common assumption of unidimensional axial flow in the annulus, which has been applied for conventional pressure drop computations. In order to address the effect of rotational speed, eccentricity of the drillstring, variation in borehole diameter due to cased and open-hole sections, and drillstring upsets, 2D and 3D finite element schemes were applied. The numerical modeling results showed good agreement with available laboratory data for laminar, axial-rotary flows of power law fluids in concentric annuli and axial flows in an eccentric annulus. The simulations revealed that the rotation of the inner cylinder causes a decrease in the pressure differential along the concentric annuli, which is due to the contribution of the tangential shear rate in the viscosity function of the generalized non-Newtonian constitutive equation for the fluid. The eccentricity caused a decrease in the pressure differential along the annulus for both purely axial and axial-rotary flows. The simulation of complex annular flow geometries revealed that the flow constraint imposed by the inner cylinder external upset introduces a significant pressure gradient increase. Despite the satisfactory performance of the numerical modeling under the scope of its assumptions of laminar regime and steady flow configuration, it was found to be limited in its ability to evaluate the pressure behavior in the annulus of the field case. This was due to the fact that in field operations, the pressure loss in the annulus is affected by flow turbulence and dynamic behavior of the drillstring.Petroleum and Geosystems Engineerin