Impact of the dogleg geometry on displacement efficiency during cementing; an integrated modelling approach

Abstract

In the dogleg section of directional wells, casing strings bend to fit the borehole trajectory, which may lead to complicated contact areas with the borehole. The severity of dogleg in combination with casing properties may make the formed annulus space so complex that cement may not displace mud remnants very well. While current simulation methods only consider constant eccentricity, we propose a model for a more general geometry defined by the well trajectory and casing properties. We have developed a three-dimensional finite element model (FEM) to simulate large-deformations of the casing while running into the dogleg. The casing deformation determines the annulus geometry that is supposed to be filled with cements. We utilized a computational fluid dynamics (CFD) model to simulate cement-mud displacement in the resultant annulus space. The effects of density ratio and rheology on the displacement efficiency are analyzed here. The obtained numerical results show the complicated contact geometry of the casing and borehole at the dogleg, which may worsen in casing strings with small thickness and large curvature doglegs. Results show that the magnitude of the stress developed along the casing can considerably increase due to friction. Complicated geometry of the annulus induced by casing deformation makes mud displacement by cement in the dogleg difficult. Continuous drilling mud pockets are formed due to the cement reverse flow. However, increasing density difference helps development of a steady and shorter displacement interface. We also noticed the phenomenon of reverse flow occurred during the cementing, which prevents sweeping or perfect displacement of the cement in the annulus space. The outcome will help us to prevent any potential damage that might happen due to large pressure fluctuations during fracturing through casing by minimum costs and avoid such potential integrity problems by setting a packer and tubing or using more appropriate cement recipes.