Comparing Soft-String and Stiff-String Methods Used to Compute Casing Centralization

Abstract

Summary Good casing centralization during cementing operations is a key factor for achieving proper mud displacement and obtaining hydraulic isolation in the annulus. To this end, centralizers are often placed along the casing to position it centrally in the borehole. The optimal number of centralizers and their spacing are determined with simulation software. Until now, the industry has typically used calculation methods derived from American Petroleum Institute (API) Spec. 10D (2002, 2004) to predict casing eccentricity in the wellbore. The calculations are based on an analytical soft-string method, which models an element of casing string between two centralizers as a bifixed-ends beam. However, in drilling operations, the use of a numerical stiff-string method to compute torque-and-drag forces is now becoming widespread. It accounts for tubular bending stiffness and provides a more realistic analysis of the stresses and loads acting upon the drillstring and the borehole. The stiff-string technique that is based on the finite-element method is thus proposed as an alternative and more effective solution for computing casing centralization for cementing operations. Measured and calculated casing centralizations were compared in several field cases. The casing eccentricity was measured after the cement placement by use of recently developed ultrasonic logging tools and diagnostics. Discrepancies between analytical and numerical calculation methods were analyzed; then, advantages and disadvantages to each method were assessed. The results of the calculation methods were used to formulate an optimal approach to casing centralization.