Optimal Spacing for Casing Centralizers (includes associated paper 18730 )

Abstract

Summary In the placement of centralizers on casing, it is necessary tointegrate information on hole trajectory, casing properties, andcentralizer performance. This paper summarizes the basic theory and algorithms for optimizing centralizer spacing based on theseconsiderations. The algorithm predicts the wall forces in athree-dimensional (3D) borehole. Although centralizer placement is important in all wells, it isespecially important for highly deviated boreholes and deep, heavy strings. With the detailed approach described here, centralizer placement designgives the operator greater confidence in the prediction of minimum standoffdistance for these types of wells. Some examples of field applications are presented. Also, cementingresults from field wells support the conclusion that good cement placementis obtained when adequate centralizers are used. Introduction Centralizing the casing in the borehole is essential toobtain effective cement placement around the casing string. This is particularly true for deviated holes. Spring-bowcentralizers or positive-standoff devices improve thecement-flow pattern for better mud displacement andprovide better cement-sheath coverage around the casingcircumference. Most flow calculations relating to muddisplacement efficiency assume the casing is centered in the wellbore. It is necessary for the engineer conductingcasing-cementing operations to integrate the axial and weight loadforces and the bending forces caused by hole curvatureinto a centralizer placement schedule that providesadequate pipe standoff based on the restoring force exertedby the centralizers. API Specification 10D contains a recommendedprocedure for the calculation of lateral forces exerted bythe casing on centralizers in a two-dimensional (2D)inclined borehole. We present an approach that permitsanalyses for 3D borehole trajectories and provides a complete design for centralizer placement. The algorithm used inthe centralizer-spacing program is based primarily on APISpecification 10D for lateral-load calculation. It usesLubinsici's 3D dogleg-severity criterion for handling thecurvature changes in the borehole. In addition, theeffect of buoyancy on casing weight and the effective lateralload on centralizers are considered in the spacing algorithm. Furthermore, pipe deflection (sagging) betweenthe centralizers is analyzed with Timoshenko's methodto arrive at the effective standoff clearance. Discussion The lateral load imposed on a casing centralizer is thecombined effect of centralizer spacing, casing weight, hole-inclination angle, hole curvature, and tension fromthe pipe hanging below the centralizer. The equations presented in API Specification 10Dsummarize the forces that act on a centralizer as follows. (1) and (2) At (3) and (4) The equations used in API Specification 10D are basedon Ref.2. Use of the positive or negative sign before thetension term or tensile load depends on the direction ofthe dogleg. The positive sign is used in the API'spublication to arrive at a consistently conservative figure for the force on a centralizer because of the various unknownsin a typical deviated hole--e.g., the degree of anglechange between two survey points. Buoyancy Effect on Casing Weight SPEDE P. 122^