Optimal Spacing for Casing Centralizers (includes associated paper 18730 )
- H.K. Lee (Amoco Production Co.) | R.C. Smith (Amoco Production Co.) | R.E. Tighe (Amoco Production Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- April 1986
- Document Type
- Journal Paper
- 122 - 130
- 1986. Society of Petroleum Engineers
- 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 3 Production and Well Operations, 1.14 Casing and Cementing, 1.6 Drilling Operations
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Summary. In the placement of centralizers on casing, it is necessary to integrate information on hole trajectory, casing properties, and centralizer performance. This paper summarizes the basic theory and algorithms for optimizing centralizer spacing based on these considerations. The algorithm predicts the wall forces in a three-dimensional (3D) borehole. Although centralizer placement is important in all wells, it is especially important for highly deviated boreholes and deep, heavy strings. With the detailed approach described here, centralizer placement design gives the operator greater confidence in the prediction of minimum standoff distance for these types of wells. Some examples of field applications are presented. Also, cementing results from field wells support the conclusion that good cement placement is obtained when adequate centralizers are used.
Centralizing the casing in the borehole is essential to obtain effective cement placement around the casing string. This is particularly true for deviated holes. Spring-bow centralizers or positive-standoff devices improve the cement-flow pattern for better mud displacement and provide better cement-sheath coverage around the casing circumference. Most flow calculations relating to mud displacement efficiency assume the casing is centered in the wellbore. It is necessary for the engineer conducting casing-cementing operations to integrate the axial and weight load forces and the bending forces caused by hole curvature into a centralizer placement schedule that provides adequate pipe standoff based on the restoring force exerted by the centralizers. API Specification 10D contains a recommended procedure for the calculation of lateral forces exerted by the casing on centralizers in a two-dimensional (2D) inclined borehole. We present an approach that permits analyses for 3D borehole trajectories and provides a complete design for centralizer placement. The algorithm used in the centralizer-spacing program is based primarily on API Specification 10D for lateral-load calculation. It uses Lubinsici's 3D dogleg-severity criterion for handling the curvature changes in the borehole. In addition, the effect of buoyancy on casing weight and the effective lateral load on centralizers are considered in the spacing algorithm. Furthermore, pipe deflection (sagging) between the centralizers is analyzed with Timoshenko's method to arrive at the effective standoff clearance.
The lateral load imposed on a casing centralizer is the combined effect of centralizer spacing, casing weight, hole-inclination angle, hole curvature, and tension from the pipe hanging below the centralizer. The equations presented in API Specification 10D summarize the forces that act on a centralizer as follows.
The equations used in API Specification 10D are based on Ref. 2. Use of the positive or negative sign before the tension term or tensile load depends on the direction of the dogleg. The positive sign is used in the API's publication to arrive at a consistently conservative figure for the force on a centralizer because of the various unknowns in a typical deviated hole--e.g., the degree of angle change between two survey points.
Buoyancy Effect on Casing Weight
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