A Case Against Cementing Casing - Casing Annuli
Authors: O'Brien Thomas B. 
Published: December 2007

Over the years a significant number of casing failures are recorded the cause of which is frequently improperly diagnosed. Although the total number of such failures is not numerically large because they occur most often in high cost wells, the consequences of each failure is serious. Generally such failures result in the well being junked and sometime lead to either down hole or surface blowouts.

Many of these failures have been in casing cemented inside other casing. Because that type cementing is not always as efficient as is apparently generally believed and because excessive pressure can be trapped in casing-casing annuli a variety of failures can result. Several failure modes are described and it is concluded that in many instances it is better to leave casing-casing annuli uncemented.

Introduction

It has long been held by many casing design engineers that fixing casing in place with cement will prevent all but burst failures and there are those who give cement sheaths credit for enhancing the burst rating of casing. That design practice is predicated on the assumption that cement sheaths will be continuous. Field tests have shown no collapse failures at forces twice the collapse rating of cemented casing. Cement with compressive strengths less than 150psi have been shown to adequately support casing in tension, Several investigators have shown that cement with 50psi compressive strength is adequate for casing support. Cement with compressive strength of 130psi was shown to prevent migration of fluids in a well bore. Even very low strength cement completely encircling casing will preclude effects of helical buckling and numerous reporters agree that for many applications adequacy of cement placement is of greater importance than is cement strength.

On the other hand partially cemented casing and cement sheaths with holidays or voids can lead to tension, collapse and buckling related failures as well as a variety of connection damage. Work with deep high pressure, high temperature wells particularly in the North Sea have led to studies which indicate the probability of a number of problems. High temperatures, hence large temperature changes coupled with high pressure loads lead to excessive pressure loading in annuli between the several protection casing strings. Large temperature changes can through longitudinal expansion and contraction cause excessive loading on well foundations on platforms. Buckling calculations show that longitudinal (tension) loading can vary from very high tension to compression at the surface in some partially cemented strings. Additionally in wells with cement filling a casing annulus into the last previous casing can experience failures in either the inner or outer string. While most of the studies mentioned above have involved wells that are expected to be subject to large temperature changes () many failures have occurred at frequently occurring 's. Some designers are now taking these potential problems into account in their design procedures.

When protection casing is cemented inside surface casing and in some other cases the cement level often falls leaving an air space at the surface. The most prevalent practice is to fill the annulus from the surface after the cement takes its initial set. There is often no method for determining how far cement has fallen and what the fill volume should be. It is reasonable to conclude that flow downward may not be uniform and at least channels may be left.

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