An Integrated Approach for Successful Primary Cementations
Authors: Hartog J.J. , Davies D.R. , Stewart R.B. 
Published: December 2007

Summary
Studies of the individual cementing variables should be combined to lead to a total cement-job design approach that results in effective zonal isolation in critical wells. For the proper design of cement columns in (particularly gas) wells, a thorough understanding of the mechanism causing loss of hydrostatic head of a cement column is needed. In addition, fluid-loss control, slurry stability, and setting behavior should be carefully designed. Attention should also be given to mud conditioning, batch mixing, scavenger slurries, and spacers. Efficient mud displacement is achieved with high cement-displacement rates. reciprocation, and suitable cement rheologies and contact times.

Introduction
Cementation of the productive zone is one of the critical pails in the completion of a well that will determine whether the production strategy planned by the reservoir engineers can be successfully carried out. Thus, unless proper zonal isolation can be achieved, it will not be possible to produce independently the different reservoirs penetrated by the well, as is often required by reservoir engineering considerations. In the case of faulty zonal isolation, it will also not be possible to perform chemical treatments in the desired intervals. Faulty zonal isolation also frequently manifests itself at the surface by the appearance of pressure on casing annuli and, in the worst case, by a blowout in which the unset cement slurry is thrown out of the casing annulus by a rapid flow of formation fluids.
Remedial (squeeze) cementations to correct uncontrolled flow behind casing are not only time-consuming and expensive, but they also weaken the integrity of the casing. Our cementing research along with other work has focused on methods to increase the success rate of primary cementing. This report sets out to explain the need for a total-job-design approach to cementation. It is shown how drilling and cementing variables together with Correct cement formulations can lead to efficient displacement of the drilling mud from the casing annulus. A theoretical model has been developed to explain how a dense cement, with more than sufficient hydrostatic head to control the formation fluids, loses its overpressure, which can result in annular (gas) flow. This theoretical model is used to define values of cement-formulation variables, which will increase the chance of successfully achieving zonal isolation.

The Annular Gas-Flow Problem

Mechanism of Gas Influx Into a Cement Column
Research at Koninklijke/Shell, as well as recent publications (see Ref. 1), have indicated that gel buildup, together with simultaneous volume reduction (caused by the cement hydration process and fluid loss to permeable formations), are the mechanisms that cause loss of hydrostatic head of the cement column. Fluid (gas) can enter the cemented annulus once the hydrostatic head has been reduced sufficiently so that overbalance is lost. Our current views on the mechanism of (gas) influx into a cemented annulus are summarized as follows.
During and immediately after pumping, the cement slurry behaves as a liquid and fully transmits hydrostatic pressure.
JPT
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