Gas Leakage in Primary Cementing - A Field Study and Laboratory Investigation
Authors: Christian W.W. , Chatterji J. , Ostroot G.W. 
Published: December 2007

With increased drilling of deep wells across high-pressure gas zones, the problem of gas migration in cemented gas-storage wells has become widespread. Laboratory tests and field results have helped to identify the significant role played by low fluid-loss additives in cement slurries for controlling or preventing gas migration.

Introduction
With deeper well completions across gas-producing horizons, especially liner cementing completions, the problems of gas leakage have become a major concern. problems of gas leakage have become a major concern. In these cases, gas leakage poses substantial problems not only in the form of potential blowouts, but also in the loss of already scarce natural resources.
To determine various factors responsible for gas cutting of cement or gas leakage through a cemented annulus, model studies simulating down-hole conditions have been conducted. An examination of the published data indicates that the recommended practices for minimizing gas leakage may be classified in two categories.
The first concerns methods to obtain better bonding of the cement to both pipe and formation surfaces. The pipe bonding is aided mechanically by the application of a resin-sand coating to the outer surface of the casing or liner or by removal of the mill varnish from the pipe. Most studies conducted to improve cement-to-formation bonding have evaluated techniques to increase mud displacement efficiency. These tests have indicated the importance of pipe centralization, the use of scratchers, and specially designed spacer fluids between the drilling fluid and the cementing composition.
Considering all the variables evaluated during these displacement tests, the one having the most pronounced effect on increasing the mud displacement efficiency has been pipe movement. Both rotation and reciprocation have been studied. Conditioning of drilling mud to decrease plastic viscosity and yield point, together with higher displacement rates during cementing, are also an integral part of primary cementing considerations in high-pressure gas zones.
The second category concerns methods that prevent entry of gas into the cemented column. Gas can only enter the cement column when the formation pressure exceeds the hydrostatic pressure at that interval; several factors that may permit reduction of hydrostatic pressure after the cement slurry is in place have been identified.
Early investigations indicated that premature setting of the cement up the hole as a result of temperature anomalies or slurry dehydration, as well as gelation or increase in cement viscosity before hydration of cement, may contribute to the problem of gas migration. However, indications are that premature dehydration of cement slurry, resulting from the lack of fluid-loss control, may be the primary cause of gas communication. This problem seems most evident where permeable zones of problem seems most evident where permeable zones of varying formation pressure occur. When hydrostatic pressure exceeds the formation pressure, slurries without pressure exceeds the formation pressure, slurries without adequate fluid-loss control may undergo extensive dehydration, building filter cake across the permeable zone and resulting in bridging the annulus. The effective hydrostatic pressure will be nullified at this point and below it in the annulus. This may result in gas migration from the higher-pressure zone toward a zone of lower pressure, thus creating a gas channel in the cement column.
The role played by fluid-loss additives in controlling gas leakage may be viewed from the time the slurry is placed in the annular space till the time it has finally set. placed in the annular space till the time it has finally set. JPT
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