It has been proven through laboratory experiments that the three types of gas migration through a cemented annulus can be eliminated by designing the correct cement mixture. The first type of void and, therefore, gas migration can occur between the casing and the cement. By adding the correct amounts of magnetite to the cement, this void, and therefore the possible cause of gas migration, can be eliminated. The second possible type of void generation is between the cement and the borehole wall where the filter cake formed at the borehole wall adversely affects the bonding process. By using a special material, Anchorage Clay, this bonding can be improved to the extent that the gas migration between the borehole and the cement can be eliminated. The third and most complicated process is the pressure changes appearing in the cement during the setting phase. A double sinus wave pressure response during this setting time generates fractures in the microstructure of the cement. The correct amount of water as well as retarders is crucial for the best results during the dehydration process of the cement. By adding the correct type of elastomers, this pressure variation during the setting of the cement can be eliminated. Elastomers are known to counter-react the pressure behavior during the setting process. This eliminates the pressure variations and, therefore, the micro cracks. The three above mentioned effects are strong functions of temperature and pressure, and the cement design for a well would have to be carefully planned since a well has both a temperature and pressure gradient with depth. This paper discusses the individual components necessary for gas-leak elimination and gives a quantitative field example where all the correct additive volumes have been designed as a function of depth. The paper gives a clear guideline for designing the total elimination of gas migration during a cement job. In addition, this paper clearly addresses all the gas migration problems related to cementing operations.
Cement slurry passes through two cycles of building its structure during the gelation period. Figure 1 shows a schematic of the duration of the first and second cycles of the total gelation time. in the first cycle, the cement slurry builds itself with time in a three-dimensional structure (thixotropic behavior). However, in some local areas, this structure collapses, releasing some trapped water. This situation takes place mainly in the presence of tight formations. When cement faces porous formation the preflush cement loses part of its water to the formation as a spurt filtrate. This results in an incomplete cement-water reaction.
The cement cake in the spurt loss area is the weakest part of the cement since it did not receive enough water to complete the reacton. The microcracks will first appear in the spurt loss area even though filtrate control materials are used with the cement.
Temperature and pressure are the two physical properties that contribute to the final cement setting. The effect of temperature has the most impact on the interface between the casing and the cement. Similarly, pressure affects the setting of the cement slurry that undergoes a phase transition from liquid to solid.