causes of gas migration through cement slurries and curring methods

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 amount 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 forms 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. 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 temperature and pressure gradients with depth. this paper discusses the individual components necessary for gas-leak elimination. the paper gives a clear guideline for designing the total elimination of gas migration during a cement job. in addition, this paper clearly addresses the gas migration problems related to cementing operations