Gas Flow in Cements

During the last few decades, several laboratory investigations and field studies have been conducted in an attempt to find solutions to the problem of gas migration after primary or remedial cement jobs. This article reviews the general findings of previous investigators and offers an updated explanation of the mechanism of gas migration. Results of our laboratory studies show that ''mobility'' of the fluids in the pore spaces during the early life of the cement, particularly after the cement structure becomes load-bearing at a given hydrostatic pressure, is the main factor that must be controlled to minimize gas migration within the cement lattice. We also show that fluid-loss control alone, though helpful, is not sufficient to stop gas migration. This paper also describes an ''impermeable'' cement system developed by applying the principles outlined in the laboratory investigation. This impermeable cement has been used in field applications in areas where severe gas migration problems have been experienced after cementing. Thus, gas migration through the cement has been prevented when this new approach is used. Several of these case histories are presented and discussed.