Gas migration after cementing is a well-known and costly problem and one which is extremely difficult to prevent. Its causes are fairly well understood and several commercial techniques are offered today to prevent it, with varying degrees of success. Despite this understanding of the phenomenon, few attempts has been made to examine the parameters which may be relevant to it and, thereby, try to predict the probability of gas migration for a given well. Evaluating this risk, on a well-by-well basis, would allow the engineer to match his job design to this risk level.In this paper, we describe a new approach to the prediction of post-placement gas flow. Our method is based on the identification of four key factors which are considered fundamental to the occurrence of gas migration. By using these four factors, we attempt to quantify the contributions made by the formation and annular configuration, the fluid hydrostatics, mud removal, and slurry performance, respectively. The factors consider such well parameters as reservoir productive capacity, annular geometry, pore pressure, hydrostatic head, mud removal efficiency, cement hydration kinetics and fluid loss.Analysis of field data from a wide variety of gas wells in the United States, Canada, Latin America, Europe, Africa, Middle East, and Far East, has allowed the calculation of semi-empirical relationships between the four factors. The wide range of field conditions, through which this method has been established permits its use in most real cases. Field exmnples are provided.