Abstract:Oil and gas upward velocity is a very important parameter reflecting the activity of reservoir. During drilling,oil and gas upward velocity influences directly the safety of drilling operation. If not accurate,it is ease lead to accidents such as well kick, blowout and so forth, and cause the unreasonable drilling fluid density damaging the reservoir. The velocity obtained by using the method of lag time, which is widely applied during drilling, have higher error. In this paper, based on the analysis of the rule of oil and gas migration in the drilling fluid, a new calculation method was obtained. The method takes into account displacement effect of drilling tools, wellbore structure, BHA among other factors, and correctes the migration time of oil and gas. This method has been tested and the result shows that it has a high accuracy.