Recycling gas drilling is a new drilling technology. The paper based on the theory of gas transient flow, a set of theoretic models of gas flow control at the export of annulus are set up in combinations of the principles of the seepage flow mechanics and the characteristics of recycling gas drilling technique. According to the basic laws which the flowing gas should comply with, the properties of various gas flows and the dynamic pressure are portrayed when the gas zone is penetrated. Two control schemes are proposed, namely constant pressure method and constant flow rate method. The Mac Cormack method and the characteristic line method are used to solve the theoretic models. The process of drilling gas zone was analysed using numerical simulation. The results of study can play a positive role in guiding the field operation.
Keywords: recycling gas drilling, transient flow, formation seepage, gas flow control scheme, numerical simulation