Abstract: The general material balance equation was applied widely in the field, but few studies about material balance equation of gas injection flooding has been conducted. When the injected gas comes into contact with the oil in place, there will be diffusion and dispersion between oil and gas system. And the volume of oil in place will expand due to the dissolved gas. As a result, the material balance equation of gas injection is different from that without regard to diffusion and dispersion between phases. According to the mechanism of gas injection, this paper classified the reservoir floods into three zones-free gas, free oil and transitional zone. Using slim tube experiment to study the relation between the transitional zone and the pressure and quantity of injected gas. And we erect the material balance equation of gas injection flooding and WAG injection flooding according to the principle of material balance and expansion. This equation has taken diffusion and dispersion between oil and gas system and the expansibility of oil into consideration, and provides an effective way for gas injection reservoir.

Key words: mterial balance equation, gas injection, miscible