Abstract: This paper presents some formulas which are based on Fraim's chart matching model for gas reservoir, combined with the nonlinear regression analysis and the relationship of PVT in CO₂ gas reservoir. They can be used to determine the OGIP and reservoir parameters according to CO₂ gas well production history. Through calculation of OGIP of Gao-53 Well in Huagou gas field, the reserves producing degree in this field has been found out, which can be as an important basis for further exploitation decision. Also, this paper presents the criterion to validate the calculation results with the semi-log figure of normalized time vs. normalized pressure being straight line or not, and provide references for acquiring proper OGIP or reserves and reservoir parameters.

Key words: CO₂ gas reservoir, production, decline, reservoir parameter, OGIP, reserve