Abstract: CO₂ can be dissolved in irreducible water, intrusion water and produced water in CO₂ gas reservoirs. This paper presents the material balance equation for CO₂ gas reservoir based on mole conservation theory. The physical property and phase change of CO₂ are closely related to the pressure and temperature conditions, and the solubility of CO₂ is affected by the formation water salinity. Using Newton interpolation polynomial, the parameters of physical properties under different reservoir conditions can be calculated, and based on the material balance equation, integrated with high pressure property of CO₂ and its changes with pressure and temperature, the CO₂ inplace or geological reserves and the average reservoir pressure in different recovery factors can also be estimated, and the phase state changes of CO₂ in the development process can be determined. The relative error of the recalculated geological reserves is 0.131%, showing this method is effective.

Key words: CO₂ gas reservoir, material balance, geological reserves, formation pressure, phase state