Study of phase behavior and physical properties of a natural gas reservoir with high carbon dioxide content

The discovery of natural gas reservoirs with high and varying CO 2 content (5% to 98%) in China has attracted worldwide attention. In this paper, the phase diagram of natural gas with different CO 2 content was calculated and drawn by PVT Sim 100 which was fitted by experimental data based on the Ruska PVT 2730 apparatus. Both the real critical parameters and the pseudo‐critical para­meters were calculated and compared. Also, the phase state of natural gas along the wellbore in a representative well under both shut‐in and flowing conditions was analyzed. In addition, to clarifying the phase transition process, videos of a gas sample showed variation between supercritical state to gas‐liquid state with isobaric cooling and heating up between 35°C and 25°C. Moreover, the compressibility factor (Z‐factor) of the natural gas with different CO 2 content was chosen as one of the key physical parameters of natural gas to measure at pressures ranging from atmospheric pressure to 45 MPa and temperatures ranging from 40°C to 140°C experimentally. Results show that the Z factor decreases with the increase of CO 2 content in natural gas at isothermal conditions, and increases with the increase of temperature. Furthermore, a new model is proposed to predict pressure and Z factor. The varying CO 2 ‐CH 4 concentrations of this reservoir are relevant to gas compositions which may arise in depleted natural gas reservoirs used for geologic carbon sequestration.