Z Factors for Carbon Dioxide with Hydrocarbon Gas Impurity

The sequestration of carbon dioxide (CO2) in depleted hydrocarbon gas reservoirs is a practical form of reducing greenhouse gas emissions. These mature fields have been able to safely store natural gas over a long period of time and are prime candidates for CO2 storage. Additionally, infrastructures and wells used in the development of these fields can also be considered for CO2 injection. However, the volumes of CO2 that can be safely stored together with the residual natural gas in these reservoirs must be accurately determined. In Reservoir Engineering, an important parameter required for the calculation of reservoir gas in place volume is the gas compressibility factor, or z factor. Z factor values must also be determined for residual natural gas /CO2 mixtures for every depleted gas reservoir selected for CO2 storage, in order to accurately estimate CO2 storage volumes and to monitor CO2 injection. Z factor varies with temperature, pressure and gas composition and can be determined experimentally from the ratio of the measured gas volume to the ideal gas volume at the measured temperature and pressure. However, the experimental method can be very time consuming, costly and only applicable to the compositions and conditions investigated. In the upstream gas industry, reservoir engineers commonly use z factors for natural gas from the well-known charts and correlations by Standing and Katz, Dranchuk and Abou-Kassem, Wichert and Aziz, and Sutton, with corrections for small amounts of hydrogen sulfide (H2S), CO2 and nitrogen (N2) which are considered as impurities of natural gas. From a review of z factor correlations, the Piper, McCain and Corredor correlation provide the most accurate method for determining z factor values for natural gas containing as much as 70 % of these impure gases. In this study the correlation by Piper, McCain and Corredor was tested for determining the z factor for CO2 by treating natural gas and other pure hydrocarbon gas as impurities of CO2. Published z values for CO2/hydrocarbon gas mixtures with CO2 concentrations ranging from 0.0 to 80.0 % were experimentally determined at various temperatures from 160°F to 400°F and at pressures ranging from 200 psia to 10000 psia for each temperature. The pressures, temperatures and gas compositions investigated in this study represent those that will be encountered during CO2 storage within depleted gas reservoirs. The results showed an average absolute percent difference (AAPD) of less than 5 between the predicted and experimental z values. The results further showed that the percent difference between the predicted and experimental data were less than 5 for temperatures ranging from 200°F to 400°F, with CO2 concentrations as high as 80 %, for all pressures. At 160°F and CO2 concentration of 80 % the difference was as high as 5% to 10 % for pressures between 1000 psia and 4500 psia. These results suggest that the correlation by Piper, McCain and Corridor can be suitably applied to calculate the volume of CO2 that can be sequestered within depleted natural gas reservoirs with residual natural gas present.