An apparatus and method are described to estimate a fraction of carbon dioxide present in a downhole fluid. The apparatus includes a carrier configured to be conveyed through a borehole penetrating the earth. The apparatus also includes a pressure sensor disposed at the carrier and configured to measure an ambient pressure of the downhole fluid and an ambient temperature sensor disposed at the carrier and configured to measure a temperature of the downhole fluid. A processor of the apparatus receives the ambient pressure and the ambient temperature measurements and solves for the fraction of carbon dioxide in the downhole fluid using a correlation function with the ambient pressure and the ambient temperature as inputs to the correlation function.