How to differentiate between CO2 and CH4 in the subsurface using rock physics? A case study from the Thrace Basin, Türkiye

Abstract Natural gas production can be complicated due to the presence of naturally occurring carbon dioxide (CO2) in a field where production takes place. Knowing beforehand the 3D subsurface distribution of in situ CO2 aids the efficient natural gas production in such fields. It is also important to quantify at a site that in situ methane gas (CH4) can be distinguished from CO2 in the reservoir unit. In this paper, a methodology describing differences between CO2 and CH4 at subsurface conditions in a gas field located in the Thrace Basin, in northwestern Türkiye, is presented. First, the thermodynamic properties of CO2 and CH4 are modeled as a function of pressure and temperature to infer the depth-dependent change of these properties. Following a thermodynamic modeling study, we then constrain a Hertz–Mindlin-based rock physics model with X-ray diffraction (XRD) and processed well log data to produce cross-plots of expected in situ elastic properties derived from different saturation cases, such as CO2–brine, CH4–brine, and CO2–CH4–brine. Furthermore, attenuation and velocity dispersion induced by patchy CO2–brine and CH4–brine saturation are quantified using White’s layered model. The analysis of the results in this case study indicates that density differences between these gases may be used to distinguish CO2 from CH4 in subsurface conditions. Finally, we tested and compared this finding with measured well log data and showed the compatibility of cross-plots derived from the theoretical rock physics model with the ones created from the field data.