Sensitivity study of C/O logging measurements by the Monte Carlo method

Carbon oxygen ratio (C/O) logging is an important method for the accurate determination of hydrocarbon saturation in the reservoir region. One of the advantages of the measurement, that it is independent of Cl content. Furthermore, the insensitivity of high energy neutrons to the casing, makes it possible to use it in cased boreholes too. In addition to the application in the oil industry, monitoring of CO₂ reservoirs is also possible. In our study, the modeling of the time-dependent coupled neutron-gamma field produced by the tool was carried out, using MCNP, a general-purpose Monte Carlo radiation transport code. The energy spectrum of gammas reaching the scintillation detector crystal were simulated in different detector positions, and different tool environments: reservoir rock, reservoir porosity, hydrocarbon saturation, and well status (cased or open). The effect of the parameters above are illustrated by vertical cross sections of the particle fluxes around the tool, and shown by the changes of the interpretation charts. By the introduction of a “goodness” factor, derived from the interpretation chart, the potential, and the limitations of C/O logging are investigated.