A Method for Characterizing Fluid Contacts in Freshwater Environments Exposed to Extrinsic Heat Utilizing Time-Spectral Pulsed Neutron Logging Measurements

Identifying freshwater pay zones using electrical resistivity logging presents challenges primarily due to the minimal resistivity contrast between hydrocarbons and freshwater. Prior research has documented the application of thermal energy to a formation to modify the characteristics of freshwater, reporting variations in resistivity measurements that facilitate the identification of fluid contacts through resistivity logging. Further exploration of an alternative measurement with distinct physics leads us to propose the utilization of pulsed neutron logs derived from time spectra when the formation temperature is elevated through an additional heating source. When a formation is subjected to non-intrinsic heat, such as targeted heating through electromagnetic irradiation, the properties of fluid components occupying the pores exhibit variations. The in-situ density change in water exceeds that observed in oil (and sometimes gas, depending on the reservoir pressure). Multidetector pulsed neutron well logging acquires time-spectral data and provides gamma-ray ratios from various combinations of multiple scintillation detectors. Two gamma-ray ratio measurements sensitive to fluid density variations (or hydrogen index) have been developed: (1) inelastic scattering of fast neutron-induced gamma-ray ratio and (2) thermal neutron capture-induced gamma-ray ratio measurements. The MCNP models reveal that these ratio measurements are particularly effective in indicating the conversion of freshwater into steam. The inelastic ratio log delineates steam formation as temperature increases, accompanied by a reduction in its response. Similarly, the capture ratio measurement also exhibits a decrease in its response; however, this reduction is comparatively less pronounced than that observed in the inelastic ratio measurement. The conventional macroscopic thermal neutron capture cross section, sigma log, demonstrates its sensitivity in the transition from freshwater to steam; however, it does not provide the same level of resolution for the freshwater-to-steam transition as the ratio measurements. Carbon/Oxygen (C/O) logging can also indicate the interfaces between hydrocarbons and freshwater; however, the sensitivity of the C/O ratio is significantly dependent on porosity and hole size. Consequently, challenges persist in identifying fluid contacts using the C/O ratio in low-porosity formations with large holes. This innovative method for precisely delineating pay zones in freshwater environments will assist petrophysicists and reservoir engineers, particularly during the exploration phase of reservoir fluid characterization. Recent enhancements of slimhole multidetector pulsed neutron well logging instruments have facilitated the faster logging of these ratio measurements. Additionally, the heat-flasked pulsed neutron tool permits pulsed neutron data acquisition in harsh temperatures exceeding 350 degrees Fahrenheit.