Monte Carlo modelling of a pulsed neutron capture tool for CO2 saturation estimation in sandstone reservoirs

One of the key factors for a successful carbon capture and storage (CCS) project, is the ongoing monitoring of the carbon-dioxide injected into the reservoir rocks. Borehole geophysics measurements are invaluable for this process. Due to the evident presence of borehole casing, the number of effective well logging methods is limited.  This means, that nuclear measurements play a highly important role owing to their relatively great depth of penetration. We present a method for PNC (Pulsed Neutron Capture) tools to estimate CO2 saturation in sandstone reservoirs independently of the water salinity. To achieve this, we utilize a carefully selected energy window in the inelastic part of the gamma spectra. The ratio created from the number of counts in this window for different detector spacings is sensitive to hydrogen content of the reservoir rock, thus indirectly to CO2 saturation as well. We estimate the energy deposited by gamma photons in the scintillation detectors of the PNC tool for different model parameters such as rock properties (e.g. porosity) and CO2 saturations. A systematic modelling of the measurement was carried out using Monte Carlo N-Particle (MCNP), a general-purpose particle transport code. These results demonstrate the potential of this method for CO2 monitoring in sandstone reservoirs.