Cased-Hole Pulsed Neutron Log response simulation with the Monte Carlo method applied for Carbon Capture, Utilization and Storage and Underground Hydrogen Storage purposes

Cased-Hole Pulsed Neutron Logging (CH PNL) measurements represent the standard solution for formation evaluation and time-lapse water/hydrocarbon saturation monitoring in cased wells. Today, this technology proves to be a valuable monitoring option also in the context of Carbon Capture, Utilization and Storage (CCUS) and Underground Hydrogen Storage (UHS) projects. In case of storage into depleted natural gas reservoirs, CH PNL tools can be used at the so-called spy wells to highlight plume arrival from the injector(s) and to possibly quantify its composition behind casing in terms of water, methane (CH4), carbon dioxide (CO2) and/or hydrogen (H2) relative saturations, according to the storage context (CCUS or UHS). However, the latter task can be not straightforward in case of mixing between CO2 or H2 and CH4 during injection. The present work starts from the different interactions occurring among the emitted neutrons (from the CH PNL tools), the borehole environment and target formation: these include inelastic scattering, elastic scattering, and neutron absorption (capture). In detail, the focus is the construction of a model capable of simulating CH PNL inelastic scattering response in these environments and for different measurement conditions, exploiting the Serpent 2 Monte Carlo (MC) particle transport code. The latter approach is fundamental due to the complex physics involved in the inelastic process with respect to the more straightforward elastic/capture interactions. After the definition and validation of the MC inelastic modeling, and the exploration of the potentiality of such cased-hole monitoring technique together with its code modeling phases, a total of three interesting monitoring scenarios have been simulated (in terms of rock lithology, porosity, formation water salinity, type of storage, pressure and temperature regime) and a critical analysis of the relative results has been conducted. The outcomes are fundamental to assess the diagnostic capability of CH PNL, together with the associated interpretation uncertainty, for CCUS and UHS purposes, in view of the final goal of planning a proper and cost-effective monitoring campaign. In addition, this thesis provides the very first known CH PNL response simulation for H2 storage in depleted gas reservoirs and augment the knowledge of neutron interactions in presence of CO2 by including inelastic scattering modeling.