Distinguishing CO2 and Hydrocarbon Gas Using Pulsed Neutron Logs for CO2 Storage Projects in Depleted Gas Reservoirs

Monitoring carbon dioxide (CO2) injection and storage is critical for geological CO2 storage projects. Among the various monitoring technologies, pulsed neutron well logging provides insights into the CO2 injection and storage profiles of near-wellbore formations on a well- centric (or well-to-well) basis. Using pulsed neutron well logs and stochastic forward models, we present a technique for differentiating between CO2 and hydrocarbon gas (i.e., methane or CH4) under subsurface conditions when CO2 is injected into depleted gas reservoirs. We made certain assumptions regarding the depleted gas reservoirs to quantify the CO2 and CH4 volumes separately. First, the water volume in the formation remained unchanged after CO2 injection. Subsequently, the CH4 volume decreased with CO2 injection, but the total volume of the two fluids remained constant. Finally, variations in the formation properties, and the attributes and behavior of each fluid were negligible. To compute the saturation of each component, we integrated (1) the inelastic gamma-ray count rate ratio-based measurement from a three-gamma-ray-detector-equipped pulsed neutron logging tool and (2) the respective stochastically simulated tool responses considering well and formation specifications and developed an advanced multiphase saturation analysis algorithm. Monte Carlo N-Particle (MCNP) simulations produced forward models for the evaluation of irreducible water- and CH4-filled formations with porosities ranging from zero to 40%. The predicted tool responses were at maximum ratio-measurement values when the formations were filled with water and CH4. The minimum predicted tool responses occurred when the porous media were filled with CO2 and irreducible water. Furthermore, the tool responses for formations with varying CH4 and CO2 volumes were simulated, expanding the characterization of the ratio measurement. The correlations between the ratio measurement and CO2 saturation at varying porosities were determined using the generated MCNP models. Forward modeling-based in-situ CO2 saturation and ratio-measurement correlations at different porosities enable the determination of the saturation of CO2 and CH4. Previous pulsed neutron data analysis methods for monitoring CO2 have been limited to estimating the total gas volume without distinguishing CO2 from a hydrocarbon gas. When the water in the porous media of depleted gas reservoirs is irreducible, the proposed method allows the calculation of the CO2 and CH4 volumes separately. Accurate well-centric CO2 and CH4 saturation data during the post-injection phase can be used as conditional data to update field-scale reservoir simulation models.