Simulation of monitoring Underground Hydrogen Storage (UHS) using nuclear borehole geophysics

Underground Hydrogen Storage (UHS) seems like a very promising new technology to balance the seasonal variance of renewable energy production. The excess energy of renewables can be used for water splitting to produce ‘green’ hydrogen, which can be stored in UHS sites. However, due to the high mobility and small molecular size of hydrogen, the risk of leakage through faults and caprock is large compared to gas or carbon-dioxide. Thus, a complex monitoring system is a critical part of every UHS project. Borehole geophysical measurements are parts of this system. One of the key monitoring parameters is the hydrogen saturation in the vicinity of wells. The aim of our research is to investigate the applicability of nuclear borehole measurements for hydrogen saturation estimation in the presence of borehole casing, independently of water salinity. To achieve this goal, systematic simulation runs were carried out using the MCNP particle transport code. We present a method based on the ratio of gamma counts acquired from two detectors of a pulsed neutron logging (PNL) tool and the results of a sensitivity study focused on the most crucial model parameters of the measurement: hydrogen saturation, porosity, lithology, borehole diameter. Model results demonstrate that the sensitivity of the method is larger in high porosity reservoirs and smaller borehole diameters but there is a still reasonable sensitivity in wells with up to 8-inch casings. Additionally, an alternative technique is presented to improve the sensitivity of the method used in sandstone reservoirs which yields 10% sensitivity increase, when the rock consists of more than 50% sandstone. The improvement is independent of porosity, that is especially advantageous in reservoirs with lower porosity (10–15%). The simulation results prove that, in deep saline aquifers, the hydrogen saturation of the reservoir rock can be monitored independently of water salinity by using only one nuclear borehole geophysical method.