Natural reservoirs are geological formations that store fluids in their pores allowing fluids to flow throughout the rock. These formations are crucial to the energy value chain, for example, they contribute to the production of hydrocarbons, transport of geothermal energy, and carbon storage. Candidate reservoirs for storing or producing fluids are assessed based on their porosity and permeability, which indicate the effective storage volume and the fluid transmissibility, respectively. Therefore, a tool is needed to measure a reservoir’s pore distribution without being influenced by the properties of the surrounding rock formation.

Nuclear magnetic resonance (NMR) techniques can measure the amount of fluid stored in natural reservoirs by altering the orientation of the spins of the hydrogen atoms by applying a radiofrequency pulse in a fixed magnetic field, then measuring the decay times of the longitudinal or transverse components of the nuclear magnetization vector relative to the applied magnetic field, T1 and T2, respectively. Measurements of the T2 distribution can be used to rapidly quantify the amount of stored fluid. Additionally, these measurements can be performed through downhole logging to scan rock formations in situ. The T2 cut-off is a threshold used to distinguish mobile fluids from immobile fluids bound in the enclosed pores of the rock; above this threshold fluids are free to move throughout the rock formation and hence the effective porosity and permeability can be derived from it. Typical T2 cut-off values for sandstone and carbonates are 33 ms and 92 ms, respectively.

NMR T2 distributions can measure the pore size distribution and fluid content without being influenced by the matrix of the rock, that is, the mineralogical component. However, data processing must be conducted with a critical eye, as there are always exceptions. For example, the rate of T2 decay is increased in rocks such as greensands owing to the paramagnetic properties of glauconite and iron minerals present in the rock. Large amounts of iron in the rock lead to decreased quantities of mobile fluid and affect the measured range of the pore size distribution, which can decrease the T2 cut-off value to as low as 10 ms. Additionally, it is difficult to define the T2 cut-off value of carbonates because the pore network is more complex than that of sandstone owing to the presence of vugs (cavities) and the high chemical reactivity of carbonate minerals. Therefore, further work is needed to explore whether NMR can be used to reliably estimate the porosity of these complex reservoirs.