Drainage mechanisms in gas reservoirs with bimodal pores – A core and pore scale study

Underground gas injection and storage is principally used to meet variations in demand and supply of natural gas and for geosequestration. Reservoir engineering analysis of underground gas flow requires accurate modelling of the reservoir pores and their displacement mechanism. Modelling of multimodal pores like carbonates is quite challenging because of the difficulty in defining how two or more pore types interface. Parallel or serial communication between reservoir pores are the commonly used configurations in reservoir models. However, the factors controlling the communication pattern are not well understood and are rarely documented in the literature. Hence, the determination of the drainage pattern and the contribution from each pore type in a multimodal pore rock in the total flow process is done with some level of uncertainty. Capillary pressure (Pc) and electrical resistivity index (RI) curves can only indicate serial pore arrangement, but the combination of both also exists in some rocks. In this study, we employed a combination of nuclear magnetic resonance (NMR) relaxation measurements with Pc and RI methods to investigate how two pore types communicate during gas injection and the factors controlling their drainage pattern. Nitrogen was used as the gas phase to displace brine from different rock types covering limestones, dolostones, and sandstones. In another set of experiments, surfactant was added to the brine to ensure drainage of the micropores. We then developed an analytical workflow used to probe the drainage pattern and the contribution of the pore types. We quantitatively delineated the contributions of the macropores and micropores in the Pc and RI measurements and the intercommunication between them for the entire drainage process. The results showed that for a bimodal rock system, macropores and micropores are drained either in series, in parallel, or both depending on the rock structure. We found that the pore communication and drainage pattern correlate with the ratio of the macro and micro pores, the rock permeability, and log mean of the T₂ relaxation of the rock. We also found that the drainage pattern controls the irreducible water saturation attained in each rock type.