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Adaptability evaluation of reservoirs needs to be performed before natural gas flooding. However, the current evaluation methods consider the reservoir pressure and minimum miscible pressure (MMP) as separate parameters to evaluate the miscibility, which ignores the correlation between reservoir pressure and MMP. Also, the effect of reservoir pressure on injectivity is also neglected, while higher pressure than MMP may not improve the miscibility but increase the difficulty of gas injection. Furthermore, evaluation range of each index is inconsistent. Therefore, this paper proposes an improved evaluation system by introducing the miscible ability and optimizing the evaluation criteria to better evaluate the adaptability of natural gas flooding in enhancing oil recovery. The miscible ability is newly defined to evaluate the impact of reservoir pressure on miscibility, and multi-level evaluation model is further developed which includes 12 indexes based on reservoir/fluid/rock properties. The evaluation criteria are established by optimizing parameter intervals. The adaptability of BQ Oil field is assessed using Analytic Hierarchy Process (AHP) and fuzzy evaluation. Field application validates the reliability of the proposed evaluation system. It can be used to quickly evaluate the adaptability of natural gas flooding, which could improve success rate of natural gas flooding and reduce the costs. • An improved system for adaptability evaluation of natural gas flooding is proposed. • Miscible ability is defined to evaluate the impact of pressure on miscibility. • Multi-level evaluation model composed of 12 indexes is developed. • Improved evaluation system can identify reservoirs suitable for natural gas flooding. [ABSTRACT FROM AUTHOR]

Underground gas storage (UGS) plays an important role in large-scale energy (natural gas) storage, while salt deposition may affect the storage and production capacity of UGS. However, salt precipitation mechanism of UGS under movable water conditions is not clear. This work aims at understanding salt precipitation on rock properties of an UGS in porous media under movable water conditions. Firstly, core flooding experiments are performed to establish the relationship between dynamic petrophysical properties and the amount of salt crystals. The dynamic process of salt precipitation at the pore-scale is further captured by microscopic visualization experiments, and the location and morphology of salt crystals at micro-scale are also observed using scan electron microscope (SEM). Results indicate that formation-water evaporation and salt precipitation improve the formation properties when the amount of deposited salt is less than 39 mg/g. After the amount of deposited salt is more than 48 mg/g, formation porosity and permeability can be reduced. The size of salt crystals under movable water (>40 μm) is much larger than that under irreducible water conditions (1 μm–5 μm). This work offers a methodology for analyzing the effect of salt precipitation on the UGS in porous media under movable water conditions. • Salt precipitation in porous media under movable water conditions are investigated in the UGS. • Macro/micro experiments are performed to simulate multi-cycle gas injection and production of the UGS. • Relationship between dynamic formation properties and the amount of salt crystals is determined. • The location and morphology of salt crystals at micro-scale are analyzed. [ABSTRACT FROM AUTHOR]