Pore structure changes and its stress-sensitive behavior in sandstone under cyclic stress: Implication for underground gas storage

The cyclic gas injection-extraction process during underground gas storage (UGS) operation may alter the pore structure and stress sensitivity in the reservoir rock, which can affect the storage capacity and operation efficiency of UGS. In this study, the pore structure alteration of sandstone from a UGS reservoir induced by a total of 50 cycles of confining stress loading-unloading was examined using the nuclear magnetic resonance (NMR), then the effect of the stress cycle numbers on the stress-sensitive behavior and compressibility of different types of pores in sandstone were also determined. At first confining stress loading-unloading, the proportion of macropores in the sandstone decreases, while micropores and mesopores increase. Changes in stress sensitivity and compressibility are more significant in macropores than that in mesopores and micropores. With increasing stress cycle numbers, the macropores in the sandstone are compressed and partially converted into micropores and mesopores, leading to a decrease in the proportion of macropores and an increase in micropores and mesopores. However, at the same stress, the porosity of sandstone gradually increases as the damage occurred and new pores formed in the sandstone with the increasing stress cycles. For each cycle, the porosity at loading is greater than that of unloading, and the recovery degree of porosity at unloading firstly increases (the 1st-10th cycles) and then gradually decreases (the 10th-50th cycles). The stress sensitivity and compressibility of macropores in the sandstone increase significantly, while micropores and mesopores increase slightly. The results indicated that the gas injectivity and storage capacity of UGS is dynamically controlled by the cyclic stress induced porosity and stress sensitivity alterations in reservoir.