Abstract: Constructing the underground gas storages in low-grade, multi-interbedded salt formations and using the pore space of the insoluble sediments at the bottom of the salt caverns excavated by solution mining to store natural gas are expected to greatly expand the storage capacity of the salt cavern. However, it is necessary to evaluate the feasibility and applicability of the insoluble sediments for gas storage. Firstly, large-size samples were prepared with insoluble sediments for computed tomography scanning, and the size and connectivity of the pores of the insoluble sediments were analyzed qualitatively based on the three-dimensional reconstruction models. Results demonstrated that the pores of the insoluble sediments have large size and adequate connectivity. Secondly, a self-developed experiment device of salt cavern insoluble sediments by solution mining was adopted to carry out the experiments of gas injection and brine displacement. The small insoluble particles and large-size interlayer blocks were mixed for preparing insoluble sediment samples, the porosities of which were measured before and after the experiments of gas injection and brine displacement, and the reasons for the change of porosity were analyzed. Finally, a prediction model of the pore volume of the insoluble sediments was established, and the theoretical equations of the mining space of salt cavern and the volume of the brine which can be discharged from the insoluble sediments were proposed. By considering the continuous accumulation of small insoluble particles and disordered accumulation of the large-size collapsed interlayer blocks, the mining space and drainable brine volume of the insoluble sediments were calculated, respectively, which were in good agreement with the field data of solution mining. This study can provide references for evaluating the volume of pore space of insoluble sediments for gas storage in low-grade, multi-interbedded salt formations.

Key words: salt cavern gas storage, multi-interbedded salt formation, insoluble sediments, porosity, volume capacity evaluation