POROSITY AND PERMEABILITY CHANGES DURING CYCLIC LOADING AND UNLOADING OF CARBONATE RESERVOIRS FOR GAS STORAGE APPLICATIONS

Underground gas storage is utilized to level seasonal variations in the supply and demand of natural gas, or hydrogen, and provide energy stability and security. In such projects, the reservoir undergoes cyclic injection, storage, and production of the gas. Cycles vary between days to years. Due to the cyclic nature of injection and production from the reservoir, it is important to understand the effect of cyclic loading and unloading, and the accompanying changes in stress, caused by pressurization and depressurization on rock porosity and permeability.
In this report, we have examined the effect of cyclicity on two Texas Cream carbonate samples. To simulate reservoir conditions, we hold the confining pressure constant while loading and unloading the pore pressure within the elastic limits. We ran several cycles for each sample and the porosity and permeability were recorded during the cycle. The results show a slight decrease in the porosity and permeability with cycling. Results also emphasize that mechanical rock damage needs to be considered with the cyclic nature of the gas storage operation as it leads to rock fatigue damage and deformation.
Most of the research in this area focuses on reservoir depletion or one cycle of loading and unloading. Therefore, these results highlight the importance of considering the changes in rock characterization with pore pressure cyclicity. The characterization of the reservoir rock porosity and permeability change is essential for modeling and to set guidelines for better reservoir management.