Prediction model for critical drawdown pressure under multi-cycle injection-production in underground gas storage based on experiments

Ensuring the safe operation of gas storage facilities requires precise management of the critical drawdown pressure (CDP). It is a key factor governing both productivity and sand control. The unique conditions of multi-cycle underground gas storage (UGS) subject the reservoir rocks to cyclical stress variations. The influence of cyclical stress can lead to the degradation of rock mechanical properties, thereby resulting in the evolving nature of the critical production pressure difference prediction model. Current research on the dynamic behavior of the CDP prediction model remains limited to numerical simulation approaches. However, there is a notable lack of experimental validation. To bridge this knowledge gap, laboratory experiments were conducted to develop a model capable of predicting the CDP under multi-cycle UGS injection-production conditions. This study first investigates the relationship between rock mechanical parameters and the number of cycles using the MNDC-2 experimental system and establishes correlations between these parameters and cycle number through experimental fitting. Subsequently, key parameters for the static CDP model—namely, Poisson’s ratio and Young’s modulus—are defined based on the Mohr–Coulomb criterion. Finally, a dynamic CDP prediction model for M gas storage is developed by integrating the static model with experimentally derived correlations. The model demonstrates the influence of the neglect of fatigue effects on CDP. The results show that in terms of rock mechanical properties, the triaxial compressive strength of the M Gas Storage decreased with increasing cycles due to the damage during the multi-cycle stress loading experiment. Within 100 cycles, the compressive strength of the core decreased by 26.8%; Poisson’s ratio of the core increased by 10.66%; and Young’s modulus of the core decreased by 19.22%. Moreover, due to the greater increase in rock fractures caused by cyclic loading during the early stages compared to the later stages, the changes in Young’s modulus and Poisson’s ratio were more pronounced in the initial phase of loading. In terms of the CDP, the dynamic critical drawdown pressure (DCDP) decreases at a slower rate compared to the static critical drawdown pressure (SCDP) as the number of cycles increases under constant formation pressure. Within 50 cycles, the maximum difference between the DCDP and SCDP reached 32.9%. In addition, under a constant number of cycles, the DCDP exhibits a slower decline rate compared to the SCDP as the formation pressure increases. Within 50 cycles, the maximum difference between the DCDP and SCDP reached 40.6%. This demonstrates the existence of differences between the DCDP and the SCDP. Therefore, the impact of cyclic effects on production must be considered during the cyclic injection and production process in gas storage.