Most of the underground gas storage （UGS） facilities built in China are rebuilt from sandstone gas reservoirs. They are subjected to increased risks of sand production due to the cumulative alternating damage to the reservoir rock structure after multi-cycle alternating injection and production. In order to effectively predict sand production risks，an alternating mechanics experiment considering multi-parameter sensitivity was designed by combining the stress field of the borehole wall of the injection-production well and the shear failure criterion of rocks. Based on the alternating mechanics experiment，the degree of rock strength damage was calculated，and a sand production critical drawdown pressure model based on rock damage degree was proposed. Research showed that the stress field of the reservoir dynamically changes with the production of the gas storage，resulting in a change in the difference between the circumferential and radial stresses of the surrounding rock of the borehole wall of the injection-production well. At the same time，the stress difference also changes along the wellbore circumference. Under the alternating loads，the cumulative damage of rocks grows with the increasing water saturation （for the same particle sizes of rocks） and larger particle sizes of rocks （for the same water saturations of rocks）. The sand production critical drawdown pressure predicted using the proposed method is 11.41 MPa in the early stage of gas production and 3.97 MPa in the late stage. This study provides a new method for determining experimental parameters in alternating mechanics，as well as a new method for judging sand production in alternating injection and production for UGS facilities to guide the UGS production planning and control.

The evaluation method of storage capacity expansion and design production realization is key to achieving efficient underground gas storage （UGS） operation and predicting the future UGS state. In order to understand the operational rules of UGS storage capacity expansion and design production realization，this paper investigates the J gas storage，a UGS of gas cap reservoir as an example. Based on tracking the basic UGS production data and analyzing the development performance of the gas reservoir，this paper analyzes the multi-cycle injection and production operation data of the UGS and evaluates the pressure，storage capacity，gas production capacity of wells and injection-production operation performance of the UGS，using the dynamic evaluation technology of UGS in gas fields. The main factors affecting the storage capacity expansion and design production realization of the J gas storage are clarified. It is suggested that the J gas storage is transitioning from the capacity expansion stage to the stable stage，but the subsequent storage capacity expansion is challenging，because the drainage well fails to drain normally with gas channeling in the reservoir. It is necessary to perform fine reservoir description of the UGS to clarify the reservoir sand body distribution pattern and identify gas channels. Meanwhile，numerical simulation shall be done to develop a reasonable injection-production scheme to avoid impacts of gas channeling in drainage wells and resume storage capacity expansion. A method for evaluating the UGS capacity expansion and production realization has been developed，which provides technical support and references for the construction of similar UGS facilities and realization of UGS storage and production capacity targets.

Key words：gas storage in gas cap reservoir；storage capacity；working gas volume；injection-production

The dynamic sealing performances of the caprock and fault are important evaluation indicators of underground gas storage （UGS） and are of great significance to the safe operation of the UGS. In view of the shortcomings of the static evaluation system of caprocks and faults for the UGS，this paper takes a carbonate gas storage in North China as an example，performs rock mechanical experiments based on well logs，and establishes the dynamic-static conversion model for rock mechanics parameters and a four-dimensional geomechanical UGS model. Based on the developed models，the initial stress field of the UGS geological body is inverted，the pressure-bearing capacity of the UGS geological body is analyzed，and the four-dimensional dynamic stress field information under the UGS injection and production conditions are obtained. Four-dimensional geomechanical calculation results show that the risk of caprock and fault failure is low under the current upper limit pressure （47 MPa）. The caprock and faults will not fail even if the upper limit pressure of the UGS is increased to 49 MPa. As per this combined with the operating capacity of the surface compressor，the upper limit operating pressure of the UGS is determined to be 49 MPa. The establishment of the four-dimensional dynamic stress field provides a scientific basis for the further pressurization and capacity expansion of the UGS.