Abstract
Different from the long-term and low-speed depletion of conventional gas fields or monotonous gas injection in carbon dioxide geological sequestration, the engineering background of multi-cycle intensive injection and production of underground gas storage (UGS) makes the dynamic sealing capacity and stability of the trap cap and pipe string under alternating load become the key problem that cannot be ignored. Firstly, the basic mechanical parameters of sandy mudstone and G-grade oil well cement were determined by conventional uniaxial tests, then, different upper load levels and alternating times were set to carry out alternating mechanical tests for sandy mudstone and G-grade oil well cement stone. The deformation, failure and stress-strain curve characteristics of the two types of rocks were analyzed, and the variation laws of the strength and elastic modulus with the upper stress and the alternating times of the two types of rocks were obtained: The strength of cement stone decreases, while that of sandy mudstone increases with the increase of upper limit stress and alternating times, and the reasons for the different laws of the two types of rocks were further analyzed. Then, after undergoing different alternating times with different upper load levels, the dynamic breakout pressure tests of two types of rocks were carried out by the step-by-step method. The results show that the breakthrough pressure of the two types of rocks decreased gradually and significantly with the increase of the alternating times. The dynamic evolution prediction model of the breakthrough pressure with the alternating times was established. And because the breakthrough pressure of the cement stone in the nondestructive state exceeds the measuring range of the equipment, the initial breakthrough pressure of the cement rock was also predicted by the prediction model of the breakthrough pressure. All of these results above can be significant reference value for the sealing evaluation of gas storage site selection and the real-time monitoring after it is put into operation.
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