Experimental Study on Failure Mechanism of Cement Sheath in Injection Production Well of Gas Storage

Gas injection and gas production from a gas storage well and pressure accumulation and unloading of the annular space of the well result in repeated loading and unloading of the wellbore, leading to the failure of the cement sheath, or even gas invasion from formation into the well. To study the failure mechanisms of the cement sheath of a gas injector (also acted as a gas producer), a laboratory experiment was conducted in which the cement sheath was subjected to the action of internal circulation pressure of 8.1 MPa and 13.6 MPa, according to the work conditions of the cement sheath in the upper part of the annular space B of a gas storage. The cement sheath was studied for the change of micro fractures and pores in it by analyzing its sealing property and permeability as well as SEM and CT analyses. In the experiment, there was no gas channeling through the cement sheath after 30 times of circulation at 8.1 MPa; when the circulation pressure was increased to 13.6 MPa, 23 times of gas channeling were found in 30 times of circulation. This experimental result is in accordance with the situation of shallow gas channeling into the annular space B. Cyclic loading may result in micro fractures in the cement sheath, and with the increase in the amplitude of the cyclic load, the permeability of the sheath sample increased at a small amplitude. The more the number of the micro fractures in the cement sheath, the wider and longer the micro fractures extended, and some micro fractures even ran through the whole sheath. Micro-annulus path around the cement sheath were resulted from the accumulation of failures of the micro structure of the cement sheath. For a well with high quality cementing job, the failure of the microstructure of the cement sheath is essentially the cause of gas channeling. Microstructure failure in a cement sheath and micro-annulus path around the cement sheath both can directly result in gas channeling, though the dominant cause of the gas channeling is affected by many factors. The experimental results have revealed the mechanisms of failure of the cement sheath under cyclic loading in a gas injection and producing well and the relevance between the cyclic loading and gas channeling, and are of instructive significance on the optimization of gas storage operation and quality control of the wellbore of a new gas storage.