Analysis and preventive measures for the downhole damage of optical fiber based optical fiber oil-water interface instrument

The position of diesel-brine interface is a key factor in the solution mining process of salt-cavern underground gas storage. If the diesel-brine interface is not positioned accurately in the process of solution mining, the cavity shape and stability will be destroyed and even the cavity may be abandoned. Optical fiber based diesel-brine interface monitoring tool is currently the main means to measure diesel-brine interface. The on-site damage situations of optical fiber based diesel-brine interface monitoring tool in Jintan Salt-Cavern Underground Gas Storage were analyzed. It is shown that diesel-brine interface monitoring tools are damaged more than 1/3 well times in different degrees. The main reason for this phenomenon is that insoluble content is higher than 80% and the insoluble drops in the process of solution mining, which leads to string oscillation to damage the optical fiber. In order to reduce of damage frequency of optical fiber based diesel-brine interface monitoring tool, it is recommended to uplift the external pipe shoe for solution mining above the insoluble interbed and apply mechanical semi-protection measures to the optical fiber when there is an interbed with insoluble content higher than 80%. Half a year after these measures are tested and applied on site in 2 wells of Jintan Salt-Cavern Underground Gas Storage, no phenomenon of optical fiber damage is discovered. It shows that the preventive measures can effectively protect the optical cable.