Geomechanical Monitoring of Caprock and Wellbore Integrity Using Fiber Optic Cable: Strain Measurement from the Fluid Injection and Extraction Field Tests

This paper presents a new method to monitor caprock and wellbore integrity at CO2 storage sites by utilizing the Distributed Fiber Optic Sensing (DFOS). DFOS has an advantage to measure temperature and strain at any point in an unprocessed optical fiber, contrary to the conventional Fiber Bragg Grating (FBG) sensing which measures temperature and strain at a limited number of discrete points along the processed fiber cable. To put the DFOS technique into the practical use at the CO2 storage sites, we measured the frequency shifts of the Rayleigh and Brillouin scattering in an optical fiber attached to a sandstone sample under hydrostatic pressure, and also measured strain of the sample by conventional strain gages simultaneously. We applied the DFOS technique to our filed experiments when injecting and extracting fluid from a shallow well. The shallow well was drilled to a depth of 300m and three optical fiber cables were installed behind well casing for detecting impacted zones along the well depth direction. With this permanent installation behind casing, we successfully detected the deformed zone during the fluid injection and extraction tests. The field results suggest that the DFOS technique can be used in geomechanical monitoring of caprock and wellbore integrity by providing deformation details such as which depth and how much the target layer deformed in subsurface.