Fiber refractometer to detect and distinguish carbon dioxide and methane leakage in the deep ocean

• We created a fiber tip refractometer for use in ocean sequestration.
• Sensor effectively distinguishes methane from carbon dioxide in seawater.
• Interferometric plots generated during the transition between species.
• Tests performed at 9.65 MPa, 20 °C.

Deep ocean injection of carbon dioxide into subsea geologic formations carries a risk of carbon dioxide leakage as well as leakage of methane from petroleum bearing formations that injection wells may penetrate. Therefore, leakage monitoring technology for this application must detect carbon dioxide and distinguish it from methane. Here we demonstrate an all-optical approach to detect and differentiate between liquid carbon dioxide and supercritical methane bubbles in synthetic seawater at 9.65 MPa. This method employs fiber tip refractometry, a fiber optic measurement technique that is sensitive to the refractive index of the surrounding medium. Carbon dioxide and methane bubbles are clearly detected as they pass the sensor tip and these species are clearly distinguished from each other. Interferometric signals are also observed in association with the transition of bubbles onto and off of the sensor tip.