Detecting Supercritical CO2 in Brine at Sequestration Pressure with an Optical Fiber Sensor

Monitoring of sequestered carbon is essential to establishing the environmental safety and the efficacy of geological carbon sequestration. Sequestration in saline aquifers requires the detection of supercritical CO2 and CO2-saturated brine as distinct from the native reservoir brine. Here we demonstrate an all-optical approach to detect both supercritical CO2, and saturated brine under sequestration conditions. The method employs a long-period grating written on an optical fiber with a resonance wavelength that is sensitive to local refractive index within a pressure- and temperature-controlled apparatus at 40 degrees C and 1400 psi (9.65 MPa). The supercritical CO2 and brine are clearly distinguished by a wavelength shift of 1.149 nm (refractive index difference of 0.2371). The CO2-saturated brine is also detectable relative to brine, with a resonance wavelength shift of 0.192 nm (refractive index difference of 0.0396). Importantly, these findings indicate the potential for distributed, all-optical monitoring of CO2 sequestration in saline aquifers.