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 CO(2) and CO(2)-saturated brine as distinct from the native reservoir brine. Here we demonstrate an all-optical approach to detect both supercritical CO(2), 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 °C and 1400 psi (9.65 MPa). The supercritical CO(2) and brine are clearly distinguished by a wavelength shift of 1.149 nm (refractive index difference of 0.2371). The CO(2)-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 CO(2) sequestration in saline aquifers.

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Long period gratings (LPGs) are coated with hafnium oxide using plasma-enhanced atomic layer deposition (PEALD) to increase the sensitivity of these devices to the refractive index of the surrounding medium. PEALD allows deposition at low temperatures which reduces thermal degradation of UV-written LPGs. Depositions targeting three different coating thicknesses are investigated: 30 nm, 50 nm and 70 nm. Coating thickness measurements taken by scanning electron microscopy of the optical fibers confirm deposition of uniform coatings. The performance of the coated LPGs shows that deposition of hafnium oxide on LPGs induces two-step transition behavior of the cladding modes.