Near-Field Seismic Scattering Explains the Variation of DAS Amplitudes Inside a Leakage of Supercritical CO2

Abstract Distributed acoustic sensors (DAS) deployed in the boreholes enable high-resolution monitoring of subsurface changes. Typically seismic monitoring uses DAS amplitudes of reflected and transmitted seismic waves recorded above and below the geological objects that undergo changes, targets. We develop a monitoring framework relying on the DAS amplitudes inside the targets. The underlying model considers the anomalies as near-field scattering of seismic waves. We applied the proposed method to a field data set, in which five deep boreholes instrumented with DAS monitored the movement of a small injection of supercritical CO2. We accurately matched the DAS anomalies for diverse types of seismic signals: controlled high-frequency seismic sources, regional earthquakes, and oceanic microseisms from remote storms and local winds. For low-frequency signals, the microseisms and remote earthquakes, the strain-rate measurements may track the evolution of the reservoir changes providing a single-hole source-free monitoring tool. Our study lays the groundwork for a joint inversion of the DAS data, in situ, and reflected and transmitted signals. Plain Language Summary: Boreholes provide the only means for direct observations of the subsurface changes associated with injections and extractions of fluids, such as geological carbon sequestration, energy storage, and petroleum production. Our study shows that boreholes instrumented with fiber-optic seismic sensors can track very subtle reservoir changes associated with these operations. Because the fiber-optic installation is cheap and permanent, we can continuously monitor the reservoir conditions to optimize the fluid injection/extraction and ensure safety. Unlike the previous seismic techniques, we use the amplitudes of seismic signals inside the reservoir to quantitatively estimate reservoir parameters. We show that our framework is both accurate and practical using the data recorded for a small injection of supercritical gas. We showed that the noise produced by remote storms and local winds, regional earthquakes, and man-controlled seismic sources obey the proposed methodology. Therefore, our technique provides a cheap, single-hole, source-free monitoring tool.