Enhanced subsea leakage detection with broadband active acoustic sensor system

Abstract

For field re-activation, improved/enhanced oil recovery through injection of gas in reservoirs, and geological storage of CO2 (carbon dioxide), robust barriers must be provided and confirmed to avoid leakage. Natural seeps from the sea floor represents an additional challenge, as many detection systems will misinterpret them as an infrastructure leakage. Active acoustics (backscattering) has been successfully used for detection of even moderate underwater hydrocarbon gas releases at long ranges, using, e.g., scientific echosounders. However, CO2 leakage behavior is particularly complex and operations in cold and deep waters make such leakages difficult to detect with active acoustics, due to its behavior and low acoustic impedance in liquid phase. Methodology for detecting CO2 even in liquid phase is a prerequisite for many deep water operations, and at present no adequate technology exists for this purpose. In order to design an active acoustic system that can detect subsea leakage of hydrocarbons and CO2 in liquid phase, the behavior and backscattering by droplets as a function of environmental parameters and acoustic frequency is simulated. Theoretical backscattering is further compared with in situ acoustic and optical measurements of CO2, CH4 (methane), and air, released and measured at depths from 1300m to surface, using a custom built gas release and measurement frame.