Monitoring Subsea CCS: Following the CO2Molecules From Top to Bottom and Ensuring They Stay in the Rock

Abstract

Abstract This paper evaluates technologies and available methods for the surveillance of flow in subsea CO2 injection systems, going from the subsea infrastructure to the injection wells, into the reservoir, and then back to the seafloor. Considerations and provisions required in the subsea system to accommodate certain surveillance technologies are highlighted. In this paper, the potential path taken by CO2 molecules are followed through a subsea storage system, with leak detection and plume surveillance in mind. Feasibility and opportunities for using monitoring methods and leak detection technologies were assessed by utilizing an assessment matrix which summarises the various advantages and disadvantages of plume monitoring technologies and leak detection methods. The following aspects were assessed for each technology: 1. Technical feasibility; 2. Technology Readiness Level (TRL) and development required; 3. Impact of the solution on the subsea system (e.g. field architecture, power and communication); 4. Relative costs; 5. Schedule (e.g. added risk due to development required). Leak detection methods having unknown efficacy and limited research in CO2 applications were screened out. Those methods which have been field tested for CO2 service or currently under lab testing were advanced to a second stage of screening by product specifications to further consider their integration into the subsea injection system design. Passive acoustic technology was found to be the best suited for detection across subsea infrastructure as it covers a desirable detection radius at the lowest cost. Screening of plume monitoring methods showed benefits and drawbacks of each method evaluated; the review concluded that different technologies should be used at different stages of a subsea CCS project. Near well bore, CO2 migration monitoring can be performed using Vertical Seismic Profiling (VSP) as it yields a high-resolution image in the vicinity of the well. When the plume eventually outgrows VSP's radius of investigation, a sparse acquisition strategy can later be used to verify conformance of reservoir model's plume migration at strategic locations. Finally, leak detection tools such as an AUV with side scan sonar and a chemically degraded trigger and beacon could be used for leakage from the reservoir through the seabed and legacy wells respectively. This paper provides a novel integrated view of CO2 escape paths, bringing together topics around CO2 leak detection from subsea injection equipment, sub-surface plume monitoring and leakage from reservoir through the seabed. The results of this research can benefit CCS projects in the concept phase influencing the design of subsea infrastructure to accommodate monitoring methods, thus guiding technology and risk assessments to support a Measurement, Monitoring and Verification (MMV) plan.