Subsea Reinjection of Methane-Rich Gas: System Design and Injection Pump Testing

Many oil and gas fields rely on gas injection for production support or disposal. However, methane gas injection in an offshore development is energy inefficient, limits throughput capacity, and requires sizeable infrastructure. Hydrocarbon transportation and processing result in pressure losses, which must be compensated for by multiple stages of compression. Subsea reinjection of methane-rich gas is a novel solution that could alleviate the downsides of the traditional topside approach. Subsea processing technologies enable the full reinjection process to occur subsea, without the gas stream having to travel to an intermediate topside facility. The potential benefits of subsea gas reinjection are significant, and extensive study work and flow loop testing was performed to verify the technical feasibility of this approach. This paper discusses the selection of a subsea gas reinjection concept, system design, benefits, engineering challenges, and successful full-scale, low-density gas pump testing. Subsea reinjection of methane-rich gas leverages qualified subsea processing technologies using arrangements that are flexible depending on operating conditions. The subsea gas reinjection system design incorporates high-pressure subsea separation to optimize efficiency and liquid yield, while subsea boosting pressurizes the supercritical gas. A comprehensive test campaign validated the ability to pump supercritical gas through subsea single-phase pumps, even for low-density gas applications. With subsea gas reinjection, system efficiency is increased because pressure losses are minimized, emissions are decreased, safety is improved, and flowline and host facility infrastructure can be reduced. Economics are optimized through facility debottlenecking and accelerated production. Subsea gas reinjection is a novel, promising concept for offshore oil and gas projects. It uniquely repurposes existing technology to improve system performance while decreasing costs, greenhouse gas (GHG) emissions, and complexity. Although system designs for the subsea injection of carbon dioxide (CO2)-rich gas exist, the boosting of low-density methane-rich gas for injection is unprecedented.