In an Offshore magazine article, Bringing greater control to hydrate management, Emerson’s Jan Olaf Gomnæs describes the issues some offshore natural gas producers face with the formation of hydrates in the producing wells. Jan describes these hydrates as:
…crystals that form in high-pressure and low-temperature flows where water and natural gas are present.
Given the right flow, temperature, and well production composition conditions, these crystals can lead to blockages. Jan highlights the process:
As the natural gas becomes encased in a lattice of ice formed from the water, the danger of blockages in tubing, flow lines, and pipelines increases.
To prevent this hydrate formation from impacting production, operators need to understand when conditions are favorable for crystal growth. They also need the tools to combat this hydrate formation when the conditions are present. These conditions can include water breakthrough, or the point where water trapped in the reservoir joins with the natural gas from the reservoir in the producing well.
Jan notes two types of hydrate inhibitors, “methanol and ethylene glycol (MEG) and low-dose hydrate inhibitors (LDHIs)…”
Roxar technology plays a key role in the management of these hydrate inhibitors. Jan describes subsea wet gas meters that provide operators with the gas and condensate flow rates needed to determine hydrate formation conditions. These devices:
…can detect changes in the water production with sensitivity better than +0.005% volume, while the absolute accuracy is +0.1% volume in high Gas Void Fraction (GVF) (greater than 98.5%) cases.
Early detection of these conditions provides operators with an opportunity to use MEG or LDHIs to inhibit hydrate formation. Subsea chemical injection valves are required for the hydrate inhibitors. Getting the correct amount of inhibitor injected in critical. Too little, and it can contribute to hydrate formation. Too much can be expensive.
Jan explains the need behind high-flow subsea chemical injection valves:
…to increase flexibility to adjust the flow, in contrast to a needle valve with narrow flow path and high flow rates. This is achieved by a giving a longer flow path but the same pressure drop. The result is a wider path that allows larger particles to flow through the valve, a lower fluid velocity, and low instances of valve erosion.
Flow measurement and control is not an easy proposition in an environment that can include sand particles from the reservoir and tremendous pressures and temperatures from the water depths in which these devices may operate. Reliability is a critical part of the design, since remotely operated underwater vehicles (ROVs) are used to install and replace the meters and valves. Minimizing moving parts, using erosion-resistant ceramics, and avoiding the need for special installation and removal tools were a few of the design considerations highlighted by Jan.
Even if you’re not in the offshore oil & gas production industry, you might find the challenges your counterparts face to be an interesting read.