Natural Gas Hydrates
Transcript
– Hydrates, and no, not that kind of hydrate. These kinds of hydrates are one of the challenges faced in the oil field when it comes to pipelines and equipment. Commonly found in low temperature and high pressure conditions, the developments of hydrates create flow assurance problems. Unlike a hydrating bottle of water, gas hydrates pose a concerned to safety and production. Don’t get me wrong. Hydration from a cool glass of water on a summer day is great. But in pipelines, well, you get the point.
Hydrates are white, solid, ice-like structures that form when pressures are really high and temperatures are low. A crystal lattice forms around a hydrocarbon molecule almost like a cage in this process. As you can see in the illustration provided, hydrocarbon molecules occupy void spaces within the lattice of water molecules. Natural gases with higher specific gravities form hydrates at lower pressures compared to those with lower specific gravities. The water molecule cage freezes at low temperatures and provides space for hydrocarbon molecules to reside. There are several types of lattice structures, one with void sizes large enough to accommodate molecules such as propane and methane, while the other can be occupied by small and medium-sized molecules such as methane and ethane. Methane is the most common trapped gas in hydrates comprising 99% of the trapped gas. Carbon dioxide, propane, and ethane can also form hydrates.
For hydrates to form, water and sea crystals must be present. They sea crystal provides as a base or a starter for hydrates to grow from. Research from professors at Stanford University tested the time it would take for hydrates to form after the introduction of a sea crystal to a water droplet in a chamber where temperature and pressures could be monitored and evaluated. By controlling the temperature and monitoring the pressure change during hydrate formation, information could be extracted about the crystal growth rate. The images shown were produced from the hydrate visualization cell.
Now I’m going to illustrate how to determine if hydrate-forming conditions exist. Here, you can see that I have drawn a simple pressure versus temperature diagram, and now I’m going to draw an arbitrary line that is representative of your specific gravity gas. If conditions are above the line, hydrates are able to form. However, if you are below the line, hydrate-forming conditions do not exist. Now, I’m going to walk through an example of how to determine if you have hydrate-forming conditions. For this problem set, we are going to say that we have a 0.7 specific gravity natural gas and is to be compressed to 500 PSIA and cooled to 50 degrees Fahrenheit.
The big question, is there a possibility of hydrate formation under these conditions? This line represents our specific gravity gas and now we are going to come in at 50 degrees Fahrenheit and 500 PSIA and make a dot. As you can see, we are above the specific gravity gas line, which is indicative of having hydrate formation. This diagram shows the hydrate portion of a phase diagram for a mixture of water and a light hydrocarbon. As you can see, high pressure and low temperature move you into the hydrate and ice formation zone. As temperatures increase, hydrate and water are now present. And if we increase our temperature and pressures even more, we are now in the hydrocarbon liquid and water generation zone. If we decrease our pressure and our temperature, ice and hydrocarbon gas can form. And if we increase our temperature and remain underneath our specific gravity gas line, water and hydrocarbon gas are able to form.
So we’ve already covered what hydrates are, how they form, and we’ve even looked at a couple pressure versus temperature diagrams. You might be wondering to yourself, well, how do we keep hydrates from forming? We’ve established that they’re not necessarily a good thing, so what steps do we take in the field to mitigate hydrates from forming?
The goal is to keep your natural gas out of the hydrate formation zone. If you think about the pressure versus temperature diagrams that we discussed earlier, we want conditions to be under the specific gravity line. This can be done in several ways. One by heating the lines the gas is traveling through. A second, by dehydrating the gas and water molecules. And a third, by utilizing chemical inhibitors such as methanol to lower the hydrate formation temperature of the gas. All of these techniques have the goals of reducing safety risks from blocked lines and improving gas flow.