Posted on: 10. 23. 26
Gas Compositions Changing Over Time
Natural gas from a wellhead will not have a constant composition throughout the life of the well. There will be varying levels of BTU content, NGL content, water vapor content and heavier hydrocarbons. So the gas coming from a well today may be completely different than what it was when first producing.
When designing most surface equipment for field applications they are designed for one case of gas composition. When gas composition varies from your design case you will see issues throughout your surface equipment. Not meeting design parameters can cause loss of liquids, temperature issues, tripping of equipment and excessive wear on mechanical components. Its important to know why gas composition varies and what effects it can have on your surface equipment.
Why does gas composition vary?
As the well depletes, wells on a pad are commingled, weather changes and other factors.
One of the biggest reasons gas composition varies is from depletion of the well. When a well is first flushed it will have a higher bottom hole pressure which forces lighter hydrocarbons such as methane to the surface. As the pressure in the well decreases throughout the life of the well you will start to see more NGL’s come to the surface. Depending on your well it can either increase or decrease the amount of rich gas coming from the well.
When multiple wells or different zones are being commingled into one manifold you will continue to see gas composition vary. New wells being added to a manifold or even a well changing artificial lift can change the gas composition coming to your surface equipment. Another factor that can change your gas composition is weather. When it gets cold in the winter months you will start to see liquid fall out of your gathering lines. During the summer your rich gas will make it to your surface equipment.
What Effects Does Gas Composition Have on Surface Equipment?
There are many ways that gas composition can affect your surface equipment.
JT Skids and Fuel Gas Conditioners
When gas composition changes on your JT skids you can see temperature changes throughout your equipment. JT skids are used to lower the temperature of your gas stream by dropping pressure. If your gas goes from rich to lean you will not see as big of a temperature drop on your JT skid. Which will cause less liquid to fall out of your gas stream. If your gas composition increases to be richer than what your skid was designed for you can see too much liquid fall out. Causing your heat exchanger to not meet design temperatures and over flow your liquid drums. If you want to learn more about how your fuel gas conditioner works check out our fuel gas conditioner crash course.
Separators and Filter Separators
Gas composition can affect your separators by increasing liquid load. When you have a sudden change in your NGL content your separators can become overloaded. Once your separators become overloaded with liquid you can start to see liquid carry over. Liquid will travel through your line and cause damage to downstream equipment.
Gas Dehydrators
As gas composition changes your water vapor content will change. You will need to run more glycol and heat to dehydrate your gas. If your gas dehydrator is not able to keep up with your water content you can start to see your sales gas go over pipeline specs.
Compressors
Gas composition can change the molecular weight of your gas. When your molecular weight increases your compressors will have to work harder to produce the same amount of gas. This can cause your engine or motor to overload.
Engines
Your engine can also be affected by gas composition. If your fuel gas is too rich your engine can start to knock. If you want to learn more about what happens when your fuel gas gets too lean check out our article.
What are some signs my equipment is not performing due to gas composition?
You may start to see your liquids recovery change, liquid carry over, engine knocking and having to manually adjust controls.
* You may see a change in your NGL recovery without any change in your gas pressure and volume.
* You can start to see your filters get dirty quicker or liquid in your sales gas.
* Your engine can start to knock or trip due to high exhaust temperatures.
* You will use more chemicals on your gas dehydrators to meet specs.
* You may have to manually open and close valves to keep your equipment running.
How to Design Your Equipment for Changing Gas Compositions
There are several ways to design your equipment for changing gas compositions.
Designing your equipment for one gas analysis can be one of the biggest mistakes you can make. Make sure your equipment has a wide range of turndown to allow for gas composition changes. Using automated controls and modular equipment will allow you to adjust to changing gas compositions. Check out our article on designing your skids for turndown.
Taking gas samples and knowing your gas composition will allow you to make adjustments to your equipment. You can change how much glycol you are running, adjust your compressor pressures and change chokes on your JT skid. Along with taking gas samples make sure you have a preventive maintenance plan in place for your equipment.
Standard vs Gas Composition Changing Design Case
| Standard Design Case | Gas Composition Changing Design Case |
| Lower Turndown (10% – 15%) | Higher Turndown (50%+) |
| Manually Adjusted Chokes and Valves | Automatically Adjusted Valves |
| Will have limits to how much liquid you can process | Can handle higher amounts of liquid |
| Fuel gas taken directly from wellhead | Fuel gas treated on a separate skid |
| Higher chance of tripping equipment | Less chance of equipment tripping |
Work With Pro-Gas LLC
When dealing with gas composition changes you want to make sure your equipment can handle it. Pro-Gas LLC has been designing and building equipment for over 90 years combined. We can help you design your next project to handle any gas composition that may come your way.
At Pro-Gas LLC we like to stay on top of our customers needs. We treat our equipment as if it were our own and monitor for any changes in performance. Check out our services to see what we can do for you or request a quote.
FAQ
Why does gas composition change?
As a well depletes your gas composition will change. When new wells are brought online or multiple zones are being commingled into one manifold your gas composition will change. Weather can also play a factor in your gas composition changing.
How does rich fuel gas affect my engine?
Rich fuel gas will cause your engine to knock. When your engine knocks it can cause your engine to overheat. Which can lead to piston failure, valve failure and tripping your engine.
What equipment can help with changing gas compositions?
Skid mounted equipment such as JT skids with automatic chokes, filter separators and fuel gas conditioners can help process changing gas compositions.
