Whats the difference between dry and wet gas?

When reading about shale plays, I keep coming across the concept of wet gas, and that this is good and more economic to develop. I know that the difference is down to chemical composition, not moisture, is that right?

Surely natural gas prices are relatively low, and whether the gas is wet or dry shouldn't make much difference in the scheme of things? Is wet gas easier to extract, is this the reason?

Comments

  • edited March 2018

    Natural gas, when it comes out of the well is a mixture of different hydrocarbons in varying proportions, and also has water, carbon dioxide and other 'waste' materials. The average person might think of wet gas having water (H2O) in it, and they'd be correct to an extent, since the raw output will have water, and wet gas is a term that refers to this initial production flow. H2O is only one of the 'wet' parts in this definition.

    We'll move on to specific answers to your questions, but it's worth noting that water is one of the first things that needs to be processed out. This is because water vapour will condense and will lead to icing up of the gas in pipelines due to weather conditions and/or high pressure. When ice forms, transportation pipes and valves can clog. Also, at the processing plant, some of the wet gases are separated by chilling below the freezing point of water, so water needs to be removed before any other 'wet gases' are separated.

    Other waste products that need to be processed out include chemicals that are hazardous to the environment and could cause an environmental penalty. An example of a hazardous chemical present in natural gas is hydrogen sulphide which is very poisonous.

    The other reason why gas can be wet

    This answer is more relevant to your question than the discussion of water vapour. Wet gas has higher levels of natural gas liquids (NGLs) and condensates. In the oil and gas industry, wet gas is often used to describe the raw unprocessed gas, but some gas is 'wetter' than others.

    When there is a high level of methane, the gas is considered dry, even if it has just been extracted from the well. Many conventional wells naturally produce dry gas that needs little processing. When we refer to dry gas, we mainly mean methane, which is used to heat homes, cook, and power some vehicles.

    If there is a significant amount of butane, ethane, pentane, liquid petroleum/gasolene or other higher hydrocarbons then the gas is termed 'wet'. These are NGLs or condensates.

    From the standpoint of BTUs, dry gas is below 1050. Wet gas is above 1050 BTUs with anything over 1350 being termed as 'super rich'.

    What about the financial aspects of the different hydrocarbons?

    NGLs and condensates are more economically valuable than methane, and when we see reserves described as BOE (barrel of oil equivalent) then this is what is being referred to. The prices of BOEs tend to track oil and are therefore more valuable.

    How wet gas is processed

    Raw unprocessed gas is sent through a pipeline to a processing plant. First, all the liquids need to be removed from the natural gas. Then the liquids are separated into their different components.

    The processing techniques vary. To separate the lighter hydrocarbons like ethane, the refiners chill the natural gas. Lighter NGLs and condensates turn into liquid and can be drained away from the methane. In order to keep methane in a gaseous state, but liquify the rest, the gas is cooled to -140 degrees fahrenheit/ -95 degrees celsius.

    Fractional distillation (or fractionation) is used to separate butane, propane and other heavier hydrocarbons. This is a similar principle to distilling alcoholic spirits as different liquids evaporate and 'boil off' at different temperatures.

    The transportation and processing stage is known as the 'midstream' sector of the oil and gas industry.

    Why not use wet gas for industrial applications?

    The reason why natural gas is processed and not used raw, is because wet gas will ruin your equipment and cause corrosion. Wet gas would probably work in an oven or vehicle, but would lead to mechanical failure over time. It's far better to separate out all the different hydrocarbons and use them in the correct tried and tested situations.

    Whats the difference between LNG and NGL?

    Its worth mentioning that liquid natural gas (LNG), and natural gas liquids (NGL) are different things. LNG is fully processed methane gas that is under very high pressure which causes it to turn into a liquid. Natural gas liquids are the wet gas or condensates that turn to liquid simply by chilling them.

    What is the connection between shale/unconventional extraction and wet gas?

    Conventional gas reservoirs nearly always produce drier gas. This is because we're tapping into a reservoir that is under great pressure, and the dry, lighter methane has already risen to the top, above oil, and condensates.

    With unconventional techniques such as fracking or multilateral drilling, we're not necessarily tapping into the top of a reservoir, and different parts of a reservoir can be joined together during drilling and fracturing.

    This is why shale wells tend to produce wetter gas. In the Eagle Ford, raw gas extracted is roughly 1/3 dry, 1/3 NGLs and 1/3 condensate.

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