Although there is no absolute limit for the concentration of C6+ in natural gas, when it is present in significant concentrations it will generally range from the sum of iso-pentane and normal-pentane to approximately double that sum. However, there are some important factors to consider when identifying the C6+ concentration and avoiding a falsely high reading.
- What is the geographic location where the natural gas stream originated?
Natural gas wells from different regions will have varying amounts of hydrocarbon liquids. For instance, production from shale formations tends to contain more natural gas liquids (NGLs) than that from more traditional sources. The formation as well as the drilling/production methods should be taken into account when determining the C6+ content.
- What is the flowing temperature and pressure of the natural gas stream that was sampled?
Natural gas streams flowing at low pressures with average-to-high temperatures can show increased concentrations of NGLs. Conversely, at high pressures and low temperatures the C6+ concentration is generally lower due to the heavier hydrocarbons condensing as liquids.
- Was the sample obtained properly?
Poorly obtained samples can lead to liquids condensing on the sample cylinder walls. At times, depending upon sampling conditions and gas characteristics, sample cylinders must be heated before pulling a sample to prevent condensation.
- Was the sample cylinder cleaned properly before collection?
Improperly cleaned sample cylinders may contain residue from previous samples, thereby potentially contaminating the new sample and invalidating analytical results.
- Does the gas chromatograph used to analyze samples have a linear response over the entire concentration range being analyzed?
Improperly calibrated gas chromatographs that are linear for each component over the expected concentration ranges.
These pragmatic considerations, in combination with a database for comparing the historical concentrations of C6+ in any given natural gas stream over time, should help analysts determine if a sample is representative or if it should be carefully re-sampled for more accurate results. For more information on C6+ determinations and the considerations summarized above, contact an experienced commercial laboratory that specializes in natural gas and NGL analysis.
"…it seems that one can obtain more truths, important to Humanity,
from Chemistry than from any other Science."
~ Samuel Hahnemann, German Chemist and Physician