The best source for natural gas properties is the American Gas Association Report 8 (AGA8). I have posted a free Excel AddIn that provides such properties in a spreadsheet. The composition of natural gas varies considerably depending on the source so the JT coefficient varies with mole fractions of the constituents as well as conditions. I also have a spreadsheet you can download from the web that calculates whether condensation will occur or not, which is what took out much of the gas supply in Texas during the recent cold snap. Compressor stations don't like condensation!