Wellhead gathering — cold separation and the hydrate check

A wet-gas wellhead through a JT choke, a cold separator and a gathering line — the flowsheet that gives HYSYS Upstream's own tools (well IPR, hydrates, water content, gas pipeline) something real to work on. All four are in the Analysis panel and none had a curated example.

From the flowsheet. 800 mol/s of wet gas lets down 120 to 60 bar across the choke, cooling itself to 294 K by Joule-Thomson alone; a chiller takes it to 250 K (2.6 MW) and the cold separator drops 88.5 mol/s of condensate (11%), sending 711.5 mol/s of sales gas down 25 km of 300 mm line for a 3.8 bar drop at 2.4 m/s.

Well deliverability (Analysis > Well IPR). Vogel's composite IPR from one test point (0.045 at 200 bar flowing bottomhole, against a 240 bar reservoir and a 180 bar bubble point) gives an absolute open flow of 0.18 and, at 120 bar flowing pressure, a deliverability of 0.125 — the curve that says whether the well can actually feed this train.

The hydrate check (Analysis > Hydrates, Water content). This is the pair that matters and the reason the two tools belong together. At 60 bar the gas is water-saturated at 21.2 lb/MMscf with free water present; Towler-Mokhatab puts the hydrate formation temperature at 291.2 K. Operating anywhere near seabed or winter temperature — 288 K, say — is 3.2 K inside the hydrate envelope, calling for about 20.5 wt% MEG to stay clear with a 3 K margin. A line that is hydraulically fine can still plug solid.

Honesty notes. Hydrate temperature is the Towler-Mokhatab gas-gravity correlation and the inhibitor dose is Hammerschmidt — both screening correlations, not a rigorous hydrate equilibrium model. Vogel's IPR is a reservoir-inflow correlation, not a reservoir simulation. The gathering line here is single-phase gas; a genuinely two-phase line is the pipeline op's Beggs-Brill branch, which needs a mixed inlet.