Module 3
Upstream & Gathering Systems
Everything in the network starts at a hole in the ground. Before any gas or oil can be sold or shipped, it has to be coaxed up the well, split into its parts, and funneled out of the field — the first time many wells become one network.
What you'll be able to do
- Describe what a well is and what the wellhead and Christmas tree do.
- Explain why produced fluid arrives at surface as a three-phase mix, not pure gas or oil.
- Walk through how a three-phase separator splits gas, oil, and water — and why outlet position matters.
- Define a gathering system and explain why it's the network's "first mile" in reverse.
- Reason about separator pressure and gathering pressure, including where the numbers genuinely vary.
From a single well to the field's first trunk line — the upstream chain at a glance.
The well and the wellhead
A well is a bore drilled from the surface down to the reservoir, lined with steel casing and locked in place with cement. Completion is the step that prepares it to actually flow — perforating the casing, running production tubing, and (in tight rock) fracturing to open up flow paths.
We keep the drilling detail brief on purpose. For a network course, what matters is simple: what comes up the well, and at what pressure.
🧭 The big idea
A well is just a controlled path from the reservoir to the surface. The whole upstream system exists to receive whatever that path delivers — and tame its pressure and its mix.
Sitting on top of the well is the wellhead: the surface assembly that seals the well and controls flow. The stack of valves and fittings bolted on top of it is nicknamed the Christmas tree, after its branching shape. It's the well's control panel — letting operators throttle, isolate, and monitor what comes up.
The mix rises up the cased bore, the wellhead seals it, and the Christmas tree's valves steer it out toward separation.
Why it comes up as a mix
Reservoirs don't store tidy, pure products. They hold hydrocarbons and water packed together in rock. So when a well flows, what actually reaches the surface is almost never pure — it's a multiphase mixture.
🥤 A concrete picture
Think of a shaken bottle of oil-and-vinegar dressing with some fizz on top: bubbles (gas), oil floating in the middle, and watery brine sinking to the bottom. The well delivers all three at once — jumbled together.
That brine is often very salty, and can carry sand. None of the three can be sold or piped efficiently while they're tangled together, so the very next job in the field is to pull them apart.
The three-phase separator
A separator is a pressure vessel that uses two free tools — gravity and residence time (letting the fluid sit long enough to settle) — to split the stream. A three-phase separator does the full split, gas / oil / water, inside one vessel.
- Gas rises and exits the top — passing through a mist extractor that knocks liquid droplets back down.
- Oil floats on water in the middle — and leaves through an oil outlet partway up.
- Water settles to the bottom — the heaviest phase, drawn off through a water outlet at the floor.
🧭 Outlet order = density order
The three outlets are stacked by weight: gas out the top, oil out the middle, water out the bottom. If you remember nothing else, remember the stack — lightest leaves highest.
🔢 Pressure & size
Gas-well separators often run at ~1,000 psi or more, and are kept physically
small (often ~2 ft diameter) precisely so the vessel can safely contain that
pressure. They can be horizontal or vertical and sit right near the wellhead.
You'll meet the live version of this split in the widget just below — change the inlet mix and watch the three outlet streams respond. (The pressure slider is operating context: it sets how the vessel is built, not how the stream divides.)
🧪 Try this
Push gas toward 100%: the gas band swells to fill the vessel and gas's share of the outlet streams grows, while oil and water shrink to slivers. That's a gas well. Now do the reverse for an oily, watery well — the layers flip. (The three mix sliders are normalized to 100%, so they need not add up exactly.)
Gathering systems — the first network
Gathering lines are the small-diameter pipelines that collect production from many individual wells and carry it to a central point — a gas processing plant, or a tank battery / trunk line for oil. They historically run at low-to-moderate pressure and are smaller than transmission pipe.
🔁 The last mile, in reverse
A distribution network fans out from one source to many homes. A gathering system does the opposite: many small feeders aggregate in to one trunk. Same tree shape — the arrows just point the other way.
Many-to-one aggregation. Lots of low-pressure feeders collect at junctions and merge into one trunk — the field's first true network.
⚠️ Don't assume "gathering = low pressure"
That's the historical default, but modern shale gathering can run at much higher pressure, sometimes approaching transmission levels. varies The range is wide — always check the specific system rather than assuming.
| Quantity | Value | Note |
|---|---|---|
| Gas-well separator pressure | ~1,000 psi+ | kept small to contain it |
| Gas-well separator diameter | ~2 ft | typical, near wellhead |
| Separator phases | 3 | gas / oil / water |
| Gathering pressure | low → high | varies historically low; shale can be high |
ℹ️ A note on oversight
Gathering lines have historically been more lightly regulated than transmission, but the U.S. PHMSA (Pipeline and Hazardous Materials Safety Administration) has expanded oversight of large gathering lines.
Key takeaways
- A well is a drilled, cased, cemented bore; completion preps it to flow. For networks, focus on what comes up and at what pressure.
- The wellhead seals and controls the well; the valve stack on top is the Christmas tree.
- Produced fluid is a three-phase mix: gas + oil/condensate + produced water (brine).
- A three-phase separator uses gravity + residence time: gas out the top (via mist extractor), oil the middle, water the bottom.
- Gas-well separators often run ~1,000 psi+ and stay physically small to contain it.
- Gathering is many-to-one aggregation into a trunk — the "first network." Pressure is historically low but varies (shale can run high).
What is the "Christmas tree" on a well?
Why: The Christmas tree is the branching valve assembly on the wellhead — it lets operators throttle, isolate, and monitor the well.
In a three-phase separator, which outlet is at the top of the vessel?
Why: Outlets stack by density: gas (lightest) out the top, oil in the middle, water (heaviest) at the bottom.
Roughly what pressure do gas-well separators often operate at?
Why: Gas-well separators commonly run at ~1,000 psi or more, and are kept physically small (~2 ft diameter) to safely contain that pressure.
Why is a gathering system described as the "last mile in reverse"?
Why: Distribution fans out from one source to many homes; gathering does the opposite — many wells aggregate into a trunk. Note that gathering pressure varies and modern shale systems can run high.