Gathering System Definition / Meaning
A gathering system is the network of pipelines, valves, separators, meters, and other equipment that collects raw hydrocarbons (oil, natural gas, and produced water) from multiple wellheads and transports them to a central processing facility, storage tank, or transmission pipeline. It serves as the critical middle link between producing wells and downstream infrastructure, handling the initial aggregation and partial conditioning of the produced fluids. Gathering systems are custom-designed based on terrain, fluid properties, production rates, and regulatory requirements.
Overview
After a well is drilled and completed, the produced fluids emerge at the wellhead at high pressure. The gathering system begins immediately downstream of the wellhead equipment, typically with a flowline that carries the multiphase mixture to a central point. From there, the system may include manifolds to combine flows from several wells, separators to split oil, gas, and water, and meters to measure production volumes. The gathered fluids are then sent via larger trunk lines to a gas processing plant or oil battery for further treatment and sale.
Key Components
A typical gathering system comprises several equipment categories. The table below lists the major components and their functions.
| Component | Function |
|---|---|
| Flowline | Small-diameter pipe (2–8 inches) connecting each wellhead to a manifold or central point. |
| Manifold | Valved assembly that combines flow from multiple flowlines into a single larger pipe. |
| Separator | Vessel that uses gravity and pressure differences to separate gas, oil, and water. |
| Meter (LACT unit) | Measures oil and water volumes for custody transfer and allocation. |
| Valves & Regulators | Control flow, isolate sections, and regulate pressure within the system. |
| Trunk Line | Larger pipe (8–24 inches) that carries aggregated fluids to a processing plant or storage. |
| Pig Launcher/Receiver | Allows insertion and retrieval of pipeline inspection gauges (pigs) for cleaning and inspection. |
Function & Purpose
The primary functions of a gathering system include:
- Collection: Bring together produced fluids from multiple wells spread over a field or basin.
- Transportation: Move raw hydrocarbons from wellhead to processing facilities efficiently and safely.
- Initial Separation: Remove free water and basic sediment (BS&W) to reduce corrosion and improve pipeline efficiency.
- Measurement: Track production from each well for royalty and regulatory reporting.
- Pressure Conservation: Maintain proper backpressure to prevent formation damage and optimize reservoir drainage.
Gathering systems also play a role in emissions control by routing flash gas from tanks to vapor recovery units (VRUs) or flare systems.
Configuration Types
Gathering systems can be designed in several configurations depending on field layout and production characteristics.
- Radial System: All flowlines converge at a central manifold. Common in small, clustered well pads.
- Trunk & Lateral System: A main trunk line runs through the field, with laterals branching to individual wells. Best for elongated or spread-out fields.
- Loop System: Flowlines form a closed loop, providing redundancy and flexibility in routing. Used in high-reliability areas.
- Gathering Station: A centralized facility where multiple flowlines converge, often with separation, metering, and compression.
Operational Considerations
Design and operation of gathering systems involve several technical factors:
- Size and Capacity: Pipes must be sized to handle peak production rates without excessive pressure drop or velocity (which can cause erosion or liquid holdup).
- Corrosion Management: Sour gas (H2S) and acid gases require corrosion-resistant alloys or chemical inhibitors.
- Hydrate Prevention: In gas systems, water and low temperatures can form hydrates that plug lines. Methanol or glycol injection is often used.
- Pressure Rating: Components must withstand maximum shut-in pressure (MAOP) from the wellhead and any pumps or compressors.
- Environmental Compliance: Leak detection, secondary containment, and spill prevention plans are mandatory in most jurisdictions.
Usage Example
A field operator in West Texas manages a gathering system serving 30 horizontal wells spread over 15 square miles. Each well flows through a 4-inch flowline to a 12-well manifold pad, where a three-phase separator removes gas, oil, and water. Oil is measured by a lease automatic custody transfer (LACT) unit and flows into a 10-inch trunk line to a central tank battery. Gas is metered and compressed into an 8-inch line feeding a nearby gas plant. Water is sent via a separate 6-inch line to a disposal well. This arrangement allows efficient centralized processing and reduces the number of truck trips.