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| Complete workflow of GGS (Gas Gathering Station) and EPS (Early Production System) in the oil and gas industry, showing how crude oil, natural gas, and produced water are processed from the wellhead to export. |
To handle these fluids efficiently, operators use Group Gathering Stations (GGS) and Early Production Systems (EPS). These surface production facilities collect hydrocarbons from multiple wells, perform primary separation, and prepare the production stream for further processing.
A Group Gathering Station (GGS) is a permanent production facility that gathers, monitors, and processes fluids from multiple wells, while an Early Production System (EPS) is a temporary facility used to start production quickly during the early stages of field development.
In this article, we will explore the working principle, major components, process flow, and key differences between GGS and EPS, along with their role in modern oil and gas production operations.
What Are GGS and EPS in the Oil and Gas Industry?
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| Group Gathering Station (GGS) & EPS: Role in Oil & Gas Industry |
A Group Gathering Station (GGS) is a permanent production facility that receives fluids from multiple wells and performs primary processing before transferring the hydrocarbons to downstream facilities. Depending on field requirements, a GGS may include well headers, bath heaters, separators, storage tanks, a heater treater, pumping systems, and a SCADA-based control room.
An Early Production System (EPS) is a temporary production facility installed during the early stages of field development. Its primary purpose is to start production quickly before permanent facilities are fully constructed. EPS is commonly used during the appraisal and early development phase of an oil and gas field before permanent production facilities are commissioned. Unlike a GGS, an EPS generally performs only primary separation and typically does not include a heater treater. The separated crude oil is usually transferred to a nearby GGS for further processing and treatment.
Although both facilities handle production fluids from oil and gas wells, a GGS serves as a long-term production hub, while an EPS focuses on early production and rapid field development.
Major Components of GGS and EPS
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| GGS & EPS Process Flow: Wellhead → GGS → Three headers (Zero/Test/Group) → Separate bath heaters & separators → Zero & Test to storage tanks → Group to Heater Treater → Recycling of Zero/Test oil → Heater Treater → Oil to CTF, Water to ETP, Gas to trunk line. EPS is temporary with only primary separation. |
Well Header System
The well header system serves as the primary collection point for production fluids arriving from multiple wells. In a typical GGS, production is routed through the Zero Header, Test Header, or Group Header depending on well conditions and operational requirements.
Bath Heaters
Bath heaters are used to heat production fluids before separation. Heating reduces crude oil viscosity, improves fluid flow, and enhances separation efficiency.
Separators
Separators perform the initial separation of production fluids. Depending on the facility design, they may operate as two-phase separators (oil and gas) or three-phase separators (oil, gas, and water).
Storage Tanks
Storage tanks provide temporary storage for crude oil before further processing or transportation. In GGS facilities, separate tanks may be provided for Zero, Test, and Group production streams.
Heater Treater
The heater treater is one of the most important processing units in a GGS. It removes remaining water and gas from crude oil, improves oil quality, and prepares the production stream for transportation. Unlike GGS facilities, an EPS generally does not include a heater treater.
Pumps and Compressors
Pumps are used to transfer crude oil between processing units and storage facilities, while compressors increase gas pressure for transportation or utilization.
SCADA Control Room
Modern GGS facilities are monitored and controlled through a SCADA-based control room integrated with PLC and DCS systems, enabling real-time monitoring, control, and safe operation of production equipment.
These components work together in a specific sequence to process production fluids from the wellhead to the final production facility, as explained in the following process flow.
Detailed Process Flow of GGS and EPS
The process flow of a Group Gathering Station (GGS) begins at the wellhead, where crude oil, natural gas, and produced water are transported through flow lines to the gathering facility. Depending on the production conditions and operational requirements, the incoming fluids are routed through different headers and processing units before being prepared for transportation and further treatment.
Step 1: Wellhead to Header System
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| Wellhead to Header System in GGS - Crude oil from out-site wellhead travels through flow line into the Group Gathering Station (GGS), where it connects to the well header system and divides into three separate headers: Zero Header (zero/atmospheric pressure), Test Header (specific pressure for well testing), and Group Header (typically 4 kg/cm² to formation pressure). Each header is connected to its own dedicated bath heater for pressure-specific processing. |
- Zero Header – For low-pressure or depleted wells.
- Test Header – For well testing and production monitoring.
- Group Header – For normal field production.
Each header is connected to a dedicated bath heater and separator to ensure efficient handling of production fluids.
Step 2: Zero Header Operations
The Zero Header handles production from wells with little or no effective formation pressure. The primary objective of this system is to recover production from marginal wells and maintain field output.
Production from the Zero Header passes through the Zero Bath Heater and then enters the Zero Separator. Since the main purpose of this system is production recovery rather than complete processing, the crude oil is routed to the Zero Storage Tank for temporary storage.
Step 3: Test Header Operations
The Test Header is used for individual well testing, production monitoring, and performance evaluation. When a well needs to be tested, its production stream is diverted from the normal production line to the Test Header.
The production fluid passes through the Test Bath Heater and enters the Test Separator, where the flow can be measured and monitored under controlled conditions. This allows operators to evaluate well productivity, fluid characteristics, and overall reservoir performance.
Unlike the main production stream, the primary purpose of the Test Header is testing and monitoring rather than full-scale processing. The crude oil is therefore routed to the Test Storage Tank for temporary storage after testing activities are completed.
The collected crude oil is not wasted. After testing and measurement requirements have been fulfilled, the oil stored in the Test Tank is transferred to the main processing stream and later routed to the Heater Treater for further treatment and separation.
After the initial separation process in the separator, the gas stream may still contain fine liquid droplets, condensate, and solid particles. Before entering the gas compressor or downstream processing units, the gas is routed through a gas scrubber, where these remaining contaminants are removed. This final cleaning stage improves gas quality, prevents liquid carryover, and protects downstream equipment from damage.
Step 4: Group Header Operations
The Group Header handles the main production stream of the oilfield and receives fluids from regular producing wells operating under normal field conditions. Unlike the Zero and Test systems, the Group Header is designed for continuous production and primary processing.
Production fluids from the Group Header first pass through the Group Bath Heater, where heating reduces crude oil viscosity and improves separation efficiency. The heated fluid then enters the Group Separator, which performs the actual primary separation process.
Depending on the facility design, the Group Separator may operate as:
- A two-phase separator, separating crude oil and natural gas, or
- A three-phase separator, separating crude oil, natural gas, and produced water.
The separated gas is routed to the field gas handling system, while the produced water is directed to the appropriate water handling facilities. The crude oil follows the main production path and may either be sent directly to the Heater Treater or temporarily stored in a Group Storage Tank before further treatment.
Unlike the Zero and Test systems, the Group Header represents the primary production stream of the field and performs the actual oil, gas, and water separation required for efficient hydrocarbon processing.
Step 5: Zero and Test Oil Recycling
An important operational practice in many Group Gathering Stations (GGS) is the recovery and recycling of crude oil collected in the Zero Storage Tank and Test Storage Tank. Since the Zero Header and Test Header are primarily used for production recovery, well testing, and performance monitoring, the crude oil stored in these tanks is not treated as waste.
After the required testing, measurement, or temporary storage activities have been completed, the collected crude oil is transferred back into the main production stream and routed to the Heater Treater for further processing. This ensures that valuable hydrocarbons are recovered and utilized rather than being left in storage.
The recycling of Zero Tank Oil and Test Tank Oil offers several operational benefits, including improved hydrocarbon recovery, reduced production losses, and enhanced overall field efficiency. By integrating these streams into the main treatment process, operators can maximize crude oil recovery while maintaining efficient production operations.
As a result, crude oil from the Zero Header System and Test Header System ultimately undergoes the same treatment process as the main production stream before being transferred to storage facilities, the Central Tank Farm (CTF), or downstream transportation systems.
Step 6: Heater Treater Operations
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| Heater Treater Operations in GGS - The Heater Treater receives crude oil from the Group Separator (and recycled oil from Zero and Test storage tanks). Inside the Heater Treater, the fluid is separated into three distinct components: stabilized crude oil, produced water, and natural gas. The treated crude oil is transferred to an oil storage tank and then pumped to the Central Tank Farm (CTF). Produced water is sent to the Effluent Treatment Plant (ETP) for treatment and disposal, while natural gas is routed to the gas trunk line for utilization or further processing. |
Inside the Heater Treater, controlled heating reduces crude oil viscosity and breaks oil-water emulsions, allowing more efficient separation of hydrocarbons. This process improves crude oil quality and prepares it for storage and transportation.
During treatment, the production stream is separated into three components:
- Crude Oil
- Produced Water
- Natural Gas
The treated crude oil is transferred to storage facilities, while the separated water and gas are routed to their respective handling systems. By removing remaining water and gas from the crude oil, the Heater Treater improves production efficiency and helps meet downstream processing and transportation requirements.
Unlike a GGS, an Early Production System (EPS) generally does not include a Heater Treater. Therefore, crude oil produced through an EPS is typically transferred to a nearby GGS for final treatment and conditioning.
Step 7: Final Product Routing
After treatment in the Heater Treater, the production stream is separated into crude oil, produced water, and natural gas. Each component is then routed to its respective handling system for storage, transportation, or further processing.
Crude Oil Handling
The treated crude oil is transferred to the Oil Storage Tank, where it is temporarily stored before transportation. Transfer pumps then move the crude oil to the Central Tank Farm (CTF) for final storage, custody transfer, and dispatch to refineries or pipeline networks.
Produced Water Handling
The separated produced water is routed to dedicated water handling facilities for treatment, disposal, or reuse in accordance with field operating requirements and environmental regulations.
Natural Gas Handling
The separated natural gas is directed to the field gas handling system or Gas Trunk Line for further processing, utilization, transportation, or reinjection depending on operational requirements.
This final routing stage completes the production process within the Group Gathering Station (GGS), ensuring that crude oil, gas, and water are safely managed and prepared for their next stage of handling.
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| Complete process flow of Group Gathering Station (GGS) and Early Production System (EPS) in upstream oil and gas production operations. |
GGS vs EPS: Key Differences
Although Group Gathering Stations (GGS) and Early Production Systems (EPS) perform similar basic functions, they differ significantly in terms of size, processing capability, operational objectives, and field application.
| Parameter | GGS | EPS |
|---|---|---|
| Facility Type | Permanent production facility | Temporary production facility |
| Purpose | Long-term hydrocarbon processing and handling | Early production and field evaluation |
| Installation | Permanent infrastructure | Modular and fast installation |
| Processing Level | Primary and secondary processing | Primary processing only |
| Heater Treater | Usually available | Generally not available |
| Production Capacity | Medium to high | Limited |
| Field Stage | Developed or mature fields | Early field development |
| Investment | Higher capital investment | Lower initial investment |
| Main Objective | Continuous production operations | Rapid production startup |
| Crude Oil Destination | CTF or downstream facilities | Usually transferred to GGS for further treatment |
In Simple Terms
A Group Gathering Station (GGS) is a permanent production facility that performs both separation and treatment of hydrocarbons before transportation. In contrast, an Early Production System (EPS) is a temporary facility designed to start production quickly and generate early revenue while permanent facilities are being developed.
In most field developments, an EPS performs only primary separation, after which the crude oil is transferred to a nearby GGS for final treatment in the Heater Treater before being sent to the Central Tank Farm (CTF).
Advantages of GGS and EPS
Both Group Gathering Stations (GGS) and Early Production Systems (EPS) play a vital role in efficient hydrocarbon production and field development. These facilities help operators maximize production, improve fluid handling, and ensure safe transportation of crude oil, natural gas, and produced water.
Advantages of GGS
- Centralized Production Management – Collects and processes production fluids from multiple wells at a single facility.
- Improved Separation Efficiency – Enables effective separation of crude oil, gas, and water through dedicated processing equipment.
- Enhanced Hydrocarbon Recovery – Recycling of Zero and Test Tank oil minimizes production losses and maximizes recovery.
- Better Crude Oil Quality – Heater Treaters remove remaining water and gas, improving oil quality for transportation and storage.
- Efficient Monitoring and Control – SCADA-based systems provide real-time monitoring and operational control.
- Reduced Operating Costs – Centralized facilities reduce the need for multiple processing units across the field.
- Long-Term Production Solution – Supports continuous production throughout the field's operational life.
Advantages of EPS
- Rapid Production Startup – Allows operators to begin production before permanent facilities are fully constructed.
- Early Revenue Generation – Produces hydrocarbons and generates cash flow during the early stages of field development.
- Lower Initial Investment – Requires less infrastructure and capital compared to a full-scale GGS.
- Faster Field Evaluation – Helps assess reservoir performance and production potential.
- Flexible Installation – Modular design allows quick deployment and relocation when required.
- Supports Future GGS Development – Provides production continuity until permanent facilities become operational.
- Reduces Project Development Time – Accelerates the transition from discovery to commercial production.
Together, GGS and EPS help optimize oilfield operations by improving production efficiency, maximizing hydrocarbon recovery, and ensuring reliable handling of production fluids from the wellhead to the final processing facility.
Applications of GGS and EPS
Group Gathering Stations (GGS) and Early Production Systems (EPS) are widely used in onshore oil and gas fields to support efficient hydrocarbon production and field development. These facilities help operators collect, separate, process, and transport production fluids from multiple wells in a safe and controlled manner.
GGS and EPS play a critical role in hydrocarbon gathering, production optimization, well testing, early field development, and the safe transportation of crude oil, natural gas, and produced fluids. By improving production efficiency and supporting reliable field operations, these facilities remain essential components of modern upstream oil and gas infrastructure.
Conclusion
Group Gathering Stations (GGS) and Early Production Systems (EPS) are essential facilities in the upstream oil and gas industry, ensuring the efficient collection, separation, and handling of production fluids from multiple wells. While an EPS enables early production and rapid field development through primary separation, a GGS serves as a permanent production hub that performs comprehensive processing and treatment of hydrocarbons.
In a typical production system, fluids from the wellhead are routed through Zero, Test, and Group Headers, followed by bath heaters, separators, storage tanks, and the Heater Treater. The recycling of crude oil from the Zero and Test systems further improves hydrocarbon recovery and minimizes production losses. Modern GGS facilities also utilize SCADA-based monitoring and control systems to ensure safe, efficient, and reliable operations.
By working together, GGS and EPS help operators maximize production, improve crude oil quality, reduce operational losses, and support long-term field development. As a result, these facilities play a critical role in transforming raw well fluids into marketable hydrocarbons ready for transportation, storage, and further processing.
Frequently Asked Questions (FAQs)
What is a Group Gathering Station (GGS)?
A Group Gathering Station (GGS) is a surface production facility that collects production fluids from multiple oil and gas wells, performs separation and treatment processes, and prepares hydrocarbons for storage, transportation, or further processing.
What is an Early Production System (EPS)?
An Early Production System (EPS) is a temporary production facility used during the initial stages of field development. It enables operators to start production quickly before permanent facilities such as a GGS are fully operational.
What is the main difference between GGS and EPS?
The primary difference is that a GGS is a permanent facility that performs both separation and treatment, while an EPS is a temporary facility that typically performs only primary separation before sending crude oil to a GGS for further processing.
What is the purpose of the Zero Header in a GGS?
The Zero Header handles production from low-pressure or depleted wells where formation pressure is insufficient for normal production flow. Its main objective is to maximize production recovery from marginal wells.
Why is the Test Header used in a GGS?
The Test Header is used for individual well testing, production monitoring, and performance evaluation. It helps operators assess well productivity and reservoir performance.
What happens to the oil stored in the Zero and Test Tanks?
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| Well Group header in GGS |
What is the role of a Heater Treater in a GGS?
A Heater Treater removes remaining water and gas from crude oil, breaks emulsions, improves oil quality, and prepares the crude oil for storage and transportation.
Does an EPS have a Heater Treater?
In most cases, an EPS does not include a Heater Treater. Crude oil from the EPS is usually transferred to a nearby GGS for final treatment and conditioning.
How is crude oil transferred from a GGS?
After treatment, crude oil is sent to storage tanks and then transferred through pumps to a Central Tank Farm (CTF), pipeline system, or other downstream facilities.
How does SCADA help in GGS operations?
A SCADA (Supervisory Control and Data Acquisition) system provides real-time monitoring and control of production equipment, helping operators improve efficiency, safety, and operational reliability.
What does GGS stand for in the oil and gas industry?
GGS stands for Group Gathering Station. It is a surface production facility that collects production fluids from multiple wells, performs separation and treatment, and prepares hydrocarbons for transportation and further processing.
