Part 1 of 10 in the series From Reservoir to Surface: Oil and Gas Production for Beginners.
The oil and gas production process is the chain of steps that moves hydrocarbons from a rock layer deep underground to a pipeline, a tank or a ship. It starts in the reservoir, continues up the well, passes through the wellhead and surface valves, and ends at processing facilities that separate and treat oil, gas and water so each one can be sold, used or safely disposed of.
This first article gives you the big picture. Each of the next nine parts zooms in on one piece of the chain.
Where Production Fits in the Oil and Gas Industry
The industry is usually split into three sectors:
- Upstream: exploration, drilling and production. This series lives here.
- Midstream: transporting and storing crude oil and gas (pipelines, tankers, storage terminals).
- Downstream: refining crude into fuels and products, and selling them.
Production begins once a well has been drilled and proven. If you are new to the subject, it helps to first read what petroleum is and how it collects in reservoir rocks.
Step 1: The Reservoir
A reservoir is a porous and permeable rock, usually sandstone or carbonate, that holds oil, gas and water in its pore spaces. Two rock properties control how much it can produce: porosity (how much space there is) and permeability (how easily fluids move through it).
The fluids are under pressure. When a well opens a path to the surface, this formation pressure pushes fluids toward the low-pressure wellbore. The natural energy that drives this flow comes from several drive mechanisms:
- Solution gas drive: gas dissolved in the oil comes out of solution and expands as pressure falls.
- Gas cap drive: a free gas layer above the oil expands and pushes oil down toward the wells.
- Water drive: water from a connected aquifer moves in and replaces the produced oil.
- Gravity drainage and compaction: less common, but important in some fields.
Step 2: The Well and Its Completion
After drilling, the well must be completed. The completion is everything installed in the well to let fluids flow safely and efficiently: casing or liner across the reservoir, perforations, production tubing, a packer and safety valves.
Fluid enters the wellbore through the completion, then rises up the production tubing. If reservoir pressure is high enough to lift the fluid column all the way to surface, the well flows naturally. If not, it needs artificial lift, such as a pump or gas lift.
Natural flow vs artificial lift
Many wells flow naturally early in their life. As reservoir pressure declines and water production increases, the fluid column becomes too heavy for the reservoir to lift. At that point, artificial lift adds energy to keep the well producing. Parts 7 and 8 of this series cover this in detail.
Step 3: The Wellhead and Christmas Tree
At the surface, the wellhead supports the casing and tubing strings and seals the spaces between them. On top of it sits the Christmas tree, an assembly of valves that controls flow out of the well and allows the well to be shut in. A choke on the tree, or just downstream of it, reduces the pressure and sets the flow rate before the fluid enters the surface lines.
Step 4: Flowlines, Manifolds and Separation
From the tree, a flowline carries the well stream to a gathering station or production facility. Streams from many wells are combined in a manifold, where an operator can route any well either to the main production line or to a test separator for measurement.
The well stream is a mixture of oil, gas and water, often with some sand. The first major processing step is the separator, a pressure vessel that uses gravity and time to split the stream into gas, oil and water. Fields often use two or three separation stages at decreasing pressures.
Step 5: Treating Oil, Gas and Water
After separation, each product goes through its own treatment:
Oil treatment
- Dehydration: removing remaining water and breaking emulsions, often with heat and chemicals.
- Desalting: washing out salt, which causes corrosion in pipelines and refineries.
- Stabilization: removing light components so the crude does not release too much gas in storage tanks.
Gas treatment
- Dehydration: removing water vapour, usually with glycol, to prevent hydrates and corrosion.
- Sweetening: removing hydrogen sulphide (H2S) and carbon dioxide (CO2) where present.
- Compression: raising pressure for export pipelines, gas lift or reinjection.
Produced water treatment
Water separated from the oil still contains small oil droplets and solids. It is cleaned and then reinjected into the reservoir for pressure support or disposal, or discharged where regulations allow.
Step 6: Storage, Metering and Export
Treated crude goes to storage tanks and is then exported by pipeline or tanker. Before it changes ownership, it passes through fiscal metering, which measures volume and quality accurately because it is the basis for sales and taxes. Treated gas is metered and exported by pipeline, or used on site as fuel.
How Engineers Monitor the System
Production engineers watch the whole chain, not just one part. Typical tools include:
- Flow tests through a test separator to measure each well’s oil, gas and water rates.
- Bottom hole pressure surveys to track reservoir pressure.
- Well performance evaluation to find wells that underperform.
- SCADA systems that collect pressures, temperatures and rates from wells and facilities in real time.
Frequently Asked Questions
What are the main stages of oil and gas production?
Fluid flow in the reservoir, flow into and up the well, flow control at the wellhead and Christmas tree, separation of oil, gas and water, treatment of each product, and finally storage, metering and export.
Is the production process the same for oil fields and gas fields?
The chain is similar, but the emphasis changes. Oil fields focus on separating and treating crude, while gas fields focus on removing liquids, water and acid gases, and on compression.
Why is water produced with oil?
Water naturally exists in reservoir rocks alongside oil and gas. As a field matures, more water usually reaches the wells, especially in water drive reservoirs or fields under water injection.
What is the difference between upstream and downstream?
Upstream finds and produces oil and gas. Downstream refines crude oil into products such as gasoline and diesel and sells them.
Conclusion
The oil and gas production process is a connected system: reservoir, well, wellhead, flowlines, separators, treatment and export. A change in one part affects the others. For example, a higher separator pressure reduces how much a well can produce. Understanding the full chain makes every detailed topic easier to follow.
Next in this series: Part 2, Types of Well Completion.
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