One of the biggest misconceptions about the oil and gas industry is that once an oil well starts producing, it continues flowing at the same rate until the oil runs out.
In reality, that’s almost never what happens.
Most oil wells produce their highest volumes during the first months or years of operation. After that, production gradually begins to decline. Some wells experience a slow decrease over decades, while others lose a significant portion of their production in just a few years.
So what causes an oil well to slow down?
Has all the oil been removed?
Not necessarily.
In many cases, a well stops producing economically long before the reservoir is completely depleted.
Let’s look at why this happens.
Oil Doesn’t Exist in Underground Caverns
Movies often show giant underground lakes filled with oil waiting to be pumped to the surface.
Real reservoirs look nothing like that.
Instead, oil is stored inside tiny pores within underground rock formations.
Imagine a kitchen sponge soaked with cooking oil.
The oil fills millions of tiny spaces between the grains of rock.
When a well is drilled, it creates a pathway that allows some of that oil to flow toward the surface.
The important word is some.
Even after production ends, a large percentage of the oil usually remains trapped underground.
Reservoir Pressure Begins to Decline
When a new well starts producing, the reservoir often has enough natural pressure to push oil toward the wellbore.
As fluids leave the reservoir, that pressure gradually decreases.
Think about opening a bottle of carbonated soda.
At first, the pressure is high.
As gas escapes, the pressure drops.
Oil reservoirs behave similarly, although over much longer periods.
Lower pressure means less energy is available to move oil through the rock.
As a result, production begins to decline.
Water Eventually Reaches the Well
Many oil reservoirs contain water beneath the oil.
As production continues, this water slowly moves toward the well.
Eventually, the well begins producing increasing amounts of water along with the oil.
This is known as water breakthrough.
At first, the water content may be small.
Over time, it often increases.
Some mature wells produce far more water than oil.
Although oil may still be present in the reservoir, separating and disposing of large volumes of produced water becomes increasingly expensive.
Gas Can Also Affect Production
Some reservoirs contain natural gas above the oil.
As pressure changes, gas may reach the well before much of the remaining oil.
This process, called gas breakthrough, reduces oil production efficiency because the gas follows the easiest path to the well.
Operators carefully monitor gas production because it provides important information about what’s happening inside the reservoir.
The Rock Limits Flow
Oil doesn’t move freely through rock.
It must travel through microscopic pores connected by even smaller pathways.
Over time, several things can reduce that flow.
These include:
- Wax deposits
- Scale
- Clay swelling
- Fine particles
- Formation damage
Even though oil remains in the reservoir, it may no longer flow efficiently toward the well.
Mechanical Problems Can Reduce Production
Not every decline is caused by the reservoir itself.
Sometimes the problem is the well.
Common mechanical issues include:
- Worn pumps
- Corroded tubing
- Leaking packers
- Sand production
- Failed valves
- Scale buildup inside tubing
Many production declines can be improved through maintenance or well interventions.
Artificial Lift Becomes Necessary
When natural reservoir pressure is no longer enough, operators often install artificial lift systems.
Common examples include:
- Pumpjacks
- Electric submersible pumps (ESPs)
- Gas lift systems
- Progressive cavity pumps
- Hydraulic pumps
These systems don’t create more oil.
They simply help move the remaining oil to the surface.
Many wells continue producing for years after artificial lift is installed.
Secondary Recovery Helps Maintain Production
If natural pressure isn’t enough, operators may inject fluids into the reservoir.
The most common method is water injection.
Injected water helps maintain pressure and pushes oil toward producing wells.
This process is called secondary recovery.
Many of the world’s largest oil fields rely on water injection to extend their productive lives.
Enhanced Oil Recovery Can Recover Even More Oil
After primary and secondary recovery, a significant amount of oil still remains underground.
To recover more, companies may use enhanced oil recovery (EOR) techniques.
Examples include:
- Carbon dioxide (CO₂) injection
- Steam injection
- Polymer flooding
- Chemical flooding
These methods improve the movement of oil through the reservoir.
Some fields have produced for decades because of enhanced recovery projects.
When Does a Well Become “Empty”?
This surprises many people.
Most oil wells are not empty when production stops.
In fact, a large percentage of the original oil often remains underground.
The problem is economics.
As production declines, operating costs remain.
Eventually the value of the oil being produced is no longer enough to justify operating the well.
At that point, the well may be shut in or permanently abandoned.
Oil Prices Also Matter
A well that’s unprofitable when oil sells for $50 per barrel may become profitable again if prices rise significantly.
For that reason, some wells are temporarily shut in rather than permanently abandoned.
Market conditions play an important role in production decisions.
Can an Old Oil Well Produce Again?
Yes.
There are several ways production can increase after declining.
Operators may:
- Replace damaged equipment.
- Stimulate the reservoir.
- Perform acidizing.
- Hydraulic fracture the formation.
- Install artificial lift.
- Drill additional horizontal sections.
- Complete nearby injection projects.
Many wells have experienced second or even third periods of productive life after improvements were made.
Common Misconceptions
“Oil wells suddenly run dry.”
Most don’t.
Production usually declines gradually over many years.
“Once a well stops producing, all the oil is gone.”
Not even close.
A large percentage of oil typically remains trapped inside the reservoir.
“Pumpjacks create oil.”
Pumpjacks only lift oil that’s already capable of reaching the wellbore.
They don’t increase the amount of oil in the reservoir.
Interesting Facts
- Some oil wells have produced continuously for more than 50 years.
- Many mature fields produce more water than oil.
- Modern reservoir simulation allows engineers to predict production declines years in advance.
- Enhanced oil recovery projects can significantly increase the percentage of oil recovered from a field.
- Even after decades of production, most reservoirs still contain unrecovered oil.
Final Thoughts
An oil well doesn’t stop producing because the reservoir suddenly becomes empty.
Instead, production declines as reservoir pressure falls, water and gas reach the well, equipment wears out, and the remaining oil becomes more difficult and expensive to recover.
That’s why modern oil production is as much about engineering as it is about geology.
Production engineers continuously monitor wells, adjust operating conditions, install artificial lift systems, and develop new recovery methods to extend the life of oil fields.
The next time you see a pumpjack slowly moving up and down, remember that it’s often helping recover oil from a reservoir that may have been producing for decades—and still has more oil left underground.
Frequently Asked Questions
How long does an average oil well produce?
It depends on the reservoir. Some wells produce for only a few years, while others continue producing for several decades with proper maintenance and artificial lift.
Why do old wells produce more water?
As oil is removed, water naturally moves toward the well. Over time, the amount of produced water often increases while oil production declines.
Can an abandoned oil well be reopened?
Sometimes. If oil prices improve, new technology becomes available, or additional reserves are identified, a previously shut-in well may become economically viable again.
What percentage of oil is left underground after production?
The exact amount varies by reservoir, but it’s common for a significant portion of the original oil to remain in the formation even after conventional production methods have been exhausted. Modern recovery techniques aim to increase the amount of oil that can be economically produced.