Oil wells are supposed to produce oil, gas, and water.
Sometimes they produce something else.
Sand.
A small amount may seem harmless. After all, the reservoir rock itself may be sandstone, so finding a few grains at the surface does not sound particularly surprising.
The problem begins when those few grains become hundreds of kilograms of solids moving through a production system.
Sand can damage pumps, erode valves, fill separators, plug piping, interfere with artificial lift, and eventually force a well to shut down.
In severe cases, the formation around the well can become unstable enough to damage the completion itself.
For some oil and gas fields, controlling sand is one of the biggest challenges in keeping wells producing economically.
Where Does the Sand Come From?
Most of the sand produced by conventional wells comes from the reservoir formation itself.
A sandstone reservoir is made from individual mineral grains that have been compacted and cemented together over geological time.
Some formations are extremely strong.
Others are poorly consolidated.
Imagine the difference between a concrete block and a pile of damp beach sand.
Both contain grains.
One has enough strength to remain intact when subjected to stress.
The other can easily fall apart.
When oil, gas, or water begins flowing through a weak formation toward the well, individual grains can break loose and enter the wellbore.
Once that happens, the produced fluids carry the sand toward the surface.
Why Does the Rock Suddenly Fall Apart?
Reservoir rock is subjected to stress from the surrounding earth.
Before production begins, the fluids inside the pore spaces help support some of that stress.
Production changes the situation.
As reservoir pressure decreases, the effective stress acting on the rock can increase.
At the same time, pressure near the well may be significantly lower than pressure farther into the reservoir because fluids are being drawn toward the well.
If the forces acting on the formation become greater than the rock can tolerate, grains can separate from the formation.
That is when sand production begins.
Weakly consolidated sandstone reservoirs are particularly susceptible to the problem. Industry research also shows that reservoir depletion and changes in drawdown can contribute to sand production. (SLB)
What Is Drawdown?
Drawdown is one of the most important concepts for understanding sand production.
Imagine reservoir pressure is 3,000 psi.
The pressure near the producing well is 2,500 psi.
The difference helps drive fluids toward the well.
Increase that pressure difference and production may increase.
But there is a limit.
Producing a weak formation too aggressively can increase the forces acting on the rock around the well.
At some point, the formation may begin failing and sand production can increase.
This creates a familiar oilfield tradeoff.
Operators want production.
They do not want to damage the well trying to get it.
Opening the Choke Is Not Always Free Production
Suppose an oil well is producing 500 barrels per day through a restricted choke.
Someone may look at it and think:
Why not open the choke and produce 800?
Sometimes that is possible.
Sometimes it creates trouble.
Increasing the flow rate can increase drawdown and fluid velocity near the well.
In a formation already close to failure, that change may be enough to begin producing significant quantities of sand.
The additional production may look good for a short period.
Then the separator starts filling with solids.
Valves begin eroding.
The artificial lift system starts having problems.
Eventually, the well may lose more production than the extra choke opening ever gained.
Good production optimization considers the life of the well, not simply today’s maximum flow rate.
Does Every Sandstone Reservoir Produce Sand?
No.
Many sandstone reservoirs produce for years without significant sand problems.
Formation strength varies enormously.
Grain size matters.
Cementation matters.
Reservoir pressure matters.
Completion design matters.
Fluid properties matter.
Water production matters.
The pressure conditions around the well matter.
Two wells only a short distance apart can behave differently.
One may remain essentially sand free.
The other may require substantial sand control.
Why Can Sand Production Start Years Later?
A well may produce cleanly for years and then suddenly begin making sand.
This sometimes confuses operators because nothing obvious has changed at the surface.
Underground, however, plenty may have changed.
Reservoir pressure may have declined.
Water production may have increased.
The stress around the well may have changed.
The completion may have deteriorated.
A sand control device may have become damaged.
Production conditions may have changed.
A well that was stable early in its life can therefore become unstable later.
Sand control is not always a problem that reveals itself on the first day of production.
Water Can Make Sand Problems Worse
Water breakthrough often changes the behavior of a producing well.
There are several reasons.
Water can change forces between formation grains.
Increasing water production can also increase the total volume of fluid moving toward the well.
In certain formations, this can contribute to instability and increased sand production.
This is one reason an older oil well may suddenly develop sanding problems at roughly the same time its water cut begins increasing.
The well is not simply producing more water.
The entire flow environment around the well may be changing.
What Is Water Cut?
Water cut is the percentage of produced liquid that is water.
Suppose a well produces 1,000 barrels of total liquid each day.
If 800 barrels are water and 200 are oil, the well has an 80 percent water cut.
Mature oil wells can produce very high water cuts.
That means enormous volumes of liquid may move through the formation even when oil production itself is relatively modest.
For a weak reservoir, those changing flow conditions can influence sand production.
Sand Can Destroy Equipment Surprisingly Quickly
Sand sounds soft when you hold it in your hand.
At high velocity, it becomes an abrasive.
Imagine repeatedly blasting steel with millions of hard mineral particles carried by oil, water, or gas.
Eventually the steel loses material.
This is erosion.
Equipment particularly vulnerable to sand erosion can include:
Chokes
Control valves
Pipe bends
Flowlines
Pump components
Wellhead equipment
Separator internals
Small restrictions can experience especially high velocities.
That means a relatively inexpensive component can become a serious failure point if sand production is ignored.
Gas Wells Can Be Especially Aggressive
High velocity gas can carry sand particles at significant speeds.
When those particles hit steel surfaces, erosion can become severe.
Changes in direction are particularly important.
A straight pipe may tolerate the flow reasonably well while the outside wall of an elbow experiences concentrated particle impact.
This is why sand producing gas wells require careful attention to erosion.
The danger is not simply that sand accumulates somewhere.
It can physically remove metal from pressure containing equipment.
What Happens to Sand in a Separator?
Once sand reaches a production separator, gravity begins doing what gravity does best.
The solids settle.
Initially this may not seem like a major issue.
But separators are designed with a certain internal volume.
As sand accumulates in the bottom, that usable volume decreases.
Eventually the sand can interfere with normal separation.
It may affect liquid outlets.
It may cover internal components.
It may complicate level measurements.
The vessel may eventually require cleaning.
Opening a production vessel and removing compacted oily sand is not a pleasant maintenance job.
Preventing excessive sand from reaching the separator is usually preferable.
Sand Can Plug a Well Too
Sand does not always make it successfully to the surface.
Some of it can accumulate inside the well.
If enough material settles, it can create fill.
That fill can cover producing intervals or restrict the path through which fluids enter the well.
Production declines.
In severe situations, intervention equipment may be required to remove the accumulated solids.
Sand production has been associated with wellbore fill, equipment failure, reduced productivity, and costly cleanout operations. (SLB)
What Is a Sand Cleanout?
If enough sand accumulates inside a well, operators may need to remove it mechanically or hydraulically.
Coiled tubing is one tool that can be used for this type of intervention.
Fluid is pumped through the coiled tubing to help circulate solids out of the well.
The exact method depends on the well and the material being removed.
A cleanout can restore production.
But it costs money and requires taking the well through an intervention operation.
If the underlying sand problem remains, the sand can eventually return.
Cleaning the well treats the result.
It does not necessarily eliminate the cause.
Sand Is Hard on Artificial Lift
Artificial lift equipment generally prefers clean fluids.
Sand rarely cooperates.
Rod Pumps
Sand can interfere with valves and moving pump components.
It can increase wear and contribute to sticking or reduced pump efficiency.
Electric Submersible Pumps
An ESP contains rapidly rotating components.
Abrasive solids moving through the pump can increase wear.
Sand can also settle when the pump stops and interfere with restarting.
Progressive Cavity Pumps
These pumps can tolerate certain solids better than some alternatives, which is one reason they are used in some challenging applications.
But that does not mean unlimited sand production is harmless.
Gas Lift
Gas lift has no downhole mechanical pump in the same sense as a rod pump or ESP, which can provide advantages in some sand producing wells.
The rest of the production system still has to deal with the solids.
Frac Sand Is a Different Sand Problem
Modern unconventional wells introduce another source of solids.
Hydraulic fracturing intentionally places large quantities of proppant into created fractures.
That proppant is often sand.
Its purpose is to keep fractures open so hydrocarbons can flow through them.
After stimulation, some proppant may return to the well during cleanup and early production.
This is commonly referred to as proppant flowback.
So when an unconventional well produces sand, the material may not necessarily be formation sand created by reservoir failure.
It may include frac sand returning from the completion.
Understanding where the solids came from matters because the appropriate response can be different.
Why Put Sand Underground If Sand Is Such a Problem?
Because frac sand performs a useful job while it remains where it was placed.
Hydraulic fracturing creates or extends fractures in the reservoir.
Without support, those fractures would tend to close as pressure changes.
Proppant holds them open.
The objective is therefore not to avoid sand everywhere.
It is to keep the proppant in the fractures and out of production equipment.
Sand becomes a problem when it moves somewhere it was never intended to go.
What Is Sand Control?
Sand control refers to methods used to prevent or manage formation sand entering a producing well.
There is no single universal solution.
Possible approaches include controlling the production rate, using screens, installing gravel packs, consolidating the formation, selecting suitable completion designs, or managing produced solids at the surface.
The correct approach depends on the reservoir and the economics of the well.
SLB describes sand control systems as methods intended to prevent migration of reservoir sand and fines into the well while preserving productivity. (SLB)
What Is a Sand Screen?
A sand screen is installed downhole to help keep formation sand out while allowing produced fluids to enter the well.
The concept sounds like a household filter.
In principle, there is a similarity.
In practice, designing a screen for an oil well is much more complicated.
The openings have to relate to the formation particle size.
The screen has to survive installation.
It has to handle pressure and temperature.
It must resist plugging.
It may need to operate for many years without being accessible for routine maintenance.
A screen installed thousands of feet underground is not something you casually pull out and clean every weekend.
Modern sand screens are engineered around formation particle sizes and the expected completion environment. (SLB)
Why Not Use an Extremely Fine Screen?
If sand is the problem, making the screen openings extremely small seems obvious.
Then another problem appears.
Plugging.
Formation fluids can carry very fine particles.
Small screen openings can become restricted, reducing production.
A screen therefore has to balance sand retention against flow capacity.
This is why engineers analyze formation grain size distributions when designing sand control.
The goal is not necessarily to stop every microscopic particle.
The goal is to create a stable system that controls damaging sand production without unnecessarily choking the well.
What Is a Gravel Pack?
A gravel pack is one of the classic sand control methods.
Despite the name, it does not mean filling the entire well with random gravel.
Specially sized gravel is placed around a screen in the completion.
The gravel helps filter and stabilize formation material while allowing oil and gas to flow toward the screen.
The gravel size is selected based on the formation sand characteristics.
The screen retains the gravel.
The gravel helps retain the formation sand.
SLB defines a gravel pack as a sand control method in which a steel screen is placed in the well and the surrounding annular space is packed with specifically sized material. (Glossary)
Gravel Packs Are Common Offshore
Sand control becomes especially important for expensive offshore wells.
An offshore well may cost tens or hundreds of millions of dollars over its development and operating life.
A failure that requires major intervention can therefore be extremely expensive.
If engineers know the reservoir is likely to produce sand, designing reliable sand control into the original completion can make economic sense.
The upfront completion becomes more complicated.
The objective is to avoid much larger problems later.
What Is a Frac Pack?
A frac pack combines aspects of hydraulic fracturing and gravel packing.
The formation is stimulated while a sand control completion is also established.
This can improve productivity while providing protection against formation sand.
Frac pack completions are used in certain formations where both productivity and sand control are important.
Like most completion choices, they are not automatically appropriate for every well.
Reservoir properties and economics determine whether the additional complexity is justified.
Can Chemicals Stop a Formation From Producing Sand?
In some situations, chemical consolidation can be used.
The objective is to strengthen the formation near the well so grains are less likely to move while still maintaining enough permeability for hydrocarbons to flow.
That balance is critical.
Bond the formation too aggressively and you could reduce the flow capacity you were trying to preserve.
Do too little and sand production continues.
Chemical consolidation has been used as both an initial and remedial sand control approach in suitable wells. (SLB)
Sometimes the Solution Is Simply Producing Less
Not every sanding problem requires expensive downhole equipment.
If sand production begins only above a certain drawdown, reducing the production rate may stabilize the well.
That is not an attractive answer when everyone wants more production.
But economics matters.
Imagine producing 1,000 barrels per day while destroying equipment and requiring frequent cleanouts.
Now compare that with producing 800 barrels per day reliably for years.
The lower rate may create more value over the life of the well.
Maximum production and optimum production are not always the same thing.
How Do Operators Know a Well Is Making Sand?
Sometimes the answer is obvious.
Sand appears in surface equipment.
Filters fill.
Separators accumulate solids.
Samples contain visible grains.
Other times the signs are less direct.
Operators may notice increasing erosion, unstable production, abnormal pressure behavior, artificial lift problems, or solids appearing during maintenance.
Specialized sand monitoring equipment can also be used where the risk justifies it.
The most useful information often comes from combining several observations rather than waiting until someone sees a pile of sand.
What Is a Desander?
A desander is equipment designed to separate solid particles from a production stream.
Different designs exist for different applications.
One common approach uses centrifugal forces to separate dense particles from the produced fluids.
Desanders can be installed at the surface.
Downhole sand management equipment also exists for certain artificial lift applications.
Surface removal does not stop the formation from producing sand.
It protects equipment farther downstream.
That distinction matters.
Sand prevention and sand management are not the same thing.
What Happens to the Removed Sand?
Produced sand is not clean beach sand.
It can be coated with oil and contaminated with produced water or other substances from the well.
It therefore has to be collected, handled, cleaned, transported, or disposed of according to the applicable operating and environmental requirements.
For facilities producing large quantities of solids, sand handling becomes its own operating system.
Someone has to remove it from vessels.
Someone has to manage storage.
Someone has to determine where it goes next.
The problem does not disappear simply because a separator successfully removed it from the oil.
Sand Erosion Can Become a Safety Issue
Production loss is expensive.
Loss of containment is much more serious.
If sand gradually erodes a choke body, elbow, flowline, or other pressure containing component, wall thickness can decrease.
Eventually the equipment may no longer safely contain operating pressure.
This is why erosion monitoring matters in known sand producing systems.
Engineers may evaluate velocities, material selection, geometry, wall thickness, inspection results, and historical sand production when determining risk.
Managing sand is therefore not simply a production optimization issue.
It can be a process safety and well integrity issue too.
Why Are Elbows Vulnerable?
Imagine sand particles travelling quickly through a straight pipeline.
Their momentum carries them forward.
Now the pipe turns ninety degrees.
The gas or liquid changes direction.
The solid particles resist that change because of inertia.
They can strike the outside wall of the bend.
Repeated particle impacts gradually remove metal.
This is why piping geometry matters when designing systems expected to handle solids.
Places where the flow changes direction can experience very different erosion rates from long straight sections.
Can Sand Production Damage the Reservoir?
Potentially.
Severe formation failure can extend beyond simply releasing a few grains.
Material around the well can become unstable.
The completion can lose support.
Casing or other downhole equipment can be affected under severe conditions.
The consequences depend on the formation and completion.
This is why predicting sanding risk before designing the well can be valuable.
Sand management is easier when it is considered during completion design rather than discovered after equipment begins failing.
Why Does One Field Need Gravel Packs While Another Does Not?
Because reservoir rocks are different.
Some formations are strong enough to remain stable without downhole sand control.
Others are extremely weak.
The production strategy also matters.
A low rate well may remain stable under conditions that would cause the same formation to fail at a higher drawdown.
Well orientation, completion type, reservoir depletion, water production, stress, and grain characteristics all influence the problem.
There is no universal rule saying every sandstone reservoir needs a screen or gravel pack.
Good completion design starts by understanding the particular rock.
Is a Little Sand Acceptable?
Sometimes.
Trying to eliminate every single grain can cost more than managing a small amount of solids.
This has led to the concept of sand management rather than always pursuing absolute sand exclusion.
If a well produces a manageable amount of sand and the surface equipment can safely handle it, allowing controlled sand production may be economically reasonable in certain situations.
The important word is controlled.
Operators need to understand how much sand is being produced, whether erosion remains acceptable, whether equipment can handle the solids, and whether sanding is becoming progressively worse.
Sand Control Can Reduce Production Too
Every restriction placed between the reservoir and the well has the potential to affect flow.
Screens can plug.
Gravel packs can create additional resistance.
Poorly designed completions can reduce productivity.
That is why sand control is not simply a question of installing the strongest possible filter.
The completion needs to control solids while allowing hydrocarbons to flow economically.
This is another classic petroleum engineering compromise.
Too little control creates sand problems.
Too much restriction can create production problems.
What Happens When a Sand Screen Fails?
If a screen becomes damaged, plugged, eroded, or otherwise loses effectiveness, sand production can increase.
The well may require remedial work.
Depending on the completion, operators may have options involving intervention, additional screens, chemical treatments, or other repair methods.
Some wells become difficult to repair economically.
This is especially important offshore, where accessing a failed completion can be extremely expensive.
Sand control equipment is expected to operate in an environment where routine replacement may not be practical.
Sand Problems Can Change the Economics of an Entire Well
Imagine a well capable of producing valuable quantities of oil.
On paper, the reservoir looks excellent.
But the well produces enough sand to destroy pumps repeatedly.
Each failure requires intervention.
Production stops while equipment is repaired.
Sand has to be removed.
The replacement pump is installed.
Then the cycle begins again.
Suddenly a good reservoir has become a poor investment.
This is why petroleum economics cannot be separated from production engineering.
The amount of oil underground is only part of the story.
You also need a reliable way to get it to the surface.
Frequently Asked Questions
Why does an oil well produce sand?
Sand can be produced when the rock surrounding the well is not strong enough to remain stable under the stresses and flow conditions created during production. Formation grains can break loose and travel into the well with the produced fluids.
Does producing a well too fast cause sand?
High drawdown and increased flow can contribute to formation failure in susceptible reservoirs. The relationship depends on reservoir strength, completion design, pressure, water production, and other conditions.
Is frac sand the same as formation sand?
No. Formation sand originates from the reservoir rock. Frac sand is intentionally pumped into fractures as proppant during hydraulic fracturing. Some frac sand can later return to the well during production.
What damage can produced sand cause?
Sand can erode valves and piping, damage artificial lift equipment, fill separators, plug the well, reduce production, and contribute to completion problems.
What is a sand screen?
A sand screen is a downhole completion device designed to allow produced fluids into the well while controlling the movement of formation sand.
What is a gravel pack?
A gravel pack uses specifically selected gravel placed around a downhole screen to help stabilize and filter formation sand while maintaining a flow path for produced fluids.
Can sand completely stop an oil well?
Yes. Sand can accumulate inside the well and restrict or cover producing intervals. Severe solids problems can also damage equipment enough to force the well out of production.
Why does water production sometimes increase sanding?
Water breakthrough can change formation behavior and increase the volume and nature of fluid flow around the well. In susceptible formations, this can contribute to increased sand production.
Can operators simply remove the sand at the surface?
Surface desanders and separators can manage produced solids, but they do not prevent formation sand from entering the well. Whether surface management is sufficient depends on the amount of sand, erosion risk, equipment limitations, and well economics.
Is some sand production acceptable?
In certain wells, controlled sand production may be economically manageable. The operator still needs to ensure erosion, equipment damage, well integrity, and solids handling remain within acceptable limits.