Gas anchors
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Low fillage, fluid pound or a gas-locked rod pump usually points to the same tool below the pump: a gas anchor. Which configuration fits depends on whether the pump can sit below the perforations, how much liquid it displaces, and whether solids or deviation are in play. This guide works through the four configurations by well condition, then covers the sizing rule, the specifications a quotation has to fix, and where each option stops working. It also sets out the records that should ship with the part and the SpectraMax lead-time bands by product class.
Gas Anchors for Rod Pumps
Your rod pump is showing low fillage or fluid pound, or it has gas locked, and you need a tool below it that keeps the gas out and a choice you can defend to whoever signs off the work. A gas anchor is usually that tool. Which configuration fits depends on whether the pump can sit below the perforations, how much liquid the pump displaces, and whether solids or deviation are in play.
This guide works through that choice by well condition, then covers sizing, specifications, where each option stops working, the records that should ship with the part, and lead times. Every SpectraMax gas anchor in it is engineered to order by Maximus OIGA Private Limited and sold by quotation, with lead-time bands of 1 to 4, 4 to 12 and 8 to 16 weeks or more by product class.
What is a gas anchor, and which well does it suit
The SLB Energy Glossary describes a gas anchor as a perforated tubular attached to the subsurface sucker-rod pump that controls the entrance of gas and helps prevent gas lock. Clegg's Southwestern Petroleum Short Course paper, "Another Look at Gas Anchors", calls the name a misnomer: the tool is a downhole gas separator, not an anchor.
A gas anchor run below the pump raises pump fillage by sending free gas up the casing annulus. That makes the pump's position against the perforations the first fact on any gas anchor enquiry, and the one we size the rest of the anchor around.
Every gas anchor, whatever its build, relies on the same gravity principle to make that separation.
How does a gas anchor separate gas before it reaches the pump
A gas anchor separates gas by gravity: gas bubbles rise faster than the liquid moves down toward the dip tube, so gas leaves through the upper ports and liquid is drawn from below. It happens in three steps:
- On the upstroke, well fluid enters through ports or slots near the top of the gas anchor.
- Each gas bubble rises and leaves through the same ports into the casing annulus.
- Liquid moves down to the bottom of the dip tube and up to the pump, while its downward liquid velocity stays below the bubble rise velocity.
The dip tube, also called the mosquito bill, is the inner tube the pump draws liquid through, open near the bottom of the outer barrel. The ports are cut in a perforated nipple or a slotted barrel, and a mud anchor below lets solids settle out.
McCoy and Podio, in "Improved Downhole Gas Separators", state the governing condition: separation holds only while the downward liquid velocity in the separation annulus stays below the gas bubble rise velocity. That condition is why every SpectraMax enquiry asks for pump size and stroke rate: together they set the downward velocity the barrel has to hold.
Each configuration of gas anchor applies this principle in a different place in the well.
What types of gas anchor are there
Gas anchors come in four main configurations: the natural gas anchor (pump set below the perforations), the poor-boy gas anchor with its dip tube, the collar-size gas separator, and the packer-type gas separator, with a mud anchor added where solids travel with the gas.
Natural gas anchor
A natural gas anchor is a placement, not a tool: the pump intake sits below the perforations, so the casing itself acts as the separator. It suits a well with enough rathole to set the intake at least 5 ft below the fluid entry zone, and it is not an option when the pump must sit above the perforations.
Poor-boy gas anchor
A poor-boy gas anchor is a perforated or slotted outer barrel with a concentric dip tube, run below the pump with no seal to the casing. It suits a pump set above the perforations at a low to moderate pump rate, and the small annulus inside the barrel caps how much liquid it can separate.
Collar-size gas separator
A collar-size gas separator has an outer barrel as large as the tubing collar allows, with ports at the top and an internal dip tube. It gives more capacity than a poor-boy barrel when the pump sits in or above the fluid entry zone, and the casing drift caps its barrel diameter.
Packer-type gas separator
A packer-type gas separator carries a cup or packer seal on its mandrel, so separation happens in the larger casing annulus above the seal. It suits pump rates beyond what an internal annulus can separate. Solids can settle on the seal cup, so the cup is matched to the well fluid and temperature.
Mud anchor add-on
A mud anchor is a lower joint or slotted barrel that lets solids settle out alongside the gas separation, for wells where sand or other solids travel with the gas. It does not separate gas on its own, so it runs below one of the four configurations above.
All four types of gas anchor are built to order in the SpectraMax line, to the casing size, tubing connection and pump rate on your enquiry, so the barrel matches your well rather than the nearest stock size. Choosing between them comes down to matching the configuration to your well condition.
Which gas anchor suits your well condition
The right gas anchor depends on three well conditions: whether the pump can be set below the perforations, the liquid rate the pump displaces against the casing size, and whether solids or deviation are present. Each row below names the condition under which its choice stops holding.
| Well condition | Configuration | Why it fits | Where it stops |
|---|---|---|---|
| Rathole below the perforations | Natural gas anchor, with the pump intake at least 5 ft below the fluid entry zone | The casing becomes the outer barrel; McCoy and Podio call it the simplest and most efficient option. | Not available when the pump must sit above the perforations. |
| Pump above the perforations, low to moderate pump rate | Poor-boy gas anchor or collar-size gas separator, with the largest outer barrel the casing drift allows | Separation happens inside the barrel, with no seal to set. | Stops when pump displacement exceeds the capacity of the annulus inside the barrel. |
| Pump above the perforations, liquid rate beyond the internal annulus capacity | Packer-type gas separator | Separation moves to the larger casing annulus above the seal. | Stops when solids settle on the seal cup, or the cup does not suit the well fluid and temperature. |
| Solids travel with the gas | Add a mud anchor below the gas anchor | Solids settle out in the lower joint instead of reaching the pump. | Does not separate gas on its own; it pairs with one of the configurations above. |
| Deviated setting depth | Any configuration, with the inclination at setting depth stated on the enquiry | Gravity separation performance changes with inclination. One Southwestern Petroleum Short Course study found its own tool most efficient at about 40 to 45 degrees and cautioned against solids placement beyond 65 degrees. | Those angles are that study's findings for its own tool; the quotation sets the port position for the inclination you state. |
Once your well matches a row, a SpectraMax quotation fixes the configuration, the barrel size and the lead-time band.
Before you choose a row, confirm the well is one where gas anchors are used at all.
Where are gas anchors used, and where are they not the right tool
Gas anchors are used in sucker-rod pumped oil wells that produce free gas at the pump intake, usually run on the same string as a tubing anchor catcher. They belong in:
- Sucker-rod pumped oil wells on a beam pumping unit with free gas at the pump intake.
- Rod-pumped wells showing gas interference: incomplete pump fillage and fluid pound on the dynamometer card.
- Wells on a progressing cavity pump, where gravity gas anchors also run below the pump intake.
A gas anchor is not the right tool when:
- The well runs an electric submersible pump, which uses a rotary gas separator (or a vortex design) built into the ESP string, a different product not covered here.
- The well flows on its own or produces on gas lift.
- There is no free gas at the pump intake.
In every well where it is used, the gas anchor's job is to stop gas interference before it becomes full gas lock.
The tubing anchor catcher is the companion tool on the same string: it holds the tubing still against pump-stroke movement while the gas anchor separates gas. SpectraMax already makes tubing anchor catchers for the same rod-pumped string, so one manufacturer can supply both anchors.
Once the gas anchor is the right tool, the next check is whether it is big enough for your pump.
How do you size a gas anchor for your pump rate
A gas anchor's liquid capacity is set by the area of the annulus where liquid moves down, multiplied by a downward velocity of no more than 6 inches per second, which is about 53 bbl/d for every square inch of annulus.
Sizing rule Capacity (bbl/d) = separation annulus area (square inches) x 53.4. The constant is 0.5 ft/s x 86,400 s/d / 5.615 cu ft per bbl / 144 square inches per sq ft. |
McCoy and Podio put the bubble rise velocity of gas in low-viscosity produced liquid (under 10 cp) at about 6 inches per second. Small bubbles rise more slowly: a Southwestern Petroleum Short Course study on downhole gas separation, citing Stokes, gives about 1 in/s for a 1/8 in bubble and about 6 in/s for a 1/2 in bubble, so the sizing treats 6 inches per second as an upper bound.
| Configuration | Annulus (sq in) | Capacity at 6 in/s (bbl/d) | Note |
|---|---|---|---|
| Natural gas anchor: 5-1/2 in 17 lb/ft casing (casing ID 4.892 in) around 2-7/8 in tubing | 12.3 | about 650 | The casing is the outer barrel. |
| Poor-boy gas anchor: 2-7/8 in barrel (ID 2.441 in) around a 1 in dip tube (OD 1.315 in) | 3.3 | about 175 | Liquid moves down inside the barrel. |
Your numbers change with casing weight and barrel size; send them and the quotation confirms the sizing.
The same well separates roughly four times more liquid when the pump can sit below the perforations.
We size against the pump displacement during the stroke, not the well's daily average, because liquid moves through the anchor while the pump is drawing. The dip tube is sized to keep friction loss preferably under 1/2 psi, the guideline McCoy and Podio give.
Those sizing figures become the dimensions a quotation has to fix.
Need expert guidance on the right solution?
Talk to our engineers about your project requirements.
What specifications define a SpectraMax gas anchor
A SpectraMax gas anchor is specified by casing size and weight, tubing and pump connection, barrel and dip tube size, length, port or slot pattern, material, and whether a seal cup or mud anchor is added.
| Parameter | What you tell us | What the quotation confirms |
|---|---|---|
| Casing size and weight | Casing OD and weight, for example 5-1/2 in, 17 lb/ft | Casing ID and drift, and the largest barrel that fits |
| Tubing and pump connection | Tubing connection and thread, and the pump's bottom connection | EUE or NUE 8-round end connections, thread form per API 5B |
| Barrel size | A preferred nominal size, or none if you want the sizing check to set it | Barrel OD and ID, sized against your pump displacement |
| Dip tube size | Pump intake size | Dip tube OD and length, checked against the 1/2 psi friction guideline |
| Length | Setting depth relative to the perforations | Assembly length and port position |
| Port or slot pattern | Gas and solids load, where known | Port or slot pattern, count and position |
| Material and options (seal cup, mud anchor) | Corrosive service (H2S, CO2), solids, and whether you want a seal cup or mud anchor | Material grade on the material test report; seal cup and mud anchor specified to your conditions |
Gas anchor barrels in the market come in nominal sizes of 3/4 in, 1 in, 1-1/4 in and 1-1/2 in, with lengths between 12 in and 20 ft. SpectraMax gas anchors are built to the requested size and length across that range.
The material is carbon or alloy steel tubular, with stainless steel for corrosive service where you specify it, and the material test report confirms the grade.
A gas anchor is not a pressure-sealing tool. Its limit is the body and connection strength of the tubular it is made from, and on a packer-type separator the seal cup is specified to the well conditions on your enquiry. The port pattern and length follow from your setting depth and gas load.
Five inputs move a gas anchor quotation: barrel size and length, which set the steel in the part; the material grade, since stainless and alloy grades for corrosive service cost more to source and machine than carbon steel; a seal cup or mud anchor joint, which adds parts; an inspection and test plan, which adds inspection hold points; and the quantity on the order. We quote in one pass when the table above is complete.
With the specification set, the remaining question is how a gas anchor compares with the other ways of handling gas.
How does a gas anchor compare to the alternatives
A gas anchor, compared with the alternatives, is the lowest-cost tool for keeping gas out of a sucker-rod pump; setting the pump below the perforations separates gas more efficiently where the well allows it, and cyclonic separators handle rates beyond a gravity anchor.
| Approach | How it handles gas | Best when | Limit |
|---|---|---|---|
| Natural gas anchor (pump set below the perforations) | The casing acts as the outer barrel and gas rises up the casing annulus | There is rathole below the perforations | Needs rathole below the perforations |
| Poor-boy gas anchor or collar-size gas separator | Gravity separation inside a ported barrel around a dip tube | The pump sits above the perforations at a low to moderate rate | Capacity set by the annulus inside the barrel |
| Packer-type gas separator | A seal cup moves separation to the larger casing annulus | The liquid rate is beyond an internal annulus | A seal cup adds a part to specify |
| Cyclonic separator | A patented class of downhole gas separator using cyclonic motion and baffles | Gas and solids loads are higher than a gravity anchor handles | A different product class from the gravity designs in this guide |
Where does each gas anchor option stop working
Every gravity gas anchor stops working when the downward liquid velocity in its annulus exceeds the bubble rise velocity, or when gas arrives in slugs faster than the ports can vent it. Past that limit, pump fillage falls and the pump can gas lock.
Three thresholds apply whichever configuration you run:
- Side-intake ports: liquid entry falls off above about 6 ft/s superficial gas velocity in the adjacent casing annulus, and the separator is starved at about 10 ft/s.
- Slug flow: gas above about 140 Mscf/d in 5-1/2 in casing gave poor or erratic pump fillage.
- Deviated and horizontal wells: separation changes with inclination, and the quotation sets the port position for the angle you state at setting depth.
The three thresholds are the findings of one Southwestern Petroleum Short Course study on downhole gas separation, for the tools it tested.
The natural gas anchor stops working when pump displacement exceeds the casing annulus capacity, about 650 bbl/d in 5-1/2 in 17 lb/ft casing around 2-7/8 in tubing. Slug flow above the threshold can still overwhelm it.
A poor-boy gas anchor or collar-size gas separator stops working when the pump rate exceeds the capacity of the annulus inside the barrel, about 175 bbl/d for a 2-7/8 in barrel around a 1 in dip tube. A larger barrel, where the casing drift allows, or a packer-type separator raises that limit.
The packer-type separator stops working when solids settle on the seal cup, or when the cup does not suit the well fluid and temperature. The seal cup is specified to the conditions on your enquiry, which is why the quotation asks for fluid, temperature and solids data.
Knowing these limits, the next question is which standards cover a gas anchor at all.
Which standards apply to a gas anchor, and what records should a supplier send
A gas anchor is not covered by a product specification of its own: API 11AX covers the sucker-rod pump and its standard fittings and excludes specialty accessories, so a gas anchor is specified by its connections, material and dimensions.
With the gas anchor outside the scope of API 11AX, the thread form of its end connections follows API 5B.
Those standards make a vendor's claims about a gas anchor something you verify in the part's own records. Ask any supplier for these five, and read each for what it proves:
| Record | What it proves |
|---|---|
| Material test report (MTR) | The chemistry and mechanical properties of the steel, tied to the mill heat it came from |
| Heat traceability | The heat number carried from the MTR onto the part, so the report belongs to the tube you received |
| Inspection and test plan (ITP) | Which checks are made at which stage of manufacture, and where your inspector can hold the job |
| Thread inspection record | The end connections gauged against the API 5B thread form named on the order |
| Certificate of conformity | The manufacturer's signed statement that the part matches the purchase-order specification |
Maximus OIGA Private Limited is the manufacturer, so it holds these records for every part it ships, and a question on any document goes to the plant that made the part rather than to a reseller. Each SpectraMax gas anchor order carries the documentation pack described next.
What documentation ships with a SpectraMax gas anchor
Each SpectraMax gas anchor order ships with a dimensional drawing, material test reports (MTR) traceable to heat numbers, a thread inspection record for the end connections, and a certificate of conformity to the specification on your purchase order.
An inspection and test plan (ITP) is issued with the order when your QA team asks to review it before manufacture. The documentation pack ships with the gas anchor, so the lead time below covers the paperwork as well.
How is a SpectraMax gas anchor supplied and delivered
Maximus OIGA Private Limited supplies the gas anchor itself: built to your specification at the Manjusar / Savli GIDC plant in Vadodara, documented, and delivered by export to order.
- Maximus OIGA Private Limited builds and validates the SpectraMax completion string in one plant, including packer systems, liner hangers and float equipment, so the gas anchor comes from the same shop as the rest of the string.
- Your pump supplier provides the sucker-rod pump, rods and pumping unit, and the gas anchor threads onto that string by the connection named on your order.
- Each order ships from Vadodara by export, with the commercial invoice, packing list and certificate of origin that the importing country's customs clearance asks for, packed with the documentation pack.
- Your rig crew or completion contractor runs the anchor on the pump string using the drawing and connection record shipped with it, and questions on the build go back to the engineer who quoted it.
Gas anchor questions engineers ask
What is the purpose of a gas separator?
A downhole gas separator keeps free gas out of the pump so the pump barrel fills with liquid. In a rod-pumped well, the gas anchor is that separator.
What is a gas-locked pump?
A gas-locked pump is a rod pump whose barrel is full of compressed gas, so the valves do not open and the pump moves no liquid. A gas anchor below the pump prevents gas lock by separating the gas before it enters.
What is a tubing anchor?
A tubing anchor, in full a tubing anchor catcher, holds the tubing still against pump strokes, which makes it a different tool from a gas anchor despite the shared name. The two often run on the same rod-pumped string.
Is a gas anchor the same as a downhole gas separator?
Yes. The two names describe the same tool and suppliers list it under either, so an enquiry for a gas separator on a rod-pumped well and an enquiry for a gas anchor reach the same SpectraMax configurations.
Where should a gas anchor be set relative to the perforations?
Below them, wherever the rathole leaves room for the pump intake. When the pump has to sit higher, the setting depth stays where the pump needs it and the barrel is sized to the pump rate instead.
If I buy the gas anchor from a specialist manufacturer, do I lose the integrated bundle?
No. A SpectraMax gas anchor is cut to the connection your pump supplier's string already uses, so the rest of the string stays as supplied; give the pump's bottom connection on the enquiry and the quotation confirms the thread.
What should a gas anchor quotation specify?
A gas anchor quotation should specify casing size and weight, tubing and pump connection, pump size and stroke rate, setting depth relative to the perforations, inclination, solids, and the documentation you need. Maximus OIGA Private Limited confirms dimensions, connection and lead-time band on the quotation.
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