Talk to us
Article / hero
Field guide

How to Shoot a Fluid Level on a Rod-Pumped Well

How an acoustic fluid level shot works on a rod-pumped well: the gas gun, collar echoes, depth from travel time, pump submergence, intake pressure and pitfalls.
By SRABS Engineering ·
Article / How is fluid level measured in a rod-pumped well?

How is fluid level measured in a rod-pumped well?

With an acoustic shot from the surface. A gas gun on the casing valve sends a pressure pulse down the casing–tubing annulus; a microphone records the echoes from each tubing collar and from the liquid surface. The time to the liquid echo, times the speed of sound in the annulus gas, halved, is the depth to the fluid. 125

The measurement has been around since the 1930s, 3 and it is still the standard for rod-pumped wells for a simple reason. Measuring pressure downhole means pulling the rods to run a gauge, which interrupts production, and permanent downhole sensors rarely pay for themselves on wells that produce at low oil rates. 1 A shot needs nothing in the well, takes minutes, and regulators in many places accept acoustic surveys for calculating well potentials and bottomhole pressures. 1

Article / Why the fluid level matters

Why the fluid level matters

The fluid level tells you how much liquid stands over the pump intake, its submergence, and from that whether the pump is outrunning the reservoir or leaving production in the ground. 3

The handbook on acoustic surveys states the "First Law of Pumping": in a pumped well, the reservoir cannot produce more liquid into the wellbore than the pump can displace into the tubing. 3 The fluid level is how you see which side of that balance a well is on:

  • Fluid near the intake: the pump is taking all the reservoir delivers and may be pumping off. That's where fluid pound starts.
  • Fluid high above the pump: the reservoir is delivering more than the pump moves. Either the pump is under-sized, or it is running below its displacement because of gas, wear or leaks. 3

The same source gives a rule of thumb for the second case: if the producing bottomhole pressure is more than 15% of static reservoir pressure, the well may be producing well below what the formation can supply. 3

Article / How a fluid level shot works

How a fluid level shot works

The gun sends a pulse into the annulus gas; every change in cross-section reflects part of it back. Tubing collars make a regular row of small echoes, and the liquid surface makes one large one. The round-trip time to each echo is the raw measurement. 25

schematic · acoustic fluid-level shot, gas-gun method
FIG. L Acoustic fluid-level shot, aligned with depthA gas gun on the casing sends a pressure pulse down the gas in the casing–tubing annulus. Each tubing collar reflects a small echo and the liquid surface reflects a large one. On the trace, aligned with depth, the collar echoes form a regular row of small kicks and the liquid level is the large kick where the row ends. The height of liquid above the pump intake is the pump submergence.FIG. LFLUID LEVEL SHOT — ALIGNED WITH DEPTHSURFACEANNULUS · GASLIQUIDTIME s ∝ DEPTH ftTRACEsubmergence1234DWG FIG. LSCALE NTSSRABS
FIG. L Acoustic fluid-level shot, aligned with depthA gas gun on the casing sends a pressure pulse down the gas in the casing–tubing annulus. Each tubing collar reflects a small echo and the liquid surface reflects a large one. On the trace, aligned with depth, the collar echoes form a regular row of small kicks and the liquid level is the large kick where the row ends. The height of liquid above the pump intake is the pump submergence.FIG. LFLUID LEVEL SHOTTRACETIME ∝ DEPTH ↓submergence1234
  1. 1gas gun on the casing: the pulse goes down the annulus gas
  2. 2collar echoes: a regular row of small kicks
  3. 3liquid level: the large kick where the collar row ends
  4. 4pump intake: the liquid above it is the submergence
  • The pulse. A gas gun mounts on the casing. In the usual mode it releases a charge of compressed gas from a chamber into the wellhead; in reverse, with the chamber at atmospheric pressure, well gas rushes into the chamber, which also creates the pulse. 5 CO₂ is a common charge gas. 6
  • The echoes. Reflections come from cross-sectional changes along the annulus: tubing collars, anchors and liners, holes, and the liquid surface. 5 A microphone housed in the gun records them. 2
  • The trace. The recorder plots echo strength against time. Each collar is a small, regular kick; the liquid level is the large kick where the collar train ends.

The pulse has to reach the liquid and come back strong enough to read. Gas pressure in the annulus affects how fast the pulse fades, 4 and sound barely travels in gas below atmospheric pressure. 1

Article / From travel time to depth

From travel time to depth

Depth to the liquid = average acoustic velocity × round-trip travel time ÷ 2. The hard part is the velocity, which changes with gas pressure, temperature and composition, so the best methods measure it on the trace itself. 2

The handbook lists four ways to get the average acoustic velocity: 2

  1. Count the collars. Collar echoes are spaced one tubing joint apart, so counting them down to the liquid kick (or timing a known number of them) calibrates the velocity in that well.
  2. Use a known reflector. An anchor, liner top or other feature at a known depth gives the velocity directly.
  3. Calculate it from gas properties. Gas gravity or composition, with pressure and temperature, through a correlation.
  4. Use experience. Past surveys in the same or similar wells.

The first two give the best estimates of distance to the liquid level. 2 In the field, counting collar kicks and multiplying by the average joint length is the everyday version of method 1. 6

Article / Step by step: shooting a fluid level

Step by step: shooting a fluid level

Mount, charge, record, fire, count, and repeat until the level is stable. 126

  1. Mount the gun on the casing valve and connect the recorder. 6
  2. Charge the gun with compressed gas. Respect it: a charged gun and a pressured casing are hazards at any pressure. 6
  3. Start the recorder, then fire. The trace has to be running before the pulse leaves the gun. 6
  4. Read the trace. Find the collar train and the large liquid kick at its end. Mark any other strong kicks (anchor, liner) before trusting the pick.
  5. Compute the depth from collar count or the velocity and travel time. 2
  6. Subtract from pump depth to get submergence. 3
  7. Shoot again after any change. After a speed or stroke change, take shots until the level stops moving before you judge the result. 6
Article / From fluid level to pump intake pressure

From fluid level to pump intake pressure

Submergence in feet isn't the same as intake pressure. Pump intake pressure is the casing pressure plus the weight of the gas column plus the weight of the liquid column over the pump, and that liquid column is usually gas-cut. 37

In the annulus of a producing well, gas overlies a column that is generally a mixture of gas and liquid, not solid liquid. 3 A gas-cut column is lighter than dead oil or water, so the same submergence means a lower intake pressure than a simple liquid gradient suggests. That is why converting a fluid level into pressures down the wellbore is its own analysis, 7 and why casing pressure and gas flow up the annulus have to be recorded with the shot.

Article / Common mistakes

Common mistakes

The usual errors are in reading the trace and choosing the velocity, and in reading a gas-cut column as solid liquid. 238

  • Picking the wrong kick. Anchors, liners and casing changes also reflect. Mark every strong kick, not just the first big one.
  • Assuming a velocity. Velocity varies from well to well and with depth in the same well. Calibrate it from collars or a known reflector whenever the trace allows. 2
  • Treating submergence as pressure. A foamy, gas-cut column can show high fluid over the pump while intake pressure is modest. 3 The handbook gives a whole section to misleading surveys in rod-pumped wells, including gradient inversion and pump fillage effects. 8
  • One shot after a change. The level keeps moving after a speed or stroke change. Repeat the shot until it is stable. 6
  • Too little pressure in the pulse. A weak pulse in a deep or low-pressure well fades before it reaches the liquid. 14
  • Ignoring the tubing. A shot down the tubing, repeated over time, can reveal a tubing leak or an ineffective pump. 9
Article / Periodic shots vs. continuous measurement

Periodic shots vs. continuous measurement

A shot is a snapshot. It shows where the fluid was when the gun fired, which is enough for a survey but not for control. Holding a pump at the right submergence as inflow changes takes a level that is always current.

Periodic shots suit diagnosis: after a pull, after a speed change, on a schedule. They fall short when the answer has to drive a decision every few minutes: a variable-speed drive that should hold submergence can't do it from last week's shot. Continuous measurement turns the fluid level from a survey into a control input.

That is the role of SRABS Echo in the SRABS system: it reports continuous bottom-hole submergence without SCADA integration. See the system for how it fits with the pump and the control layer, and the fluid pound and gas lock guides for what a current fluid level lets you prevent.

Article / The bottom line

The bottom line

A fluid level shot is the cheapest look downhole a rod-pumped well gets. Calibrate the velocity on the collars, pick the liquid kick carefully, and convert to intake pressure with the gas in mind. When the level has to drive control instead of a report, it needs to be continuous.

If you're running wells on a timer and an occasional shot, test continuous submergence control on one well.

Loan us a well →
Article / FAQ

FAQ

How is fluid level measured in an oil well?

Usually with an acoustic shot: a gas gun on the casing sends a pulse down the annulus, and the echo from the liquid surface, timed and converted with the speed of sound in the annulus gas, gives the depth to the fluid.

What is pump submergence?

The height of liquid in the annulus above the pump intake: pump setting depth minus the depth to the fluid level. It tells you whether the pump is starving or the well could be produced harder.

How do you calculate depth from a fluid level shot?

Depth equals average acoustic velocity times round-trip travel time, divided by two. The velocity is best calibrated on the trace itself, by counting tubing collar echoes or using a reflector at a known depth.

Why count tubing collars on a fluid level trace?

Collars are one joint apart, so their echoes calibrate the speed of sound in that well's annulus gas. That gives a better depth than a velocity calculated from assumed gas properties.

Can a fluid level shot be wrong?

Yes. Common causes are picking an anchor or liner echo as the liquid, assuming the wrong acoustic velocity, and reading a foamy, gas-cut column as solid liquid when converting to pressure.

Is pump submergence the same as pump intake pressure?

No. Intake pressure also includes casing pressure and the gas column, and the liquid over the pump is usually gas-cut, so the same submergence can mean very different intake pressures.

Article / Sources

Sources

  1. Acoustic Fluid Level Measurements in Oil and Gas Wells, Chapter 3, "Fundamentals of Acoustic Fluid Level Surveys," Petroleum Extension Service (PETEX). https://utw10286.utweb.utexas.edu/images/flippingbooks/acoustic_fluid_measure/files/basic-html/page10.html
  2. Acoustic Fluid Level Measurements in Oil and Gas Wells, Chapter 5, "Methods for Determining Distance to the Liquid Level," Petroleum Extension Service (PETEX). https://utw10286.utweb.utexas.edu/images/flippingbooks/acoustic_fluid_measure/files/basic-html/page12.html
  3. Acoustic Fluid Level Measurements in Oil and Gas Wells, Chapter 8, "Applications of Fluid Level Measurements to Pumping Wells," Petroleum Extension Service (PETEX). https://utw10286.utweb.utexas.edu/images/flippingbooks/acoustic_fluid_measure/files/basic-html/page15.html
  4. Acoustic Fluid Level Measurements in Oil and Gas Wells, Chapter 4, "Acoustic Fluid Level Equipment and Procedures," Petroleum Extension Service (PETEX). https://utw10286.utweb.utexas.edu/images/flippingbooks/acoustic_fluid_measure/files/basic-html/page11.html
  5. US Patent 5,834,710, "Acoustic pulse gun assembly" (1998). https://patents.google.com/patent/US5834710
  6. "Shooting Fluid Levels," Oily Stuff, 20 December 2019. https://www.oilystuff.com/single-post/2019/12/20/shooting-fluid-levels
  7. Acoustic Fluid Level Measurements in Oil and Gas Wells, Chapter 6, "Calculating Wellbore Pressure Distribution from Acoustic Fluid Level Surveys," Petroleum Extension Service (PETEX). https://utw10286.utweb.utexas.edu/images/flippingbooks/acoustic_fluid_measure/files/basic-html/page13.html
  8. Acoustic Fluid Level Measurements in Oil and Gas Wells, Index ("misleading acoustic level surveys," "gradient inversion," "pump fillage" under rod-pumped wells), Petroleum Extension Service (PETEX). https://utw10286.utweb.utexas.edu/images/flippingbooks/acoustic_fluid_measure/files/basic-html/page27.html
  9. Rowlan, L., Fernandez, G., "Troubleshoot Oil and Gas Wells Using Acoustic Level Shots," Southwestern Petroleum Short Course. https://www.swpshortcourse.org/paper/2022026-troubleshoot-oil-and-gas-wells-using-acoustic-level-shots