SOLID SLUG TESTING
Tech Note by: Adam Hobson
Solid slug testing is a straightforward and cost-effective method for estimating hydraulic conductivity in wells. It involves quickly inserting or removing a solid object—a slug—to create a sudden change in water level, and
then measuring how the water recovers back to static conditions. With a high-resolution pressure transducer like the Level TROLL 700, you can collect reliable water level data that can be used to estimate formation
hydraulic conductivity with confidence.
A solid slug test initiates a rapid change in head (water level) by physically inserting or removing a slug into/from the water column. This can be done as a falling head test (slug in)or a rising head test (slug out). The water level disturbance causes flow between the well and surrounding formation. The Level TROLL 700 records the water level change over time, which can then be analyzed using theoretical models to estimate the near-well hydraulic conductivity (K) of the formation. Figure 1 shows a schematic of the different stages of a solid slug test and Figure 2 shows an example of water level changes during a solid slug test.
Figure 1. Schematic of the stages of a solid slug test
Figure 2. Example of water level changes during a solid slug test
Equipment
The equipment for solid slug testing consists of the slug, to change the water level, and a means to
measure the change in water level. This typically includes:
Level TROLL 700 pressure transducer
Rugged Twist-Lock Cable
TROLL Com Plus
Smartphone or tablet (iOS or Android) with VuSitu application installed
Water Level Meter 100 or 200
Well dock or other means to secure the cable
Solid slug, sized appropriately for test
Solid Slugs
Solid slugs are often made from metal or PVC pipes filled with sand, but solid versions are also used.
Solid slugs are available commercially or can be constructed. To make a solid slug, obtain a section of
PVC pipe that will fit down the well allowing space for the pressure transducer cable. Cap one end
and fill the pipe with sand or other material so that it will sink. Cap the other end and place an eye bolt
on one end to attach a rope or cable to lower and raise the slug. Alternatively, manufactured slugs
can be purchased or rented.
Instrument Considerations
Water level measuring devices for slug testing need to be accurate, fast, simple to setup and use, and
efficiently manage data. Pressure transducers, such as the Level TROLL 700, along with the VuSitu
mobile app, are well suited for slug testing applications.
Range & Accuracy – Water level measurements for slug testing typically require a high level of
accuracy to capture small changes. Level TROLLs have an accuracy of 0.05% full-scale and are
calibrated across the full pressure and temperature range of the instrument, meaning that if the unit is
within its operating range, it will maintain its stated accuracy. For the highest degree of accuracy, it is
recommended to select the smallest pressure range sensor that will accommodate the maximum
depth the sensor will be exposed to. Additionally, Level TROLL 700 sensors are offered as vented
sensors, which is preferable for high accuracy applications. While non-vented sensors can be used for
slug testing, changes in barometric pressure will cause changes in water level readings which could
lead to errors in analysis. If a non-vented sensor is used, barometric pressure must also be measured
with a second device and subtracted from the non-vented sensor reading. Since two devices are
used, the accuracy of both must be considered, further reducing the accuracy of the final water level
measurement.
Logging Rate and Data Storage – Water levels can change rapidly when conducting a slug test.
Tests run in moderate to high K formations can recover within seconds or minutes, depending on the
initial displacement. The Level TROLL 700 has a logging rate of up to 4 times per second and an
onboard data storage capacity of 250,000 data records, making it ideal for rapid data detection.
Simple to Set Up and Use – The Level TROLL 700 works with the VuSitu mobile app, which allows for
easy setup of the device and logging, viewing of real-time data during a test, and efficient data
management.
Water level meters, such as the Water Level Meter 100 or 200, are another common tool for
measuring water levels during a slug test. They are simple to use and are needed to measure the
initial depth to water. However, manual readings often cannot be made fast enough to adequately
capture water level changes during a slug test, and data collection requires writing or typing in
readings and times which can be inefficient and prone to errors.
Setup and Test Initiation
Determine Level TROLL position
- Ensure the well is properly developed and free of obstructions.
- Measure and record the depth to water (DTW) with a Water Level Meter 100 or 200.
- Measure and record the depth to bottom of well (total depth, TD ).
- Calculate the slug volume for a cylindrical, solid slug by measuring the radius and length of the slug.
- Calculate the expected initial displacement from the slug volume and inside casing radius.
- Calculate static length of water column in the well from total depth and depth to water:
- Calculate the expected maximum height of water in the well (Lwc max):
- Determine the appropriate position of the Level TROLL 700 within the well. The pressure transducer should be placed sufficiently below the top of the water column to avoid interference with the slug and to not exceed its depth limit (Figure 1). The Level TROLL should not be placed at the very bottom of the well where it could encounter silt.
Deploy the Level TROLL
- Connect the Level TROLL 700 instrument and TROLL Com Plus to the Rugged TwistLock Cable. Prior to deployment, confirm all physical and digital connections. Check
that all Twist Lock Connectors are properly seated and tightened. Use the VuSitu
mobile app to connect to the TROLL Com Plus and the Level TROLL and confirm
communication with the Level TROLL. - Submerge the Level TROLL to the pre-determined depth. Pro Tip: Match the end of
pressure transducer with the end on the water level tape and lower into well
simultaneously to estimate position of transducer or use the depth reading in Live
Readings View. - Using a well dock or other means, secure the Rugged Twist-Lock Cable to the top of
the well so that it cannot move. Any movement by the Level TROLL instrument is
undesirable and will be recorded in the data as a change in pressure/level. - Wait at least 20 minutes for the transducer to thermally equilibrate, for the cable to
relax, and let the water level return to its original height after the Level TROLL was
placed in the water. Confirm DTW with Water Level Meter.
Set Up a Log using VuSitu
Using VuSitu on a mobile device (Android or iOS), create a new log on the Level TROLL instrument. In VuSitu, navigate to logging and create a new log. Follow the on-screen instructions and pay careful attention to the sections highlighted below. For solid slug testing, some key considerations in the log setup are:
Set Level Mode: From the Set Level Mode screen, set the mode to Surface Elevation and select Set First Reading to 0 (or any other point that you want) to view the relative water level changes during the test. With Surface Elevation, the reading is directly proportional to the height of water above the sensor (depth) such that an increase in one causes an increase in the other.
With slug tests, water level is often measured as the change from static conditions, which often is set as zero. In this scenario, an increase in water level (slug-in test) will appear as a positive number and a decrease in water level (slug out) as a negative.
Logging Method and Frequency: Linear logging is recommended for most slug tests as data are recorded at a constant frequency,
which allows recording of multiple tests (slug in and slug out) with one log. For high to moderate hydraulic conductivity formations
that will respond quickly, consider using a data recording frequency of 1 reading per second or faster.
The Level TROLL 700 can record data as fast as every 250 milliseconds (msec) or 4 times per second. For extremely slow responding formations, such as aquitards that may take hours to
recover, the data recording frequency can be reduced.
Start and Stop Method: Choose a manual start and stop
method to allow flexibility with when the test starts and stops.
Running the Test
- Ensure that the Level TROLL 700 has been installed in the well such that it will remain submerged for the duration of the test, that it will avoid contact with the slug, and that it is off the bottom of the well.
- For falling head (slug in) tests, place the bottom of the slug just above the water level. Measure and mark the drop distance on the cord and fasten the cord prior to test. For a rising head (slug out) test, place the top of the slug just below the water level. Measure the removal distance and prepare to fasten the cord after removal.
- Start the internal data log. Begin the log at least 30 seconds prior to initiation for moderate to high K formation.
- For less permeable formations, pretest monitoring should last hours or days to recognize background head trends.
Pro tip: Insert or remove the solid slug in one quick, smooth motion to create a near instantaneous water level change. In moderate to low hydraulic conductivity formations (< 2 x 10m/s), initial displacement should be between approximately 0.3 and 0.9 meters to produce a reasonable signal-to-noise ratio. In higher hydraulic conductivity formations, a smaller initial displacement should be used to minimize non-Darcian (i.e., turbulent) flow losses. Note that slug size, and therefore initial displacement, does not affect the test radius.
Data Visualization
During the test, the “Live Readings View” in VuSitu can be used to see the water level data either as a value or as a chart with time. Live Readings allows water level recovery to be visualized and assessed in real-time and permits switching between the list view and the graphical view of the data.
Data Transfer
Once the log is stopped, it can be downloaded by selecting the log and tapping download. The log is saved to the mobile device and can be exported or shared via email, data sharing or other file transfer tools. If the user is logged into HydroVu and the device has an internet connection, the data will be automatically uploaded to the user’s account with Cloud Connect. Once exported, the data can be analyzed by specialized software or a custom solution.
References: Butler, J.J., The Design, Performance, and Analysis of Slug Tests, 2nd Edition, CRC Press, Taylor & Francis Group, 266 pp., 2020. Midwest Geosciences Group, Field Guide for Slug Testing and Data Analysis, www.midwestgeo.com, 2007. Accessed October 2025.