Frequent Signal Loss at Over 200 Meters? How the Inspection of More Than 40 Wells Was Completed
About 3 months ago, Mr. Liu, the general manager of a third-party company in Northwest China, contacted us.
They were preparing to undertake a well condition inspection project involving more than 40 wells, with well depths ranging from more than 60 meters to more than 200 meters. The project was stuck because of the equipment—the ordinary downhole camera he had could barely work in shallow wells, but once it reached deeper wells, the signal frequently cut out, making it uncertain whether the project could be secured.
Later, after switching to the AIMOLI AML921 digital downhole camera, Mr. Liu successfully secured the project.
In the end, the inspection of all 40+ wells was completed, with the images from each well exported together with depth scales, which could be directly submitted to the reviewing party for verification and also provided a solid foundation for subsequent repair and remediation.
What is really worth paying attention to in this case is not simply “which equipment was used,” but a practical well condition inspection question:
Why did the ordinary downhole camera frequently lose its signal at more than 200 meters? What problem did the digital downhole camera actually solve? And how could the data from the well condition inspection meet the requirements for subsequent review and repair and remediation?
I. Frequent Signal Loss at Over 200 Meters: The Problem Is Not Simply “Depth”
Let’s start with an easily overlooked fact: more than 200 meters is not considered a deep well in the industry.
Conventional equipment usually does not experience systematic image distortion until depths of several hundred meters or more. For Mr. Liu’s 40+ wells, the deepest was only more than 200 meters, yet the signal frequently cut out. This indicates that depth was not the only variable.
1. Why Does an Ordinary Downhole Camera Frequently Lose Signal in Deep Wells?
The root cause lies in the transmission method.
Most ordinary downhole cameras use analog signals: high-frequency components attenuate faster over long cables, while signals experience attenuation and distortion along the cable, together with group-delay distortion. By the time the signal reaches the surface, the result may be a scrambled image, snow, or signal interruption.
As depth increases, the signal-to-noise ratio drops another level.
Extending the cable solves the problem of “whether it is long enough,” but it cannot solve the problem of “whether the signal-to-noise ratio is sufficient.”
Digital transmission takes a different approach: the image is first converted into a digital signal downhole and then transmitted upward. It does not attenuate with distance in the same way, and the data can also be stored and remotely reviewed.
For projects involving downhole camera inspection, monitoring well condition inspection, and well casing condition inspection, this difference affects not only the image quality at the site, but also whether inspection data can be stored, exported, and reviewed.
2. In Addition to the Transmission Method, There Are 4 Hardware Variables
Signal interruption in a downhole camera does not necessarily have only one cause, such as “the cable is too long.” In actual projects, the following four hardware and field-environment variables can simultaneously affect inspection results:
| Variable | Impact Mechanism |
|---|---|
| Cable diameter and number of cores | Directly determines attenuation resistance. A common problem with small-diameter two-core armored cables is that long cables experience rapid signal attenuation and weak interference resistance |
| Connectors and sealing | Water pressure in deep wells increases linearly with depth. Failure of connector sealing can directly interrupt the signal or even damage the probe |
| Probe pressure resistance and sealing | Housing material and sealing structure determine the upper limit of usable depth |
| Wellbore environment | Gravel and metal debris on the well wall can wear the cable sheath; corrosion of the steel wire armor layer can damage the internal conductors |
Therefore, image interruption is usually not caused by a single item exceeding its limit, but rather by:
the cable, connectors, sealing, and environment all approaching their critical limits at the same time.
This is also why, when determining whether a downhole camera system is suitable for an actual engineering project, it is not enough to look only at the “maximum working depth.” The transmission method, cable configuration, probe pressure resistance, sealing structure, and actual wellbore environment must also be considered.
II. What the Client Needed Was Not Just Video, but Reviewable Digital Data
Well condition inspection is not merely an “optional extra.”
DZ/T 0307—2023, Code for Operation and Maintenance of Groundwater Monitoring Network, requires that well depth be remeasured at least once a year; every 5 years, it is recommended to use visual equipment that can enter the well casing to inspect the condition of the well casing once; the well should be cleaned once every 5 years; and one of the criteria for qualified well cleaning is that visual inspection shows that the inner wall of the well casing is smooth and the sediment in the well is less than 30 cm.
This means that the value of well condition inspection is not simply “filming the well,” but generating inspection data that can support subsequent judgment.
1. There Are Quantitative Criteria for Well Casing Damage, Misalignment, and Sedimentation
What is more stringent than “having an image” is the quantitative criteria.
DB41/T 2321—2022, Technical Specification for Borrowed Wells for Groundwater Monitoring Stations (Henan Province local standard, implemented on 2022-12-14, currently in force) stipulates in Chapter 5:
| Inspection Item | Judgment Criteria | Recommended Action | Clause |
|---|---|---|---|
| Well casing damage | Single damage length >1 m, or cumulative damage length >5 m | Not suitable for borrowing; repair required | 5.2.2 |
| Well casing misalignment or separation | Determined as such if present | Not suitable for borrowing | 5.2.2 |
| Sedimentation in the well (based on completed well depth) | ≤5% / 5%~25% / >25% | Directly available for borrowing / Available for borrowing after sediment removal / Not suitable for borrowing | 5.3.2 |
Clause 5.4 of the specification also stipulates that the water-permeability sensitivity test shall be conducted in accordance with SL 183—2005, and wells that meet the requirements may be directly borrowed without well cleaning.
2. Why Is a Depth Scale So Important?
This means:
The thickness of sedimentation and the length of damage cannot be determined based on experience or guesswork; they need to be presented as verifiable numbers.
Therefore, in downhole camera inspection, simply “having video” does not mean that the data is usable.
If image clarity is insufficient, abnormal locations cannot be accurately identified, or the video does not have corresponding depth scales, it will be difficult for subsequent reviewers to verify the specific abnormal locations.
Whether the image is clear enough can therefore directly determine whether the project can pass acceptance.
This was exactly the part that had held Mr. Liu back in the beginning.
For third-party inspection companies, a suitable downhole camera inspection system needs to solve more than just “whether it can go down the well.” It also needs to answer:
- Can it obtain stable downhole images?
- Can it accurately correspond to well depth?
- Can it completely record abnormal locations?
- Can it save and export the data for each well?
- Can it allow the reviewing party to conduct verification?
III. After Switching to AML921
The equipment we used this time was the AML921 digital downhole camera.
The following are our product parameters:
| Item | Our Product Parameters |
|---|---|
| Probe | 316 stainless steel housing, diameter ≤50 mm |
| Camera | 360° annular viewing window + downward view, 2 megapixels |
| Maximum Working Depth | 500 m (deeper requirements can be customized according to the project and confirmed by the technical agreement) |
| Standard Cable | 100 m composite cable (built-in load-bearing steel wire rope, polyurethane sheath, meter-marked scale) |
| Applicable Water | Maximum allowable turbidity 50 NTU |
| Operating Temperature | -25 ℃ ~ 60 ℃ |
1. How Were the 40+ Wells Inspected on Site?
The field process was not complicated:
Step 1: First, compare the depth scale in a shallow well and calibrate the readings.
Step 2: Then lower the equipment into each well according to the well-depth sequence.
Step 3: During the inspection, record abnormal locations while recording the video.
Step 4: Complete the image records for each well and organize them according to well number.
In the end, the inspection of 40+ wells was completed one by one, and the images with depth scales were exported for each well and could be directly submitted to the reviewing party for verification.
For this type of batch well condition inspection project, whether the equipment can stably complete continuous operations is very important.
The reason is that the project does not involve inspecting just one well, but 40+ wells. If the equipment frequently loses signal, the images cannot be completely saved, or the depth information cannot be matched, a problem with an individual well may ultimately become a data delivery problem for the entire project.
IV. Where Does the Equipment Cost More, and Where Is the Value?
Afterward, Mr. Liu calculated two costs with us.
First Calculation: Price
AML921 was somewhat more expensive than the ordinary downhole camera he had originally used.
This is a real difference in acquisition cost.
Second Calculation: Project Delivery and the Value of Subsequent Data
The project was successfully secured, and the inspection data from 40+ wells was accurate, providing a very good foundation for the subsequent repair and remediation project.
His exact words were:
“Worth every penny.”
The second calculation is the key.
The value of well condition inspection does not lie in “shooting several sections of video,” but in whether it can support the next decision:
- Which well needs cleaning?
- Which well needs casing repair?
- Which well’s filter pipe is already completely blocked and needs to be abandoned and reconstructed?
Only when the images are clear, the depth scale is accurate, and the data can be exported for verification can the subsequent plan have a basis.
The extra money spent on the equipment is not in the same order of magnitude as the cost of one wrong decision or one rework.
Therefore, for projects that involve third-party inspection, engineering acceptance, or subsequent repair and remediation, when selecting a downhole camera, it is not enough to compare the purchase price. It is also necessary to comprehensively consider:
inspection depth, signal stability, image quality, depth scale, data export, and batch operation capability.
V. What Can AML921 Do, and What Can It Not Do?
Every downhole camera has clearly defined application boundaries.
Understanding what the equipment can do is equally important as knowing what it cannot do.
1. It Does Not Support Well Deviation Measurement
This function is currently under development. At present, a separate device is required if well deviation data is needed.
Therefore, if a project also requires well deviation data, AML921 alone cannot complete the task.
2. It Cannot Replace Core Sampling
The downhole camera is good at identifying:
- Fracture development;
- Locations of fractured zones;
- Macroscopic characteristics of filling materials.
However, mineral composition and mechanical parameters still require core samples.
Therefore, downhole camera inspection and core sampling are different testing methods and cannot completely replace each other.
3. It Is Not Designed for Ultra-Deep Wells
Kilometer-level exploration wells and oil and gas wells belong to another category of products and require a different model.
The application range of AML921 should be based on the actual product parameters and project technical agreement. A standard well condition inspection device should not be directly regarded as specialized equipment for ultra-deep wells or oil and gas wells.
Frequently Asked Questions (FAQ)
1. How Often Should a Monitoring Well Undergo Visual Well Condition Inspection?
DZ/T 0307—2023 recommends using visual equipment to inspect the well casing once every 5 years, while also cleaning the well once every 5 years; acceptance of newly constructed wells and investigation of abnormal water quality are scenarios that can trigger inspection at any time.
Therefore, in actual inspection frequency, in addition to referring to the requirements of the specification, it is also necessary to determine the timing based on specific scenarios such as newly constructed well acceptance, operation and maintenance, and abnormal water quality.
2. At What Level of Sedimentation Does a Well Need Sediment Removal?
According to Clause 5.3.2 of DB41/T 2321—2022, sedimentation thickness is calculated based on the completed well depth:
- ≤5%: Can be directly borrowed without sediment removal;
- 5%~25%: Sediment removal is required before borrowing;
- >25%: Not suitable for borrowing.
The threshold must be based on the depth scale of the visual equipment.
This is also why a downhole camera needs to provide not only clear images, but also accurate depth information.
3. Does Image Interruption Always Mean That the Depth Is Too Great?
Not necessarily.
In this project, the deepest well was only more than 200 meters, yet the signal frequently cut out—the cable diameter, connector sealing, probe pressure resistance, and wellbore environment often approached their critical limits simultaneously.
Ordinary analog transmission had already experienced significant attenuation at this length. Simply extending the cable was usually ineffective:
The length was sufficient, but the signal-to-noise ratio did not keep up.
Therefore, when troubleshooting image interruption in a downhole camera, the cause cannot simply be attributed to “the well is too deep.”
4. How Deep Can a Downhole Camera Go?
It depends on the model and configuration.
The AML921 used in this project has a rated maximum working depth of 500 m; for deeper wells, the cable configuration needs to be customized according to the project, and the rated value should not be treated as the actual usable depth.
The actual usable depth also needs to be confirmed based on the specific well conditions, cable configuration, probe performance, and project technical agreement.
5. Can a Downhole Camera Replace Core Sampling?
No.
A downhole camera is good at identifying:
- Fracture development;
- Locations of fractured zones;
- Macroscopic characteristics of filling materials.
However, mineral composition and mechanical parameters still require core sampling.
Therefore, if a project needs to obtain core information such as mineral composition and mechanical parameters, corresponding core sampling and testing methods are still required.
6. Is a Downhole Camera a Method Recognized by the Specifications?
Yes.
The terminology and definitions section (Clause 3.3) of DB41/T 2321—2022 directly includes the term “downhole camera”; DZ/T 0307—2023 recommends using visual equipment to inspect well casing conditions once every 5 years.
From the perspective of practical application, the value of a downhole camera is not simply to obtain video footage from inside the well, but to provide a basis for well condition assessment, review and verification, and subsequent repair and remediation through visual images + depth information.
Data Sources and Scope
The standards referenced are DZ/T 0307—2023, DB41/T 2321—2022, HJ 164—2020, and SL 183—2005; AML921 parameters are our published product parameters, and purchases should be subject to the technical agreement.
The customer’s information in this case has been anonymized, and the time is presented using a relative description.
