FIBER OPTICS IMPROVE DOWNHOLE VIDEO

Phillip K. Schultz
Westech Inc.
Ventura, Calif.

Charles Cobb
Otis Engineering Corp.
Dallas

Fiber optics provide downhole videos with a tremendous bandwidth, permitting a rugged, small, 7/32-in. OD cable to be used in long lengths, even in wells with high surface pressure.

Run on these fiber optic cables is a small diameter, 1 11/16-in. OD camera that can operate in most oil field tubulars. The cameras are constructed of heat-treated, quality alloys and improved ocular materials that permit operations in 10,000 psi environments.

The cameras have lens polishing compounds that prevent oil and condensate buildups even after lengthy encounters with these fluids.

With these developments, real-time fiber optic downhole video systems are capable of providing excellent quality video pictures of deep well conditions.

Deployed by wire line or coiled tubing, the downhole video provides for visually inspecting and recording conditions that have in the past largely been left to speculation.

Applications include:

Inspecting perforations, casing, and tubing
Viewing fluid injection, and oil and gas production
Locating downhole fish.

As the cliche goes: "A picture is worth a thousand words." A real-time moving picture is worth even more.

SYSTEM DEVELOPMENT

The first downhole cameras were used primarily in shallow water wells in the 1970s. Having vision downhole proved to be very helpful, hence using cameras in the water well industry became commonplace.

The earliest downhole video systems used in the petroleum industry were modified water well systems. There were many limitations; resolution or picture quality was modest, and the frequency of equipment failure was considerable. However, the images of downhole conditions that were recorded were not available through any other means.

Real-time viewing of natural fractures, gas and oil production, fluid flows while injecting, casing failures, leaks, and downhole fish was finally available. The results were intriguing.

A plan to design and build a downhole video system specifically designed to operate in an oil and gas well environment was developed in the early 1980s. One of the first oil field downhole video systems had a target depth of 10,000 ft, a 2 1/4 in. overall diameter, which was small enough to traverse 2 7/8-in. tubing, and a maximum working pressure of 5,000 psi.

The telemetry system enabled transmitting a good quality video signal along a 10,000-ft length of 7/16-in. coaxial logging cable. The new 2 1/4-in. tools proved to be much more reliable than their predecessors, and by the mid-1980s much progress had been made.

In the early video systems, acrylic materials were used as a protective cover for the light source. After testing various materials, including quartz, polycarbonate, and high-temperature, chemical-resistant glass, quartz was found to provide a much better protective cover for the camera's light source.

Quartz was also found to be superior as the optical port. This resulted in ocular material able to work in 10,000-psi environments.

By using newly developed miniature cameras with state-of-the-art electronics and high-strength steel housings, the camera assembly diameter has been reduced to 1 11/16 in. and the maximum working pressure increased to 10,000 psi.

DOWNHOLE TOOLS

The downhole video camera is comprised of four subassemblies: the cable head, electronic tray, housing or pressure barrel, and the light head (Fig. 1).

The cable head with fishing neck on top includes a cable packoff and electrical connectors. The electronic tray includes the electronics package and camera. The housing has internal supports for the electronic tray and an optic port in the lower end.

The light head attaches to the lower end of the housing. The light source is generally provided by either a series of lamps placed around the lens (ring head), or a set of rods that extend a lamp out in front of the lens (standard light head) (Fig. 2a).

The ring head (Fig. 2b) is often used on fishing jobs. However, it is not well suited for working in fluid because particulate matter suspended in the fluid will cause glare that could result in poor visibility.

Another drawback of the ring head is the lamps, placed around the camera lens. These create additional heat that limits operational time.

In smaller diameter cameras, a light-emitting diode (LED) ring head can be used to greatly reduce glare and heat.

The standard light head is most commonly used when working in fluid or when inspecting the walls of the well, casing, or tubing. The distance between the optical port and the light source is determined by the internal diameter to be viewed.

Often it is desirable to view a small diameter such as production tubing and a larger diameter such as casing during the same logging run. This has led to the development of a double light configuration (Fig. 2c).

The light closest to the optical port is used when viewing the tubing. When entering the casing, the downward facing light is accessed. Light heads are usually dc powered through conductors in the cable, and their intensity is adjustable from the surface.

SURFACE EQUIPMENT

The surface video-related equipment is powered from a 110-120 y ac power source, usually a 3-5 kw diesel engine powered generator set. Components included in the surface equipment package include:

Two television monitors to display downhole images in real-time or display recorded images as required. The camera depth is normally continuously displayed on the monitors while the camera is in the hole.
Two video cassette recorders (VCRs) to record downhole images continuously on standard or commercial grade video tape while the camera is in operation.
A video typewriter with which the operator can write messages or pertinent data on the monitors and tape during or after video logging. With two monitors, two VCRs, and the typewriter, video tapes can be edited and reproduced as required on location or in the yard.
A title generator to generate the numbers and letters for display and recording.
A power panel to provide and control electrical power for all electrical components in the system. This also includes a control for the intensity of the downhole light head.
A depth panel to provide the depth signal for display on the monitors. This includes controls for adjusting or zeroing the depth counter.
A slip ring contactor to provide a revolving contact to transmit electrical power from the source to the cable on the drum, and a contact to transmit the video signal from the cable to surface equipment.
Surface pressure control equipment that includes lubricator sections, usually in 8-ft sections, blowout preventers to seal around the cable, and a manual packoff or grease-seal stuffing box. Manual packoffs or "stripper heads" are normally used on wells with up to 1,000 psi surface pressure. A grease-seal stuffing box is normally used on higher pressure wells. A grease injection pump is required with the grease-seal stuffing box.
The logging (wire line) unit can either be truck or skid mounted and usually is hydraulically powered. Maximum line speeds are normally about 200 fpm. Normal viewing speed is about 20 fpm.

OPERATIONS

When using downhole video, a transparent medium is required. In gas wells this is usually not a problem. However, condensation in gas wells has been known to impair viewing.

In oil wells or during drilling, the opaque fluid must be displaced. This is most commonly done by pumping filtered brine. However, other transparent mediums are used such as No. 1 diesel, nitrogen, or sea water.

Recent developments include the use of coiled tubing (Fig. 3) to lower the downhole camera into the well. In this application, the cable is threaded through the coiled tubing. This not only allows for ease of operation in high angle or horizontal wells, but also provides a means for displacing opaque or dirty fluids at the desired point of inspection.

Frequently, oil is encountered during the logging run even while displacing with a transparent medium. Historically this has been a problem especially when working in wells producing heavy oil.

A recently developed lens-polishing compound is providing astonishing results. During actual job site testing, the downhole camera is lowered into a well, and the well is then put on production.

During a test, water and oil were visibly seen passing the camera, and often only pure oil was encountered. At other times, transparent water only was observed.

Over the 5-hr test period, the lens and light dome remained clean.

In another well, a 2,000-ft section of oil was traversed and once a transparent medium was encountered, perfectly clear video images were seen. Again, 5 hr of downhole operation occurred with no oil adherence to the optic port or light dome.

Further lab testing indicated that the lens polish will also improve visibility when working in gas-fluid wells by eliminating or greatly reducing condensate buildup on the optic port.

VIDEO APPLICATIONS

Any downhole situation where a visual inspection or observation is desirable could be considered an application. Some of the more common applications are:

Leak detection - Normally the resolution of the camera is good enough to detect the turbulence created by a leak or to identify different fluids leaking into the well bore. Particulate matter flowing out through a hole is usually easily detected.
Damaged tubulars - Parted or collapsed tubing and casing usually appear distinctly in the video monitor.
Scale buildup - Video logs clearly indicate the severity of scale buildup in downhole tubulars, flow control devices, perforations, and locking recesses in landing nipples.
Formation fractures and their orientation-Video logging provides visual images of the size and extent of formation fractures. A gyroscope can be run in conjunction with the video log to provide directional information.
Fishing operations - Fishing is one of the most widely used applications for downhole video service. Operators use downhole video to identify the fish and usually shorten their fishing job.
Perforation inspection - Downhole video has been used to observe perforations for plugging. Perforations can be observed while the well is flowing or while fluids or gas are injected through the perforations.
Corrosion surveys - Observing corrosion damage in downhole tubulars with real-time video avoids having to do after-the-job interpretation of collected data.
Lost production-Real-time viewing with video can identify causes for loss of production such as sand bridges, fluid invasion, or malfunctioning downhole flow controls.

TELEMETRY

A formidable problem continued to prevent the operational adaptability of the system. Existing technology could provide for smaller cameras, higher pressure ratings, better methods of lighting, heat shielding, and improved optics.

In theory, nearly any well was within the technological boundaries afforded. However, in practice, the telemetry system limited usefulness.

When running in wells up to 10,000 ft, the best real-time coaxial downhole video system requires a 7/16-in. diameter coaxial cable. Wells deeper than 10,000 ft and up to approximately 13,000 ft require a 1/2-in. coaxial cable.

As illustrated in Fig. 4, the force created by the well pressure acting on the cross-sectional area of a 7/16 in. or 1/2-in. cable is extremely difficult to overcome, even with modest wellhead pressure.

Operationally, running large diameter cables like these presents expensive, nearly insurmountable, obstacles. For example, Fig. 4 indicates that when running in a well with 2,000 psi at the surface, using a 1/2-in. cable and a 1 11/16-in. tool string, at least 36 ft of high-density tungsten weight bars will be needed to overcome the pressure. A 55-ft lubricator and a crane large enough to handle the bars will also be required.

However, even with the problems caused by the large diameter coaxial cable, the real-time images provided by a video system were preferred over a system that provided a series of still pictures.

In 1989, a development project was undertaken to address this telemetry issue. Fiber optic telemetry appeared to offer solutions to most of the problems inherent with coaxial telemetry.

In August 1989, a battery-powered prototype system was completed. The initial test well runs were made to an approximate depth of 4,000 ft. With minor adjustments, the system performed well.

The next step included some actual well runs on the Alaskan North Slope. Various problems were encountered that were primarily attributed to the battery power. However, the overall fiber optic concept worked reliably in these actual well runs. Very good pictures of perforations and tubing were recorded, and a coiled tubing fish was identified at approximately 12,000 ft.

The final run attempted with the prototype system resulted in the failure of the prototype cable.

From the prototype system it was confirmed that the cable must include a power link as well as the fiber which is used for data transmission.

A cable design was chosen that uses a traditional logging cable double armoring. To increase strength, a small 7/32-in. cable diameter was chosen. The cable-breaking strength is rated at 4,700 lbf. Signal attenuation is reduced by approximately 50% compared to 7/16-in. coaxial cable. Thus, resolution is outstanding with a range in excess of 20, 000 ft.

CASE HISTORIES

A plugged slotted liner, coiled tubing fish, corrosion survey, milling job, and inspection of a liquefied natural gas storage well are some of the instances in which the video cameras on fiber optic cables have been run.

In the case of the plugged slotted liner, the production from an oil well declined to the point that the well would no longer flow.

After several attempts to bring the well in failed, the oil remaining in the hole was displaced with clear fluid and a video camera run. The video monitor clearly showed that the slots in the slotted liner were plugged with scale.

After jet cleaning and back flushing, the camera was rerun. The monitor then clearly showed that the slots were open.

In another case, coiled tubing had broken and fallen downhole in 51/2-in. production tubing. After repeated efforts to latch the fish failed, the video camera was run to determine the condition of the fish.

The video camera showed the coiled tubing fish to be pushed against the high side of the production tubing. The visual image allowed the fishing operator to select the needed fishing tool.

In one case, successive tubing surveys indicated conflicting degrees of corrosion severity. A video camera was run to visually inspect the corrosion. The video survey clearly pictured the severity of the corrosion.

During a milling job, an operator was unsuccessful at fishing lost drill pipe after several milling runs. His overshot was not able to go as deep as his mill.

After running the video camera, the monitor clearly showed that a window had been milled through the casing and down beside the fish.

In another case, deliverability in a liquefied natural gas storage well had dropped below normal rates. The video camera was run and provided a clear sharp image that brine water had invaded the well bore. The interface between the brine water and the LNG could actually be seen on the monitor.

ACKNOWLEDGMENTS

The authors recognize Matt Riorden of MBR Engineering, and Don Perkins, John Goiffon, Bob Rademaker, and Keith Olszewski of Otis Engineering Corp. for their ongoing contributions in the development and testing of the fiber optic video system. The authors also thank the management of Westech Geophysical Inc. and Otis Engineering Corp. for permission to publish this article.