For many years operators have wished for the ability to recover downhole data without the need for cables.

The system

The Expro Group's CaTS telemetry system is a field proven system that finally delivers data from downhole in producing wells without the need for cables. Based on EM technology, the system transmits extremely low frequency EM waves from the downhole tool along the metal of the completion to a surface pick-up. The system can either be wireline deployed into existing wells as a retro-fit device or deployed in new wells in mandrel form as part of the completion.

The system is battery powered and as such can transmit a certain number of data packets for a given battery power. The number of data packets that can be sent is predominantly dependent on the depth of the tool, but the completion, formation resistivity and surface facilities all have an influence. To date the system has transmitted data from 10,000 ft (3,030 m) to surface without the need for any repeaters, and it is believed that depths of up to 15,000 ft (4,570 m) are achievable with the present tool.

Field case studies

The first generation tool has a quartzdyne sensor and is wireline deployed. It has been installed in a variety of situations in producing wells where there is no reservoir monitoring system. For example, a tool has been installed in a Nigerian field at a depth of 5,900 ft (1,799 m). In this case, there was an issue regarding security at the wellhead and the client wanted no equipment located on the wellhead or around it. The system, due to its unique technology, is able to transmit up the tubing, through the wellhead and along the pipeline to a gathering station. The signal is then detected at the gathering station more than 3 miles (5 km) away and the reservoir information then transmitted to the client's office.

In Brunei, a client wanted to optimize beam pumps and tried a variety of technologies to obtain pressure measurements from below the pump, none of which worked satisfactorily. A tool was installed below the beam pump and, due to its immunity to acoustic or electrical noise, successfully transmitted data to surface.

Simpler retro-fit applications have been done in producing wells, such as installations to replace failed permanent gauges and reservoir applications where no data monitoring system was installed.
The second generation tool is currently in development and will include a reduced specification version aimed at the tens of thousands of wells on land where today there is little or no reservoir monitoring. Removing the cost of cabling and clamps plus minimal installation costs means that has the ability to provide cost-effective reservoir monitoring in situations where presently such systems could not be justified on cost. Due to the nature of its technology and downhole power requirements, the system does not compete with traditional permanent monitoring (PM) systems which can provide high data rates for the life of a well. In contrast, the tool aims to deliver a few pressure readings a day for several years. In any case, it is questionable in long-term reservoir monitoring whether any more than one or two pressure readings a day are necessary.

High rate gas wells/multilaterals

For more complex applications, operators are looking at installing the equipment in high rate gas wells and multilaterals where cable cannot easily be run. In these situations the 10,000 ft to 15,000 ft depth limitation is easily overcome by using a downhole pick-up. This is simply an additional cable run from surface and grounded to the completion, either at the subsurface safety valve or another suitable point in the well. The system gauge then transmits between the tool and the pick-up point.
For example, in large bore high rate gas wells with 95/8-in. completions, it is typically not possible to install permanent monitoring systems in the lower completion and as a result any gauges may be several thousand feet from the sand face. This leads to unknowns in the pressure measurement due to gravity and friction effects. A tool can be installed at the sand-face and a traditional permanent monitoring downhole (PMD) system installed in the upper completion. The tool then transmits from the sand-face to a pick-up at the PMD gauge and the pressure data obtained is used to calibrate the PMD gauge for the effects of gravity and friction.

Similar approaches can be used in multilaterals or long screen completions. A tool can be installed in each of the laterals with either a traditional PMD system above the junction or just a simple downhole pick-up giving the operator pressure, or indeed flow, from each of the laterals. Similarly, multiple tools can be spaced across a long horizontal screen to give accurate pressure measurements along the well bore.

Abandoned wells

In addition to reservoir monitoring from producing wells, the system has been used as a monitoring device for abandoned or suspended wells. Globally each year there are many subsea appraisal wells which are drilled and then abandoned without any long-term reservoir data being obtained. Traditionally the only way to obtain data was to install memory gauges and then suspend the well. An expensive rig would then have to be mobilized to recover the gauges - not very cost effective in deepwater. The tool presents a cost effective alternative method of obtaining that critical reservoir data.

The tools are installed and the well suspended or abandoned in whatever way is required. The presence of bridge plugs or cement has no effect on the performance of the technology. A pick-up and recorder box is installed on the subsea wellhead with an industry standard acoustic transmission system. The installation on the wellhead is very simple and can be performed by remotely-operated vehicle (ROV). All that is then required is for a surface vessel to come by and recover the data from the subsea recorder at whatever intervals may be required by the operator. Data obtained from such wells can be critical in making field development decisions.

Tight gas application on land

Another client has used the system in a similar application on land in the Rocky Mountains in Wyoming, United States. In tight gas developments, well spacing and interconnectivity are critical. The operator had a well with nine zones in a producing field that was going to be abandoned. During the abandonment process, nine tools were installed and each zone isolated with bridge plugs and the well abandoned as per regulatory requirements. The tools have since been transmitting pressures to surface and then via radio link to the client's office, thus allowing the operator to monitor the impact of nearby producing wells and thereby to determine connectivity.

Such was the success of this job that a second installation is under consideration. However, in this case it will be in a newly drilled well with up to 20 zones. The well will initially be used as a monitoring well and then turned into a producer. This creates the issue of "how do you get pressure measurements from the outside of the casing in an unperforated well?" The answer is simple - use a combination of the system plus elements of casing conveyed perforating system.

A reduced size casing is run, allowing the gauges to be mounted on the outside of the casing, and the well is cemented as normal. To allow pressure transmission between the formation and the pressure gauge, a perforating module is mounted on the outside of the casing in conjunction with the gauge. The perforating module charges are installed so as to only perforate into the formation and not the casing, thus leaving the wellbore integrity complete. The gun is then initiated creating a path from the gauge to the formation. The gauges will then send reservoir pressure to surface from the outside of the casing. The operator then can go back in and complete the well when he chooses, perforating only those zones required. The gauges will then continue to transmit pressure from both the zones that have been perforated, as well as those which have not.

While initially the tool has been used solely for reservoir monitoring using pressure and temperature sensors, the technology can be applied to virtually any downhole sensor. For example, it could be combined with a Venturi gauge to provide single phase flow measurements for injector or gas wells, or downhole sand detectors could be integrated into the system.

Downhole control

Given that the system can both transmit and receive data, it can also be used for downhole control. With a simple surface transmitter, the tool can receive data and work instructions from surface. This enables data transmission intervals to be reset from surface and specific instructions to be sent. This ability to control downhole equipment from surface without the need for cabling is a key enabling component of retro-fit flow control.

Wireline deployed, retro-fit flow control devices have many potential applications. In the near term, the ability to replace failed downhole surface safety valves without the negative impact of storm chokes, is a key objective. The system can be combined with a single-shot sliding sleeve and run on wireline with a packer to provide downhole flow control. Development programs are ongoing to enable this technology to replace storm chokes. In the longer term the vision is to develop a retro-fit intelligent well.
Today the take-up of intelligent wells is lower than anticipated and two of the major reasons behind this are the up front capital requirements and the long term reliability concerns. The design of the intelligent well and the associated capital investments have to be made before the well starts producing and, therefore, before the reservoir is fully understood. In an ideal world, a cost-effective retro-fit-able flow control device could be installed as and when the need occurs, thus reducing risk and up-front investment. In addition the retro-fit nature eliminates many of the long-term reliability concerns of intelligent wells.

Clearly, retro-fit flow control devices are only one part of the overall picture. Also required are cost-effective intervention and deployment methods. To this end, the company has been developing tractor systems for deployment and subsea rigless intervention systems to enable step-changes in subsea intervention costs.

The list of applications for cableless telemetry systems continues to grow as operators begin to understand the capabilities of the system and then think of the problems for which it could provide a solution. However, like all new technologies in the oil industry, it faces many challenges not least of which is the inherent conservativeness of our industry towards evolving technology.

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