How to Specify Connectors for Downhole Applications
Demanding requirements for downhole applications start with high pressure and high temperature and go on to encompass many other factors. Here, AMETEK reviews the main considerations for choosing a connector for these harsh applications.
There are few environments that test the extreme functionality of a harsh environment connector more than those found deep down in an oil or gas well. Demanding requirements for downhole applications start with high pressures and high temperatures (HPHT) and go on to encompass many other factors. When designing an HPHT bulkhead connector, the following parameters must be taken into consideration: pressure, pressure direction, temperature, electrical properties (insulation resistance, dielectric-withstanding voltage, current-carrying capacity), and corrosion resistance.
Pressure and Pressure Direction
In downhole applications, typical pressures range from 20kPSI to 35kPSI, and super-high-strength alloys are utilized in drilling and logging tools in order to maintain structural integrity. Alloys such as MP35N, Inconel 718, and Inconel X-750 are commonly used in the connector design. Engineers have developed specialized glass formulations to work with these super alloys to create a robust glass-to-metal-seal (GTMS) pressure barrier. At the connector-to-device interface, a redundancy-seal design is often utilized. Sealing features such as dual O-ring grooves with Viton O-rings and thermoplastic back-up rings are necessary to prevent well pressure from breaching the bulkhead connector and damaging the instruments and electronics within the tool. For additional reliability, metal seals can be incorporated into the connector design. Pressure direction will dictate seal location and the design of load-bearing surfaces.
As the pressure requirement increases, so does temperature. Temperature in a downhole application will typically range from +300°F to +500°F. Having the correct material combination is the difference between success and failure. There are few off-the-shelf glass formulations that will withstand the high temperatures or the thermal shock, which means a tool designer will usually turn to a specialist in the field of glass formulation.
The electrical properties of a connector are often considered a given; however, under HPHT conditions, these requirements are often the biggest hurdle to overcome in a design. Frequently constrained by the physical envelope of the connector, an elaborate interior seal design and highly engineered processes are needed to achieve the desired result. GTMS design rules are often stretched beyond generally accepted limits in order to achieve full electrical functionality in HPHT conditions. It takes years of experience to compile a technology tool box with the necessary means to achieve the desired electrical properties of an HPHT connector.
As if all of this was not enough to overcome, the downhole tool designer frequently has to add in the formidable task of resisting corrosion in the well. “Sour” wells, those that include an unhealthy level of hydrogen sulphide, have special material requirements that bring their own set of challenges to the GTMS process. It is common today to see material selection on a customer drawing governed by an NACE (National Association of Corrosion Engineers) specification, and the connector designer has to be able to work with the properties of an approved alloy.
Operating successfully and safely in any aspect of the challenging environment of the oil and gas world requires specialized connectors. In downhole operations, the high cost of exploration, drilling, and completion of a well places the highest demands on connector performance and reliability, and it is essential that the connector designer follows well-proven mechanical engineering principles.
This article was contributed by AMETEK Interconnect.