Capacitors have a wide spectrum of applications in electronic circuits for downhole tools. These components are used in power supply circuits, timing and wave-shaping circuits, and filtering circuits, to name a few. The performance of capacitors is greatly affected by operating conditions. Exposing these components to extreme environmental conditions can reduce their life or damage them. Some of the characteristics that are affected when capacitors are subjected to high temperatures include capacitance, insulation resistance, and dissipation factor. When selecting a capacitor for a downhole tool, it is vital to consider its tolerance to extreme environmental conditions.

The oil and gas industry employs high tech equipment for exploration and extraction of hydrocarbons. The conditions in today’s oilfields demand components that can withstand extreme temperatures, high pressures, and extreme mechanical shock and vibrations. Due to these harsh conditions, components used in drilling tools are highly susceptible to failure. Virtually all easily accessible oil reserves have been exhausted, and exploration companies are drilling deeper to get hydrocarbons. This trend is demanding a new generation of electronic components with higher tolerances to extreme environmental conditions. Capacitor manufacturers are pushing the boundaries of innovation to provide components with higher temperature ratings, higher capacitance stability, longer service life, and better mechanical durability.

Capacitors for use in deep-well logging and monitoring tools

Capacitors are essential components in electronic circuits for sonic logging tools, density logging tools, gamma-ray logging tools, and resistivity logging tools, to name a few. Capacitor manufacturers have made significant advancements in capacitor manufacturing technology to meet the growing demand for components with high temperature ratings and reliability. These capacitors are also suitable for other applications that expose electronic components to harsh conditions such as aerospace and automotive applications.

Electronic components used in deep-well drilling tools are exposed to extreme temperatures, pressures, and shock and vibrations. The downhole oil and gas industry demands highly reliable and physically robust capacitors. Film capacitors and aluminium electrolytic capacitors have lower maximum temperature ratings (usually up to 150°C), and are generally unsuitable for high temperature downhole applications (there are, however, some exceptions to this rule e.g. the Exxelia 911P series of plastic film capacitors). Ceramic and tantalum capacitors can achieve higher temperature ratings, and they are commonly used in high temperature deep-well drilling tools.

The maximum temperatures and pressures for drilling operations have been increasing as oil drillers go deeper. In the past, the maximum temperature ratings for drilling operations ranged between 150°C and 175°C. In comparison, temperatures exceeding 200°C and pressures beyond 25kpsi have been recorded in some of today’s wells. The conditions are expected to become even harsher as exploration activities move towards locations with more hostile conditions. In drilling operations, it is the measurement-while-drilling (MWD) tools that are exposed to the harshest temperatures. Wired measurement tools, completion tools, and production monitoring tools are exposed to temperatures that are slightly lower.

Measurement-while-drilling tools are usually connected next to the drill bit. This position exposes their electronic components to extreme heat, vibrations, and mechanical shock. In deep gas reservoirs, temperatures can be as high as 250°C. In hostile drilling locations, extreme shocks of up to 2000Gs and vibrations of up to 15G have been recorded. Measurement-while-drilling tools work with logging-while-drilling (LWD) tools to collect downhole data. MWD and LWD tools require capacitors that can withstand extremes of temperature, pressure, and mechanical shock.

Wired logging tools are usually attached to a wire and lowered into a well. These tools use high temperature electronic components and batteries to gather and store data about an existing well. Electronic circuits for wired logging tools are exposed to high temperatures of up to 225ºC. The temperature increases sharply from ambient to maximum temperature within a fraction of an hour. As compared to measurement-while-drilling tools, wired measurement tools are not exposed to extreme shock and vibrations.

Completion tools are used to monitor various conditions of a well including flow, density, pressure, and temperature. These tools can be exposed to temperatures that are as high as 175ºC. As compared to MWD tools, electronic circuits for completion tools are not exposed to extreme shock and vibrations. Unlike MWD, LWD, and wired logging tools, production monitoring tools have longer exposures to harsh conditions. These tools require capacitors that can withstand harsh conditions and provide high reliability and durability.

Ceramic capacitors for downhole applications

The performance characteristics of multilayer ceramic capacitors (MLCCs) vary depending on the materials used and construction technology. Stacked C0G and X7R capacitors are commonly used for high temperature applications. These capacitors are designed to withstand high operating temperatures (usually up to 200ºC) and extreme shock and vibrations. Unlike traditional X7R capacitors, stacked ceramic capacitors have reduced capacitance loss at high temperatures. High temperature solders are used to attach the leads of these ceramic capacitors to the body. Furthermore, stacked ceramic capacitors have low ESL and ESR, and they are optimized to allow soldering flexibility.

X7R multilayer ceramic capacitors are semi-stable, relatively cheap, and have good volume efficiency.  For temperatures up to 125ºC, the capacitance of these passive components is relatively constant. At high temperatures, the capacitance of these ceramic components drops depending on the applied voltage. At 200ºC, capacitors built with X7R dielectric material lose 45% of their capacitance. When tested at 260ºC, the loss increases to 60%. As compared to C0G capacitors, these capacitors have a higher temperature coefficient of capacitance (TCC). The X7R dielectric material allows designing of compact components for applications that have space limitations.


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The capacitance of C0G dielectric multilayer ceramic capacitors is stable and does not vary significantly with voltage and temperature. The capacitance of these components remains relatively constant for temperatures up to 200ºC. These capacitors have relatively low temperature coefficient of capacitance. However, they have lower capacitance values than X7R devices.

Apart from high temperatures and pressures, ceramic capacitors for use in deep-well drilling are also exposed to extreme shock and vibrations. These conditions make ceramic capacitors more susceptible to cracking. Capacitor manufacturers have been reducing the case sizes of these capacitors to reduce their vulnerability to cracking under harsh conditions. Using high-break-strength dielectrics also helps to enhance the mechanical robustness of ceramic capacitors. Compared to X7R dielectrics, C0G materials have a higher resistance to cracking.

The demand for capacitors that can withstand harsher environmental conditions is growing.  Manufacturers are exploring materials and construction processes that can enable them to produce ceramic capacitors with higher temperature ratings and more resistance to high pressures and mechanical shock. Stacked ceramic capacitors with a temperature rating of 200ºC are suitable for most deep-well drilling tools. Manufacturers are currently working to develop stacked multilayer ceramic capacitors with temperature ratings of 230ºC, 260ºC, and above.

Tantalum capacitors for downhole applications

Tantalum capacitors offer good mechanical robustness, impressive stability, and high reliability, and they are widely used in deep-well drilling tools. These capacitors come in different temperature ratings to meet the diverse needs of today’s applications. High temperature solid and wet tantalum capacitors are optimized to deliver high reliability under hostile environmental conditions. Advanced high-temperature tantalum capacitors can withstand exposure to extreme temperatures up to 200ºC. Hermetically sealed tantalum capacitors, a new generation of high temperature solid tantalum capacitors, have higher tolerances to harsh environment. These SMD capacitors have high temperature ratings (usually above 200ºC), and they are suitable for downhole logging environment.

Solid tantalum capacitors are designed to deliver high reliability while operating at high temperatures. Moulded tantalum capacitors with a temperature rating of 175ºC are widely used in high reliability circuits for use in engine control units, steering control systems, and so on. 200°C-rated solid tantalum capacitors are suitable for electronic circuits for use in downhole drilling operations. Tantalum capacitors for use in high temperature applications require voltage derating.

Wet tantalum capacitors with a temperature rating of 200°C are commonly used in downhole drilling tools. These capacitors have high ripple current capabilities, low equivalent series resistance (ESR), high bulk capacitance, and low leakage current. High temperature wet tantalum capacitors are mostly used for energy storage and filtering applications.

As oil drilling operations move towards harsher environments, the demand for tantalum capacitors with higher temperature ratings is increasing. This has pushed manufacturers to explore new materials and capacitor manufacturing technologies. Using a hermetically sealed structure, manufacturers have managed to produce tantalum capacitors with temperature ratings of above 200°C. Tantalum capacitors with a temperature rating of 230°C have been developed by enclosing the element in a hermetically-sealed SMD casing. In addition, these capacitors have demonstrated high reliability and impressive tolerance to mechanical shock and vibrations.

Conclusion

Capacitors are fundamental components in high-tech tools used in downhole oil and gas drilling. Oil exploration and production activities expose electronic sensors and components to extreme temperatures, pressures, and shock and vibrations. These harsh conditions make capacitors more susceptible to failure. The harsh conditions in today’s oilfields demand electronic circuits with highly reliable components that can withstand extremes of temperature, pressure, and vibrations. Multilayer ceramic and tantalum capacitors for downhole applications are designed to offer high reliability under hostile conditions. Capacitor manufacturers are increasingly advancing manufacturing technologies to meet the growing demand for components with higher tolerances to harsh downhole environment.

The Capacitor Faks DATA HUB has the largest collection of down-hole rated capacitor data sheets in the industry. Visit the DATA HUB and filter on “Down Hole” from the Grade drop-down box; enhance your search by adding suitable options from the Dielectric and Mounting Style drop-down boxes, as well as “Stacked” from the Form Factor drop-down for ultra high temperature stacked MLC types.