Downhole Piezo Gauge Overview
posted on 15 January 2019 by James Wright
Piezo refers to the “piezoresistive” effect of the silicon sensor. A piezoresistive semiconductor will see a change in resistivity due to a change in mechanical stress. Hence, a change in pressure or temperature results in a change in resistance. Some people call these tools “sapphire” or “strain” gauges. In every case, the same piezoresistive silicon sensor is used.
- Downhole pressure and temperature measurement
- Pressure up to 25,000 psi maximum
- Accuracy of up to +/- 0.022% full scale (Typically 0.03%)
- Resolution of 0.0003% full scale
- Temperature up to 177 °C
- Accuracy of +/-0.25 °C
- Resolution 0.005 °C
- 1 million data sets of memory
- 1 second to once a day sampling
- Special gauges can sample 10 times a second, or even up to
- 32,000 times a second
- Pressure triggers are available
- Accelerometer sensors can be added to the board to measure vibrations
- External fast response temperature sensors can be added
Piezo is less expensive - Less than half the price of a quartz pressure gauge.
Piezo is small – Which means it responds quickly to changes in pressure and temperature. This makes piezo gauges suitable for gradients.
Piezo is fast – We have piezo circuits which can sample up to 32,000 times per second. Perforating events can be captured with this high data acquisition rate.
The piezo resistive sensor is a small Silicon drum (shown below on top of a pencil) that uses changes in resistance to measure pressures and temperatures.
Looking closely at the chip above you can see a pattern that has been created on the top of the silicon. That pattern is a diffusion layer in the silicon that has been etched away. The diffusion layer is a semiconductor which has piezoresistive properties. This means that forces applied to the semi-conductor will result in a change in resistance of the material. There is a cavity in the middle of the silicon/glass sandwich and like a balloon, the die collapses under pressure, and the resistance values of the semi-conductor change.
DataCan’s piezo tools are offered in a variety of packages and with different options. It is important to understand each design. Critical mechanical package elements include: Outer Diameter and Battery Pack Size, Sensor Seal (O-Ring or Welded), Carrier Fitting (O-Ring or Metal to Metal), Battery Housing Seal. As well, each having different packages, DataCan offers custom boards which include: Pressure Trigger, Fast Sampling (10 samples per second), Accelerometer Sensor (to measure vibrations downhole), RTD Sensor (for fast temperature sensing), Smart Battery Technology (you can connect a smart battery to our download cable and measure its remaining life).
Outer Diameter – Options include 0.50”, 0.75”, 1.25”, and 1.375” OD.
The 0.50” size is a relatively new design and is great for applications with extreme space restraints. AAA battery packs are used in the 0.50” OD tool, which means the tool can only be run for a short period of time of 1 week or less.
The 0.75” size is our most economical package and works well for gauge carrier, coiled tubing, and DST operations. AA battery packs are used in the 0.75” OD tools, which limits the duration the tool can be “on” downhole, typically no more than 2 to 3 months.
DataCan now offers four different 0.75” OD packages. The 0.75” classic gauge is characterized by its o-ring sealed sensor; a very simple and reliable design. When additional memory is required, the 0.75” gauge with 4 million sample capacity is also available. It has expanded memory which allows for a high sample rate for an extended period of time.
The 0.75” welded piezo is characterized by its electron beam welded sensor and high pressure rating. And lastly, the 0.75” piezo with accelerometer is also available. Similar to the classic gauge but with an accelerometer sensor to measure any vibrations downhole.
The 1.25” gauge has a larger diameter barrel which allows for a large enough battery to run the gauge for a long duration job. DataCan offers four different 1.25” OD packages. The 1.25” classic gauge is characterized by its o-ring sealed sensor. The battery pack and housing for this tool is considered a standard or basic; a good economy tool.
The Welded III design is very similar to the 1.25” classic design, but in this case, the sensor is welded. Over time, DataCan will phase out the original 1.25” and replace it with the Welded III package. The 1.25” Welded Piezo RTD design has a RTD sensor for fast temperature sensing. It has a higher reliability with its shock mounted electronics and electron beam welded pressure sensor.
The 1.375” Welded Piezo IV is our flagship piezo tool. This tool is rugged and robust. It has all of the best design characteristics in mind. The sensor is welded and available for easy maintenance. The board is shock mounted for vibration and drops. The “C” sized battery pack is locking and large enough for long duration jobs. And the three o-rings and metal to metal seal prevent leaks from entering. This is DataCan’s most robust piezo pressure gauge.