Oil well pressure/temperature monitoring solution based on fiber Bragg grating

1. Overview

In the process of oil field development, people need to know the detailed information about the persistence and state of the fluid in the well during the production or water injection process. This requires the use of oil logging, whose reliability and accuracy are crucial, and traditional electronic-based sensors cannot work in the harsh environment of the well, such as high temperature, high pressure, corrosion, geomagnetic and geoelectric interference. Fiber Bragg grating sensors can overcome these difficulties. They are insensitive to electromagnetic interference and can withstand extreme conditions, including high temperature, high pressure (more than tens of megapascals) and strong shock and vibration. They can measure the wellbore and well site environmental parameters with high precision. At the same time, fiber Bragg grating sensors have quasi-distributed measurement capabilities. They can simultaneously measure the temperature and pressure of the well without interfering with the oil well parameter field, and can measure the temperature and pressure profile of the well. They can obtain data that cannot be obtained by traditional testing methods, providing a more ideal means for dynamic monitoring of oil wells. It is a test method that cannot be replaced by other logging and a breakthrough in testing technology. This technology is very suitable for dynamic monitoring of oil well production and provides a new set of ideas for testing technology. If the monitoring system is combined with control technology, it has great potential economic value for improving oil and gas recovery and strategic management of oil reservoirs.

The fiber Bragg grating sensing system mainly includes three parts: the sensing part, the transmission fiber and the fiber Bragg grating demodulator. The demodulator broadband light source emits broadband light λ, which is reflected back as a narrowband light λB after passing through the coupler to the fiber Bragg grating sensor. λB is measured by the wavelength detection device through the coupler and then converted into pressure or temperature value. The wavelength value of λB is related to the grating itself. The four fiber Bragg gratings in the figure reflect back four different wavelengths, so four wavelengths can be detected.

2. Technical index requirements

1. Six-stage pressure/temperature sensor connected in series, each stage includes one temperature and one pressure test point;

2. Temperature test index: range -10℃～175℃, accuracy ±0.5℃;

3. Pressure test index: range 0～40MPa, accuracy ±0.5%FS;

4. Optical cable diameter: <Φ15mm;

5. Sensor diameter: ≤Φ35mm;

6. The sensor is placed underground for long-term monitoring and works for 3 to 5 years.

3. System composition

Each sensor has two fiber Bragg gratings, one for measuring temperature and the other for measuring pressure. A 1m optical cable is led out from both ends of the sensor. The lead-out optical cable of the sensor is connected to the optical cable downhole through a connector. In this way, six sensors are connected in series through the optical cable downhole. The spacing between sensors can be determined as needed. A counterweight of about 30kg is added below the sensor to ensure that the sensor and the optical cable are vertically lowered into the well. The top of the sensor is connected to the demodulator in the ground monitoring room through an optical cable. The demodulator collects signals to calculate pressure and temperature, and then saves the data and displays it on the monitor.

4. Pressure/Temperature Sensor Characteristics

The function of the pressure/temperature sensor is to convert pressure and temperature into the wavelength signal of the fiber grating, which is the key part of the system. The sensor we designed has the following characteristics:

⑴ The technical indicators of the sensor are: Pressure: range 0～40MPa, accuracy ±0.5%FS, resolution 0.1%FS. Temperature: range -10℃～175℃, accuracy ±0.5℃, resolution 0.1℃. (When the resolution of the ground demodulator is 1pm and the accuracy is ±5pm)

⑵Adopt all stainless steel structure, resistant to pressure and corrosion

⑶ Maximum operating temperature of downhole sensor: 175℃

⑷Outer diameter of sensor: Φ30mm

⑸ The sensor is a double-ended optical fiber and can be connected in series, up to 6 levels (each level includes a pressure and a temperature test component)

⑹Long service life: 3~5 years[page]

5. Demodulator Characteristics

The demodulator is an instrument used to collect and analyze fiber Bragg grating signals, calculate and display temperature and pressure. It includes high-power broadband light source, high-precision spectrum analyzer, coupler, computer for data analysis and calculation and other accessories. Its demodulation accuracy directly affects the test accuracy of the system.

Technical indicators of our company's demodulator:

Number of channels: 1, 2, 4, 8, etc.

Wavelength range: 1525～1565nm

Resolution: 1pm

Repeatability: ±5pm

Dynamic range: >30dB

Minimum resolution interval: 0.6nm

Scan frequency: 1Hz

Working temperature: -10～60℃

Ambient humidity: 90%

6. Optical cables and connectors

The optical cable is a special double-core optical cable customized for use in oil wells (only one is used, the other is spare). It is protected by a stainless steel tube with a diameter of 3.5mm and a thickness of 0.4mm. It has the characteristics of compression resistance, tensile resistance, corrosion resistance and high temperature resistance, and can meet the requirements of long-term use underground.

Since the sensor and the optical cable need to be connected, the optical fiber in the sensor and the optical fiber in the optical cable can be fused, but the external oil and water will enter the sensor through the fusion of the optical cable, which will damage the sensor over time, so a special connector is needed to protect the connection point. We specially designed the connector for underground use.